Mastering Physics HW 4 Ch 18, 19
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Transcript of Mastering Physics HW 4 Ch 18, 19
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 118
HW 4 Ch 18 19
Due 1159pm on Thursday October 1 2015
To understand how points are awarded read the Grading Policy for this assignment
Refrigerator Prototypes Ranking Task
Six new refrigerator prototypes are tested in the laboratory For each refrigerator the electrical power needed for it to operate and the maximum heatenergy that can be removed per second from its interior are given
Part A
Rank these refrigerators on the basis of their performance coefficient
Rank from largest to smallest To rank items as equivalent overlap them
Hint 1 How to approach the problem
A refrigerator is a devic e t hat uses work to remove heat energy from a c old reservoir and deposit it into a hot reservoir By conservation of energy the energy deposited in the hot reservoir is the sum of the work done on the refrigerator and the energy removed from the cold reservoir
A good refrigerator (with a large performance coeffic ient) will remove a large amount of heat energy from the cold reservoir for a s mall amount of work input
Hint 2 Definition of the performance coefficient
The performance coeffic ient of a refrigerator is defined as the ratio of the heat energy removed from the cold reservoir to the work
input to the refrigerator
Recall that power is defined as work per unit time
ANSWER
Correct
Part B
The six refrigerators are placed in six identical sealed rooms Rank the refrigerators on the basis of the rate at which they raise the temperature of th
P
Δ 983156 Q
C m a x
983147 Q
C
W
983147 =
Q
C
W
Typesetting math 95
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 218
room
Rank from largest to smallest To rank items as equivalent overlap them
Hint 1 Temperature and Conservation of energy
A refrigerator uses electrical energy to remove heat from its interior and expel it into the environment B y conservation of energy the energyexpelled into the room must be the sum of the energy extracted from the interior of the refrigerator and the energy input to run the refrigeratorNotice that the rate at which the temperature of the room rises is directly proportional to the rate at which energy is expelled into the room
ANSWER
Correct
plusmn PSS 191 Heat-Engine Problems
Learning Goal
To practice Problem-Solving Strategy 191 for heat-engine problems
A heat engine uses the closed cyc le s hown in the diagram below The working substance ismoles of monatomic ideal gas Find the efficiency of such a cycle Use the values for pressureand volume shown in the diagram and assume that the process between points 1 and 3 isisothermal
PROBLEM-SOLVING STRAT EGY 191 Heat-engine problems
MODEL Identify each process in the cycle
983150
η
7242019 Mastering Physics HW 4 Ch 18 19
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VISUALIZE Draw the pV diagram of the cycle
SOLVE There are several steps in the mathematical analysis
Use the ideal gas law to complete your knowledge of and at one point in the cycleUse the ideal gas law and equations for specific gas processes to determine and at the beginning and end of each processCalculate and for each processFind by adding for each process in the cycle If the geometry is simple you can confirm this value by finding the area enclosedwithin the pV curve
Add just the positive values of to find
Verify that This is a self-consist ency check to verify that you havent made any mistakes
Calculate the thermal effic iency and any other quantities you need to complete the solution
ASSESS Is Do all the signs of and make sense Does have a reasonable value Have you answered the question
Model
Part A
The cycle used by the engine is composed of three processes a process at constant pressure between point 1 and 2 a process at constant volumebetween points 2 and 3 and an isothermal process between points 3 and 1 What are the processes between points 1 and 2 and between points 2 an3 respectively
ANSWER
Correct
Visualize
The pV diagram is already given in the problem introduction You may want to make a copy of the diagram in your notes so that you can add further information to it as you work through the next part
Solve
Part B
Find the effic iency of the heat engine
Express your answer as a decimal number to three significant figures
Hint 1 The efficiency of a heat engine
The efficiency of a heat engine is given by the ratio of the work done by the engine (work output ) to the energy transferred into theengine (heat input )
Hint 2 How to find the work done in one cycle
There are two ways to compute the net work done by the engine during one full cycle Using a geometrical approach calculate the areaenclosed by the pV curve for the cycle is equal to the value of this area Alternatively identify all the processes in one full cyc le andcompute the work done by the engine in each process Find by adding for each process in the cycle
Hint 3 First method Use calculus to compute areas
Because the region enclosed by the pV curve shown in this problem does not have a simple shape you need to use your knowledge of calculus to compute its area Recall that the area between the graphs of two functions and over a certain interval [ ] is given
by
For this particular problem take the function whose graph is the curve for the process as the function and the function whose graph is
983150 983152 V T
983152 V T
Q W
s
Δ E
t h
W
o u t
W
s
Q Q
H
( Δ = 0 E
t h
)
n e t
η
( Δ = 0 E
t h
)
n e t
W
s
Q η
isochoric and isobaric
isobaric and isochoric
isobaric and isothermal
isothermal and isochoric
η
η W
o u t
Q
H
η = W
o u t
Q
H
W
o u t
W
o u t
W
s
W
o u t
W
s
A 983142 ( 983160 ) 983143 ( 983160 )
a b
A = ( 983142 minus 983143 ) 983140 983160 int
b
a
3 rarr 1 983142
7242019 Mastering Physics HW 4 Ch 18 19
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the curve for the process as Use the ideal gas law to find the correct mathematical expressions for these functions Perform the
integration over the V interval [3 9]
Hint 4 Second method Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isobaric processes
In an isobaric process at pressure the work done by the syst em in which the volume changes by an amount is given by
ANSWER
Hint 5 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isochoric processes
Recall that an isochoric process is a process in which the volume does not change Consequently no work is done on or by the system
ANSWER
Hint 6 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isothermal processes
In an isothermal process at temperature the work done by the sys tem when the volume changes from to is given by
where is the number of moles in the gas and is the gas constant
Hint 2 How to solve this problem when moles and temperature are not given
Leave (the number of moles of gas) and (the temperature of the gas during the process ) as symbols in your calculation Youwill find in the end that you need a numerical value for the product Use the ideal gas law to compute this product in terms of thepressure and volume at either point 1 or point 3
ANSWER
Hint 7 Find which processes contribute to the heat input
During which processes in the cycle is heat delivered to the engine
123
Enter the letter(s) of all the correct answers in alphabetical order Do not use commas For example if you think all three processes
deliver heat to the engine enter ABC
1 rarr 2 983143
( c ) 1 0
2
m
3
1 rarr 2
W
1 2
1 rarr 2
983152 Δ V
= 983152 Δ V W
983155
=W
1 2
J
2 rarr 3
W
2 3
2 rarr 3
=W
2 3
J
3 rarr 1
W
3 1
3 rarr 1
T V
i
V
f
= 983150 R T l n ( ) W
983155
V
f
V
i
983150 R
983150 T 3 rarr 1
983150 R T
=W
3 1
J
1 rarr 2
2 rarr 3
3 rarr 1
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ANSWER
Hint 8 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isochoric processes
The heat transfer during an isochoric process in which the temperature of the gas changes by an amount is
where is the number of moles in the gas and is the heat capacity at constant volume
Hint 2 The properties of monatomic gases
For monatomic gases the heat capacity at constant volume is
where is the gas constant
Hint 3 How to compute the change in temperature
Even though the temperature of the gas is not given in the problem at certain points in the cycle can be expressed in terms of someknown quantities using the ideal gas law Thus use this law to find the temperature at points 2 and 3 in the pV diagramand then compute Note that you wonrsquot need to know how many moles are in the gas because when you calculate theamount of heat transferred in the process the number of moles will cancel out
ANSWER
Hint 9 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isothermal processes
Since in an isothermal process the thermal energy of the gas does not change is zero From the first law of thermodynamics itfollows that the heat transfer during an isothermal process is equal to the work done by the syst em or In other words all of the heat input during an isothermal process is converted into work done by the gas
ANSWER
ANSWER
Correct
If you prefer to express the effic iency as a percentage in this case you would say that the engine is 206 effic ient
Assess
Part C
2 rarr 3
Q
2 3
2 rarr 3
Δ T
Q = 983150 Δ T C
V
983150 C
V
= C
V
3 R
2
R
T
983152 V =
983150 R T
Δ T = minus T
3
T
2
983150
=Q
2 3
J
3 rarr 1
Q
3 1
3 rarr 1
Δ E
t h
Q = W
s
=Q
3 1
J
= 0206η
7242019 Mastering Physics HW 4 Ch 18 19
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Here is a table of energy transfers for this problem with some entries missing
Process (J) (J) (J)
What must be the heat input in process to satis fy the condition that
Express your answer in joules
Hint 1 How to approach the problem
To compute the heat transfer occurring during the isobaric process you can either use the generic expression for heat transfer in isobaric
processes where is the heat capacity at constant pressure or you can apply directly the first law of thermodynamics
to the process To do that youll need to know the change in thermal energy of the gas during the isobaric processRead from the table the changes in thermal energy during the isochoric and the isothermal processes and then calculate for the isobaricprocess using the fact that for a complete cyc le
ANSWER
Correct
You can easily verify that your results make sense The net heat transfer per cyc le is which is equal to the
work done by the engine in one cyc le By the first law of thermodynamics it follows that which is what we would expect foa cyclic process
An Air Conditioner Refrigerator or Heat Pump
The typical operation cycle of a common refrigerator is shown schematically in the figure Boththe condenser coils to the left and the evaporator coils to the right contain a fluid (the workingsubstance) called refrigerant which is typically in vapor-liquid phase equilibrium The compressor takes in low-pressure low-temperature vapor and compresses it adiabatically to high-pressurehigh-temperature vapor which then reaches the condenser Here the refrigerant is at a higher temperature than that of the air surrounding the condenser coils and it releases heat by
undergoing a phase change The refrigerant leaves the condenser coils as a high-pressure high-temperature liquid and expands adiabatically at a controlled rate in the expansion valve As thefluid expands it cools down Thus when it enters the evaporator coils the refrigerant is at alower temperature than its surroundings and it absorbs heat The air surrounding the evaporator cools down and most of the refrigerant in the evaporator coils vaporizes It then reaches thecompressor as a low-pressure low-temperature vapor and a new cycle begins
Part A
Air conditioners operate on the same principle as refrigerators Consider an air conditioner that has 700 of refrigerant flowing through it s circuiteach cycle The refrigerant enters the evaporator coils in phase equilibrium with 540 of its mass as liquid and the rest as vapor It flows throughthe evaporator at a constant pressure and when it reaches the compressor 95 of its mass is vapor In each cycle how much heat is absorbed
by the refrigerant while it is in the evaporator The heat of vaporization of the refrigerant is 150times10 5
Express your answer numerically in joules
Hint 1 How to approach the problem
To transform a given mass of liquid refrigerant to vapor requires the addition of a certain quantity of heat that depends on the heat of vaporization of the refrigerant and the mass of refrigerant transformed to vapor Since the refrigerant is in phase equilibrium when it enters theevaporator it is already at boiling temperature and all the absorbed heat is used in the liquid-vapor phase change Note that the pressure is keptconstant in the evaporator Also a small fraction of refrigerant is still liquid when it leaves the evaporator so the refrigerant must still be inphase equilibrium at that point and no change in temperature has occurred in the evaporator
W
s
Q Δ E
t h
1 rarr 2 minus 1 2 0
2 rarr 3 0 1 8 0 1 8 0
3 rarr 1 1 9 8 1 9 8 0
Q
1 2
1 rarr 2 ( Δ = 0 E
t h
)
n e t
Q
1 2
Q = 983150 Δ T C
983152
C
p
Δ = Q minus E
t h
W
s
1 rarr 2
Δ E
t h
Δ = 0 E
t h
= -300Q
1 2
J
= + + = 7 8 J Q
n e t
Q
1 2
Q
2 3
Q
3 1
W
o u t
Δ = 0 0 J E
t h
k g
Q
c
J k g
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 2 Find the percentage of refrigerant transformed to vapor
Consider a refrigerator where in each cycle 540 of refrigerant enters the evaporator as liquid in liquid-vapor phase equilibrium and 95leaves as vapor Assume that the pressure of the refrigerant is kept constant while it is in the evaporator What is the percentage of the
total mass of refrigerant transformed to vapor in the evaporator per cycle
ANSWER
ANSWER
Correct
Part B
In each cycle the change in internal energy of the refrigerant when it leaves the compresser is 120times105 What is the work done by the motorthe compressor
Express your answer in joules
Hint 1 Adiabatic compression
Recall that the compressor performs an adiabatic compression that is no heat exchange occurs while the refrigerant is in the compressorThen use the first law of thermodynamics to determine
ANSWER
Correct
Part C
If the direction of the refrigerant flow is inverted in an air conditioner the air conditioning unit turns into a heat pump and it can be used for heatingrather than cooling In this case the coils where the refrigerant would condense in the air conditioner become the evaporator coils when the unit isoperated as a heat pump and vice versa the evaporator coils of the air conditioner become the condenser coils in the heat pump Suppose youoperate the air conditioner described in Parts A and B as a heat pump to heat your bedroom In each cycle what is the amount of heat released
into the room You may assume that the energy changes and work done during the expansion process are negligible compared to those for other processes during the cycle
Express your answer numerically in joules
Hint 1 How to approach the problem
When the air conditioner is operated in the cooling mode in each cycle it absorbs an amount of heat from the air inside the room as you
calculated in Part A and gives off a certain amount of heat to the outside When it is operated in the heating mode instead the direction of the flow of the refrigerant inside the coil circuit is simply inverted but the operation cycle remains the same In the heating mode then thesame quantity is now absorbed from the air outside the room and the amount of heat is released to the air inside the room Note that
the compressor does the same amount of work regardless of the mode of operation and use the first law to determine
Hint 2 Find the right expresssion for the first law of thermodynamics
Suppose you have an ideal sys tem that absorbs heat from a cold reservoir and releases heat to a hot reservoir The net input of work
required by this system for operation is Which of the following expressions is correct
Hint 1 Cyclic processes
Remember that the net internal energy change in a cyclic process is zero since the system has the same temperature when it returns tothe starting point
983149
=983149
= 515times105 Q
c
J
J W
W
= 120times105 W J
Q
h
Q
c
Q
h
Q
c
Q
h
Q
h
Q
c
Q
h
W
i n
7242019 Mastering Physics HW 4 Ch 18 19
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ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 918
Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
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Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
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Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
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Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
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To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
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Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
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- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
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room
Rank from largest to smallest To rank items as equivalent overlap them
Hint 1 Temperature and Conservation of energy
A refrigerator uses electrical energy to remove heat from its interior and expel it into the environment B y conservation of energy the energyexpelled into the room must be the sum of the energy extracted from the interior of the refrigerator and the energy input to run the refrigeratorNotice that the rate at which the temperature of the room rises is directly proportional to the rate at which energy is expelled into the room
ANSWER
Correct
plusmn PSS 191 Heat-Engine Problems
Learning Goal
To practice Problem-Solving Strategy 191 for heat-engine problems
A heat engine uses the closed cyc le s hown in the diagram below The working substance ismoles of monatomic ideal gas Find the efficiency of such a cycle Use the values for pressureand volume shown in the diagram and assume that the process between points 1 and 3 isisothermal
PROBLEM-SOLVING STRAT EGY 191 Heat-engine problems
MODEL Identify each process in the cycle
983150
η
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VISUALIZE Draw the pV diagram of the cycle
SOLVE There are several steps in the mathematical analysis
Use the ideal gas law to complete your knowledge of and at one point in the cycleUse the ideal gas law and equations for specific gas processes to determine and at the beginning and end of each processCalculate and for each processFind by adding for each process in the cycle If the geometry is simple you can confirm this value by finding the area enclosedwithin the pV curve
Add just the positive values of to find
Verify that This is a self-consist ency check to verify that you havent made any mistakes
Calculate the thermal effic iency and any other quantities you need to complete the solution
ASSESS Is Do all the signs of and make sense Does have a reasonable value Have you answered the question
Model
Part A
The cycle used by the engine is composed of three processes a process at constant pressure between point 1 and 2 a process at constant volumebetween points 2 and 3 and an isothermal process between points 3 and 1 What are the processes between points 1 and 2 and between points 2 an3 respectively
ANSWER
Correct
Visualize
The pV diagram is already given in the problem introduction You may want to make a copy of the diagram in your notes so that you can add further information to it as you work through the next part
Solve
Part B
Find the effic iency of the heat engine
Express your answer as a decimal number to three significant figures
Hint 1 The efficiency of a heat engine
The efficiency of a heat engine is given by the ratio of the work done by the engine (work output ) to the energy transferred into theengine (heat input )
Hint 2 How to find the work done in one cycle
There are two ways to compute the net work done by the engine during one full cycle Using a geometrical approach calculate the areaenclosed by the pV curve for the cycle is equal to the value of this area Alternatively identify all the processes in one full cyc le andcompute the work done by the engine in each process Find by adding for each process in the cycle
Hint 3 First method Use calculus to compute areas
Because the region enclosed by the pV curve shown in this problem does not have a simple shape you need to use your knowledge of calculus to compute its area Recall that the area between the graphs of two functions and over a certain interval [ ] is given
by
For this particular problem take the function whose graph is the curve for the process as the function and the function whose graph is
983150 983152 V T
983152 V T
Q W
s
Δ E
t h
W
o u t
W
s
Q Q
H
( Δ = 0 E
t h
)
n e t
η
( Δ = 0 E
t h
)
n e t
W
s
Q η
isochoric and isobaric
isobaric and isochoric
isobaric and isothermal
isothermal and isochoric
η
η W
o u t
Q
H
η = W
o u t
Q
H
W
o u t
W
o u t
W
s
W
o u t
W
s
A 983142 ( 983160 ) 983143 ( 983160 )
a b
A = ( 983142 minus 983143 ) 983140 983160 int
b
a
3 rarr 1 983142
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the curve for the process as Use the ideal gas law to find the correct mathematical expressions for these functions Perform the
integration over the V interval [3 9]
Hint 4 Second method Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isobaric processes
In an isobaric process at pressure the work done by the syst em in which the volume changes by an amount is given by
ANSWER
Hint 5 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isochoric processes
Recall that an isochoric process is a process in which the volume does not change Consequently no work is done on or by the system
ANSWER
Hint 6 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isothermal processes
In an isothermal process at temperature the work done by the sys tem when the volume changes from to is given by
where is the number of moles in the gas and is the gas constant
Hint 2 How to solve this problem when moles and temperature are not given
Leave (the number of moles of gas) and (the temperature of the gas during the process ) as symbols in your calculation Youwill find in the end that you need a numerical value for the product Use the ideal gas law to compute this product in terms of thepressure and volume at either point 1 or point 3
ANSWER
Hint 7 Find which processes contribute to the heat input
During which processes in the cycle is heat delivered to the engine
123
Enter the letter(s) of all the correct answers in alphabetical order Do not use commas For example if you think all three processes
deliver heat to the engine enter ABC
1 rarr 2 983143
( c ) 1 0
2
m
3
1 rarr 2
W
1 2
1 rarr 2
983152 Δ V
= 983152 Δ V W
983155
=W
1 2
J
2 rarr 3
W
2 3
2 rarr 3
=W
2 3
J
3 rarr 1
W
3 1
3 rarr 1
T V
i
V
f
= 983150 R T l n ( ) W
983155
V
f
V
i
983150 R
983150 T 3 rarr 1
983150 R T
=W
3 1
J
1 rarr 2
2 rarr 3
3 rarr 1
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ANSWER
Hint 8 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isochoric processes
The heat transfer during an isochoric process in which the temperature of the gas changes by an amount is
where is the number of moles in the gas and is the heat capacity at constant volume
Hint 2 The properties of monatomic gases
For monatomic gases the heat capacity at constant volume is
where is the gas constant
Hint 3 How to compute the change in temperature
Even though the temperature of the gas is not given in the problem at certain points in the cycle can be expressed in terms of someknown quantities using the ideal gas law Thus use this law to find the temperature at points 2 and 3 in the pV diagramand then compute Note that you wonrsquot need to know how many moles are in the gas because when you calculate theamount of heat transferred in the process the number of moles will cancel out
ANSWER
Hint 9 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isothermal processes
Since in an isothermal process the thermal energy of the gas does not change is zero From the first law of thermodynamics itfollows that the heat transfer during an isothermal process is equal to the work done by the syst em or In other words all of the heat input during an isothermal process is converted into work done by the gas
ANSWER
ANSWER
Correct
If you prefer to express the effic iency as a percentage in this case you would say that the engine is 206 effic ient
Assess
Part C
2 rarr 3
Q
2 3
2 rarr 3
Δ T
Q = 983150 Δ T C
V
983150 C
V
= C
V
3 R
2
R
T
983152 V =
983150 R T
Δ T = minus T
3
T
2
983150
=Q
2 3
J
3 rarr 1
Q
3 1
3 rarr 1
Δ E
t h
Q = W
s
=Q
3 1
J
= 0206η
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Here is a table of energy transfers for this problem with some entries missing
Process (J) (J) (J)
What must be the heat input in process to satis fy the condition that
Express your answer in joules
Hint 1 How to approach the problem
To compute the heat transfer occurring during the isobaric process you can either use the generic expression for heat transfer in isobaric
processes where is the heat capacity at constant pressure or you can apply directly the first law of thermodynamics
to the process To do that youll need to know the change in thermal energy of the gas during the isobaric processRead from the table the changes in thermal energy during the isochoric and the isothermal processes and then calculate for the isobaricprocess using the fact that for a complete cyc le
ANSWER
Correct
You can easily verify that your results make sense The net heat transfer per cyc le is which is equal to the
work done by the engine in one cyc le By the first law of thermodynamics it follows that which is what we would expect foa cyclic process
An Air Conditioner Refrigerator or Heat Pump
The typical operation cycle of a common refrigerator is shown schematically in the figure Boththe condenser coils to the left and the evaporator coils to the right contain a fluid (the workingsubstance) called refrigerant which is typically in vapor-liquid phase equilibrium The compressor takes in low-pressure low-temperature vapor and compresses it adiabatically to high-pressurehigh-temperature vapor which then reaches the condenser Here the refrigerant is at a higher temperature than that of the air surrounding the condenser coils and it releases heat by
undergoing a phase change The refrigerant leaves the condenser coils as a high-pressure high-temperature liquid and expands adiabatically at a controlled rate in the expansion valve As thefluid expands it cools down Thus when it enters the evaporator coils the refrigerant is at alower temperature than its surroundings and it absorbs heat The air surrounding the evaporator cools down and most of the refrigerant in the evaporator coils vaporizes It then reaches thecompressor as a low-pressure low-temperature vapor and a new cycle begins
Part A
Air conditioners operate on the same principle as refrigerators Consider an air conditioner that has 700 of refrigerant flowing through it s circuiteach cycle The refrigerant enters the evaporator coils in phase equilibrium with 540 of its mass as liquid and the rest as vapor It flows throughthe evaporator at a constant pressure and when it reaches the compressor 95 of its mass is vapor In each cycle how much heat is absorbed
by the refrigerant while it is in the evaporator The heat of vaporization of the refrigerant is 150times10 5
Express your answer numerically in joules
Hint 1 How to approach the problem
To transform a given mass of liquid refrigerant to vapor requires the addition of a certain quantity of heat that depends on the heat of vaporization of the refrigerant and the mass of refrigerant transformed to vapor Since the refrigerant is in phase equilibrium when it enters theevaporator it is already at boiling temperature and all the absorbed heat is used in the liquid-vapor phase change Note that the pressure is keptconstant in the evaporator Also a small fraction of refrigerant is still liquid when it leaves the evaporator so the refrigerant must still be inphase equilibrium at that point and no change in temperature has occurred in the evaporator
W
s
Q Δ E
t h
1 rarr 2 minus 1 2 0
2 rarr 3 0 1 8 0 1 8 0
3 rarr 1 1 9 8 1 9 8 0
Q
1 2
1 rarr 2 ( Δ = 0 E
t h
)
n e t
Q
1 2
Q = 983150 Δ T C
983152
C
p
Δ = Q minus E
t h
W
s
1 rarr 2
Δ E
t h
Δ = 0 E
t h
= -300Q
1 2
J
= + + = 7 8 J Q
n e t
Q
1 2
Q
2 3
Q
3 1
W
o u t
Δ = 0 0 J E
t h
k g
Q
c
J k g
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Hint 2 Find the percentage of refrigerant transformed to vapor
Consider a refrigerator where in each cycle 540 of refrigerant enters the evaporator as liquid in liquid-vapor phase equilibrium and 95leaves as vapor Assume that the pressure of the refrigerant is kept constant while it is in the evaporator What is the percentage of the
total mass of refrigerant transformed to vapor in the evaporator per cycle
ANSWER
ANSWER
Correct
Part B
In each cycle the change in internal energy of the refrigerant when it leaves the compresser is 120times105 What is the work done by the motorthe compressor
Express your answer in joules
Hint 1 Adiabatic compression
Recall that the compressor performs an adiabatic compression that is no heat exchange occurs while the refrigerant is in the compressorThen use the first law of thermodynamics to determine
ANSWER
Correct
Part C
If the direction of the refrigerant flow is inverted in an air conditioner the air conditioning unit turns into a heat pump and it can be used for heatingrather than cooling In this case the coils where the refrigerant would condense in the air conditioner become the evaporator coils when the unit isoperated as a heat pump and vice versa the evaporator coils of the air conditioner become the condenser coils in the heat pump Suppose youoperate the air conditioner described in Parts A and B as a heat pump to heat your bedroom In each cycle what is the amount of heat released
into the room You may assume that the energy changes and work done during the expansion process are negligible compared to those for other processes during the cycle
Express your answer numerically in joules
Hint 1 How to approach the problem
When the air conditioner is operated in the cooling mode in each cycle it absorbs an amount of heat from the air inside the room as you
calculated in Part A and gives off a certain amount of heat to the outside When it is operated in the heating mode instead the direction of the flow of the refrigerant inside the coil circuit is simply inverted but the operation cycle remains the same In the heating mode then thesame quantity is now absorbed from the air outside the room and the amount of heat is released to the air inside the room Note that
the compressor does the same amount of work regardless of the mode of operation and use the first law to determine
Hint 2 Find the right expresssion for the first law of thermodynamics
Suppose you have an ideal sys tem that absorbs heat from a cold reservoir and releases heat to a hot reservoir The net input of work
required by this system for operation is Which of the following expressions is correct
Hint 1 Cyclic processes
Remember that the net internal energy change in a cyclic process is zero since the system has the same temperature when it returns tothe starting point
983149
=983149
= 515times105 Q
c
J
J W
W
= 120times105 W J
Q
h
Q
c
Q
h
Q
c
Q
h
Q
h
Q
c
Q
h
W
i n
7242019 Mastering Physics HW 4 Ch 18 19
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ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
7242019 Mastering Physics HW 4 Ch 18 19
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Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
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Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
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VISUALIZE Draw the pV diagram of the cycle
SOLVE There are several steps in the mathematical analysis
Use the ideal gas law to complete your knowledge of and at one point in the cycleUse the ideal gas law and equations for specific gas processes to determine and at the beginning and end of each processCalculate and for each processFind by adding for each process in the cycle If the geometry is simple you can confirm this value by finding the area enclosedwithin the pV curve
Add just the positive values of to find
Verify that This is a self-consist ency check to verify that you havent made any mistakes
Calculate the thermal effic iency and any other quantities you need to complete the solution
ASSESS Is Do all the signs of and make sense Does have a reasonable value Have you answered the question
Model
Part A
The cycle used by the engine is composed of three processes a process at constant pressure between point 1 and 2 a process at constant volumebetween points 2 and 3 and an isothermal process between points 3 and 1 What are the processes between points 1 and 2 and between points 2 an3 respectively
ANSWER
Correct
Visualize
The pV diagram is already given in the problem introduction You may want to make a copy of the diagram in your notes so that you can add further information to it as you work through the next part
Solve
Part B
Find the effic iency of the heat engine
Express your answer as a decimal number to three significant figures
Hint 1 The efficiency of a heat engine
The efficiency of a heat engine is given by the ratio of the work done by the engine (work output ) to the energy transferred into theengine (heat input )
Hint 2 How to find the work done in one cycle
There are two ways to compute the net work done by the engine during one full cycle Using a geometrical approach calculate the areaenclosed by the pV curve for the cycle is equal to the value of this area Alternatively identify all the processes in one full cyc le andcompute the work done by the engine in each process Find by adding for each process in the cycle
Hint 3 First method Use calculus to compute areas
Because the region enclosed by the pV curve shown in this problem does not have a simple shape you need to use your knowledge of calculus to compute its area Recall that the area between the graphs of two functions and over a certain interval [ ] is given
by
For this particular problem take the function whose graph is the curve for the process as the function and the function whose graph is
983150 983152 V T
983152 V T
Q W
s
Δ E
t h
W
o u t
W
s
Q Q
H
( Δ = 0 E
t h
)
n e t
η
( Δ = 0 E
t h
)
n e t
W
s
Q η
isochoric and isobaric
isobaric and isochoric
isobaric and isothermal
isothermal and isochoric
η
η W
o u t
Q
H
η = W
o u t
Q
H
W
o u t
W
o u t
W
s
W
o u t
W
s
A 983142 ( 983160 ) 983143 ( 983160 )
a b
A = ( 983142 minus 983143 ) 983140 983160 int
b
a
3 rarr 1 983142
7242019 Mastering Physics HW 4 Ch 18 19
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the curve for the process as Use the ideal gas law to find the correct mathematical expressions for these functions Perform the
integration over the V interval [3 9]
Hint 4 Second method Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isobaric processes
In an isobaric process at pressure the work done by the syst em in which the volume changes by an amount is given by
ANSWER
Hint 5 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isochoric processes
Recall that an isochoric process is a process in which the volume does not change Consequently no work is done on or by the system
ANSWER
Hint 6 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isothermal processes
In an isothermal process at temperature the work done by the sys tem when the volume changes from to is given by
where is the number of moles in the gas and is the gas constant
Hint 2 How to solve this problem when moles and temperature are not given
Leave (the number of moles of gas) and (the temperature of the gas during the process ) as symbols in your calculation Youwill find in the end that you need a numerical value for the product Use the ideal gas law to compute this product in terms of thepressure and volume at either point 1 or point 3
ANSWER
Hint 7 Find which processes contribute to the heat input
During which processes in the cycle is heat delivered to the engine
123
Enter the letter(s) of all the correct answers in alphabetical order Do not use commas For example if you think all three processes
deliver heat to the engine enter ABC
1 rarr 2 983143
( c ) 1 0
2
m
3
1 rarr 2
W
1 2
1 rarr 2
983152 Δ V
= 983152 Δ V W
983155
=W
1 2
J
2 rarr 3
W
2 3
2 rarr 3
=W
2 3
J
3 rarr 1
W
3 1
3 rarr 1
T V
i
V
f
= 983150 R T l n ( ) W
983155
V
f
V
i
983150 R
983150 T 3 rarr 1
983150 R T
=W
3 1
J
1 rarr 2
2 rarr 3
3 rarr 1
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ANSWER
Hint 8 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isochoric processes
The heat transfer during an isochoric process in which the temperature of the gas changes by an amount is
where is the number of moles in the gas and is the heat capacity at constant volume
Hint 2 The properties of monatomic gases
For monatomic gases the heat capacity at constant volume is
where is the gas constant
Hint 3 How to compute the change in temperature
Even though the temperature of the gas is not given in the problem at certain points in the cycle can be expressed in terms of someknown quantities using the ideal gas law Thus use this law to find the temperature at points 2 and 3 in the pV diagramand then compute Note that you wonrsquot need to know how many moles are in the gas because when you calculate theamount of heat transferred in the process the number of moles will cancel out
ANSWER
Hint 9 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isothermal processes
Since in an isothermal process the thermal energy of the gas does not change is zero From the first law of thermodynamics itfollows that the heat transfer during an isothermal process is equal to the work done by the syst em or In other words all of the heat input during an isothermal process is converted into work done by the gas
ANSWER
ANSWER
Correct
If you prefer to express the effic iency as a percentage in this case you would say that the engine is 206 effic ient
Assess
Part C
2 rarr 3
Q
2 3
2 rarr 3
Δ T
Q = 983150 Δ T C
V
983150 C
V
= C
V
3 R
2
R
T
983152 V =
983150 R T
Δ T = minus T
3
T
2
983150
=Q
2 3
J
3 rarr 1
Q
3 1
3 rarr 1
Δ E
t h
Q = W
s
=Q
3 1
J
= 0206η
7242019 Mastering Physics HW 4 Ch 18 19
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Here is a table of energy transfers for this problem with some entries missing
Process (J) (J) (J)
What must be the heat input in process to satis fy the condition that
Express your answer in joules
Hint 1 How to approach the problem
To compute the heat transfer occurring during the isobaric process you can either use the generic expression for heat transfer in isobaric
processes where is the heat capacity at constant pressure or you can apply directly the first law of thermodynamics
to the process To do that youll need to know the change in thermal energy of the gas during the isobaric processRead from the table the changes in thermal energy during the isochoric and the isothermal processes and then calculate for the isobaricprocess using the fact that for a complete cyc le
ANSWER
Correct
You can easily verify that your results make sense The net heat transfer per cyc le is which is equal to the
work done by the engine in one cyc le By the first law of thermodynamics it follows that which is what we would expect foa cyclic process
An Air Conditioner Refrigerator or Heat Pump
The typical operation cycle of a common refrigerator is shown schematically in the figure Boththe condenser coils to the left and the evaporator coils to the right contain a fluid (the workingsubstance) called refrigerant which is typically in vapor-liquid phase equilibrium The compressor takes in low-pressure low-temperature vapor and compresses it adiabatically to high-pressurehigh-temperature vapor which then reaches the condenser Here the refrigerant is at a higher temperature than that of the air surrounding the condenser coils and it releases heat by
undergoing a phase change The refrigerant leaves the condenser coils as a high-pressure high-temperature liquid and expands adiabatically at a controlled rate in the expansion valve As thefluid expands it cools down Thus when it enters the evaporator coils the refrigerant is at alower temperature than its surroundings and it absorbs heat The air surrounding the evaporator cools down and most of the refrigerant in the evaporator coils vaporizes It then reaches thecompressor as a low-pressure low-temperature vapor and a new cycle begins
Part A
Air conditioners operate on the same principle as refrigerators Consider an air conditioner that has 700 of refrigerant flowing through it s circuiteach cycle The refrigerant enters the evaporator coils in phase equilibrium with 540 of its mass as liquid and the rest as vapor It flows throughthe evaporator at a constant pressure and when it reaches the compressor 95 of its mass is vapor In each cycle how much heat is absorbed
by the refrigerant while it is in the evaporator The heat of vaporization of the refrigerant is 150times10 5
Express your answer numerically in joules
Hint 1 How to approach the problem
To transform a given mass of liquid refrigerant to vapor requires the addition of a certain quantity of heat that depends on the heat of vaporization of the refrigerant and the mass of refrigerant transformed to vapor Since the refrigerant is in phase equilibrium when it enters theevaporator it is already at boiling temperature and all the absorbed heat is used in the liquid-vapor phase change Note that the pressure is keptconstant in the evaporator Also a small fraction of refrigerant is still liquid when it leaves the evaporator so the refrigerant must still be inphase equilibrium at that point and no change in temperature has occurred in the evaporator
W
s
Q Δ E
t h
1 rarr 2 minus 1 2 0
2 rarr 3 0 1 8 0 1 8 0
3 rarr 1 1 9 8 1 9 8 0
Q
1 2
1 rarr 2 ( Δ = 0 E
t h
)
n e t
Q
1 2
Q = 983150 Δ T C
983152
C
p
Δ = Q minus E
t h
W
s
1 rarr 2
Δ E
t h
Δ = 0 E
t h
= -300Q
1 2
J
= + + = 7 8 J Q
n e t
Q
1 2
Q
2 3
Q
3 1
W
o u t
Δ = 0 0 J E
t h
k g
Q
c
J k g
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 2 Find the percentage of refrigerant transformed to vapor
Consider a refrigerator where in each cycle 540 of refrigerant enters the evaporator as liquid in liquid-vapor phase equilibrium and 95leaves as vapor Assume that the pressure of the refrigerant is kept constant while it is in the evaporator What is the percentage of the
total mass of refrigerant transformed to vapor in the evaporator per cycle
ANSWER
ANSWER
Correct
Part B
In each cycle the change in internal energy of the refrigerant when it leaves the compresser is 120times105 What is the work done by the motorthe compressor
Express your answer in joules
Hint 1 Adiabatic compression
Recall that the compressor performs an adiabatic compression that is no heat exchange occurs while the refrigerant is in the compressorThen use the first law of thermodynamics to determine
ANSWER
Correct
Part C
If the direction of the refrigerant flow is inverted in an air conditioner the air conditioning unit turns into a heat pump and it can be used for heatingrather than cooling In this case the coils where the refrigerant would condense in the air conditioner become the evaporator coils when the unit isoperated as a heat pump and vice versa the evaporator coils of the air conditioner become the condenser coils in the heat pump Suppose youoperate the air conditioner described in Parts A and B as a heat pump to heat your bedroom In each cycle what is the amount of heat released
into the room You may assume that the energy changes and work done during the expansion process are negligible compared to those for other processes during the cycle
Express your answer numerically in joules
Hint 1 How to approach the problem
When the air conditioner is operated in the cooling mode in each cycle it absorbs an amount of heat from the air inside the room as you
calculated in Part A and gives off a certain amount of heat to the outside When it is operated in the heating mode instead the direction of the flow of the refrigerant inside the coil circuit is simply inverted but the operation cycle remains the same In the heating mode then thesame quantity is now absorbed from the air outside the room and the amount of heat is released to the air inside the room Note that
the compressor does the same amount of work regardless of the mode of operation and use the first law to determine
Hint 2 Find the right expresssion for the first law of thermodynamics
Suppose you have an ideal sys tem that absorbs heat from a cold reservoir and releases heat to a hot reservoir The net input of work
required by this system for operation is Which of the following expressions is correct
Hint 1 Cyclic processes
Remember that the net internal energy change in a cyclic process is zero since the system has the same temperature when it returns tothe starting point
983149
=983149
= 515times105 Q
c
J
J W
W
= 120times105 W J
Q
h
Q
c
Q
h
Q
c
Q
h
Q
h
Q
c
Q
h
W
i n
7242019 Mastering Physics HW 4 Ch 18 19
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ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
7242019 Mastering Physics HW 4 Ch 18 19
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Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
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Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
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Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
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the curve for the process as Use the ideal gas law to find the correct mathematical expressions for these functions Perform the
integration over the V interval [3 9]
Hint 4 Second method Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isobaric processes
In an isobaric process at pressure the work done by the syst em in which the volume changes by an amount is given by
ANSWER
Hint 5 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isochoric processes
Recall that an isochoric process is a process in which the volume does not change Consequently no work is done on or by the system
ANSWER
Hint 6 Find the work done during process
How much work is done by the gas during process
Express your answer in joules
Hint 1 Work in isothermal processes
In an isothermal process at temperature the work done by the sys tem when the volume changes from to is given by
where is the number of moles in the gas and is the gas constant
Hint 2 How to solve this problem when moles and temperature are not given
Leave (the number of moles of gas) and (the temperature of the gas during the process ) as symbols in your calculation Youwill find in the end that you need a numerical value for the product Use the ideal gas law to compute this product in terms of thepressure and volume at either point 1 or point 3
ANSWER
Hint 7 Find which processes contribute to the heat input
During which processes in the cycle is heat delivered to the engine
123
Enter the letter(s) of all the correct answers in alphabetical order Do not use commas For example if you think all three processes
deliver heat to the engine enter ABC
1 rarr 2 983143
( c ) 1 0
2
m
3
1 rarr 2
W
1 2
1 rarr 2
983152 Δ V
= 983152 Δ V W
983155
=W
1 2
J
2 rarr 3
W
2 3
2 rarr 3
=W
2 3
J
3 rarr 1
W
3 1
3 rarr 1
T V
i
V
f
= 983150 R T l n ( ) W
983155
V
f
V
i
983150 R
983150 T 3 rarr 1
983150 R T
=W
3 1
J
1 rarr 2
2 rarr 3
3 rarr 1
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ANSWER
Hint 8 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isochoric processes
The heat transfer during an isochoric process in which the temperature of the gas changes by an amount is
where is the number of moles in the gas and is the heat capacity at constant volume
Hint 2 The properties of monatomic gases
For monatomic gases the heat capacity at constant volume is
where is the gas constant
Hint 3 How to compute the change in temperature
Even though the temperature of the gas is not given in the problem at certain points in the cycle can be expressed in terms of someknown quantities using the ideal gas law Thus use this law to find the temperature at points 2 and 3 in the pV diagramand then compute Note that you wonrsquot need to know how many moles are in the gas because when you calculate theamount of heat transferred in the process the number of moles will cancel out
ANSWER
Hint 9 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isothermal processes
Since in an isothermal process the thermal energy of the gas does not change is zero From the first law of thermodynamics itfollows that the heat transfer during an isothermal process is equal to the work done by the syst em or In other words all of the heat input during an isothermal process is converted into work done by the gas
ANSWER
ANSWER
Correct
If you prefer to express the effic iency as a percentage in this case you would say that the engine is 206 effic ient
Assess
Part C
2 rarr 3
Q
2 3
2 rarr 3
Δ T
Q = 983150 Δ T C
V
983150 C
V
= C
V
3 R
2
R
T
983152 V =
983150 R T
Δ T = minus T
3
T
2
983150
=Q
2 3
J
3 rarr 1
Q
3 1
3 rarr 1
Δ E
t h
Q = W
s
=Q
3 1
J
= 0206η
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 618
Here is a table of energy transfers for this problem with some entries missing
Process (J) (J) (J)
What must be the heat input in process to satis fy the condition that
Express your answer in joules
Hint 1 How to approach the problem
To compute the heat transfer occurring during the isobaric process you can either use the generic expression for heat transfer in isobaric
processes where is the heat capacity at constant pressure or you can apply directly the first law of thermodynamics
to the process To do that youll need to know the change in thermal energy of the gas during the isobaric processRead from the table the changes in thermal energy during the isochoric and the isothermal processes and then calculate for the isobaricprocess using the fact that for a complete cyc le
ANSWER
Correct
You can easily verify that your results make sense The net heat transfer per cyc le is which is equal to the
work done by the engine in one cyc le By the first law of thermodynamics it follows that which is what we would expect foa cyclic process
An Air Conditioner Refrigerator or Heat Pump
The typical operation cycle of a common refrigerator is shown schematically in the figure Boththe condenser coils to the left and the evaporator coils to the right contain a fluid (the workingsubstance) called refrigerant which is typically in vapor-liquid phase equilibrium The compressor takes in low-pressure low-temperature vapor and compresses it adiabatically to high-pressurehigh-temperature vapor which then reaches the condenser Here the refrigerant is at a higher temperature than that of the air surrounding the condenser coils and it releases heat by
undergoing a phase change The refrigerant leaves the condenser coils as a high-pressure high-temperature liquid and expands adiabatically at a controlled rate in the expansion valve As thefluid expands it cools down Thus when it enters the evaporator coils the refrigerant is at alower temperature than its surroundings and it absorbs heat The air surrounding the evaporator cools down and most of the refrigerant in the evaporator coils vaporizes It then reaches thecompressor as a low-pressure low-temperature vapor and a new cycle begins
Part A
Air conditioners operate on the same principle as refrigerators Consider an air conditioner that has 700 of refrigerant flowing through it s circuiteach cycle The refrigerant enters the evaporator coils in phase equilibrium with 540 of its mass as liquid and the rest as vapor It flows throughthe evaporator at a constant pressure and when it reaches the compressor 95 of its mass is vapor In each cycle how much heat is absorbed
by the refrigerant while it is in the evaporator The heat of vaporization of the refrigerant is 150times10 5
Express your answer numerically in joules
Hint 1 How to approach the problem
To transform a given mass of liquid refrigerant to vapor requires the addition of a certain quantity of heat that depends on the heat of vaporization of the refrigerant and the mass of refrigerant transformed to vapor Since the refrigerant is in phase equilibrium when it enters theevaporator it is already at boiling temperature and all the absorbed heat is used in the liquid-vapor phase change Note that the pressure is keptconstant in the evaporator Also a small fraction of refrigerant is still liquid when it leaves the evaporator so the refrigerant must still be inphase equilibrium at that point and no change in temperature has occurred in the evaporator
W
s
Q Δ E
t h
1 rarr 2 minus 1 2 0
2 rarr 3 0 1 8 0 1 8 0
3 rarr 1 1 9 8 1 9 8 0
Q
1 2
1 rarr 2 ( Δ = 0 E
t h
)
n e t
Q
1 2
Q = 983150 Δ T C
983152
C
p
Δ = Q minus E
t h
W
s
1 rarr 2
Δ E
t h
Δ = 0 E
t h
= -300Q
1 2
J
= + + = 7 8 J Q
n e t
Q
1 2
Q
2 3
Q
3 1
W
o u t
Δ = 0 0 J E
t h
k g
Q
c
J k g
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 718
Hint 2 Find the percentage of refrigerant transformed to vapor
Consider a refrigerator where in each cycle 540 of refrigerant enters the evaporator as liquid in liquid-vapor phase equilibrium and 95leaves as vapor Assume that the pressure of the refrigerant is kept constant while it is in the evaporator What is the percentage of the
total mass of refrigerant transformed to vapor in the evaporator per cycle
ANSWER
ANSWER
Correct
Part B
In each cycle the change in internal energy of the refrigerant when it leaves the compresser is 120times105 What is the work done by the motorthe compressor
Express your answer in joules
Hint 1 Adiabatic compression
Recall that the compressor performs an adiabatic compression that is no heat exchange occurs while the refrigerant is in the compressorThen use the first law of thermodynamics to determine
ANSWER
Correct
Part C
If the direction of the refrigerant flow is inverted in an air conditioner the air conditioning unit turns into a heat pump and it can be used for heatingrather than cooling In this case the coils where the refrigerant would condense in the air conditioner become the evaporator coils when the unit isoperated as a heat pump and vice versa the evaporator coils of the air conditioner become the condenser coils in the heat pump Suppose youoperate the air conditioner described in Parts A and B as a heat pump to heat your bedroom In each cycle what is the amount of heat released
into the room You may assume that the energy changes and work done during the expansion process are negligible compared to those for other processes during the cycle
Express your answer numerically in joules
Hint 1 How to approach the problem
When the air conditioner is operated in the cooling mode in each cycle it absorbs an amount of heat from the air inside the room as you
calculated in Part A and gives off a certain amount of heat to the outside When it is operated in the heating mode instead the direction of the flow of the refrigerant inside the coil circuit is simply inverted but the operation cycle remains the same In the heating mode then thesame quantity is now absorbed from the air outside the room and the amount of heat is released to the air inside the room Note that
the compressor does the same amount of work regardless of the mode of operation and use the first law to determine
Hint 2 Find the right expresssion for the first law of thermodynamics
Suppose you have an ideal sys tem that absorbs heat from a cold reservoir and releases heat to a hot reservoir The net input of work
required by this system for operation is Which of the following expressions is correct
Hint 1 Cyclic processes
Remember that the net internal energy change in a cyclic process is zero since the system has the same temperature when it returns tothe starting point
983149
=983149
= 515times105 Q
c
J
J W
W
= 120times105 W J
Q
h
Q
c
Q
h
Q
c
Q
h
Q
h
Q
c
Q
h
W
i n
7242019 Mastering Physics HW 4 Ch 18 19
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ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 918
Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1118
Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1218
Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 518
ANSWER
Hint 8 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isochoric processes
The heat transfer during an isochoric process in which the temperature of the gas changes by an amount is
where is the number of moles in the gas and is the heat capacity at constant volume
Hint 2 The properties of monatomic gases
For monatomic gases the heat capacity at constant volume is
where is the gas constant
Hint 3 How to compute the change in temperature
Even though the temperature of the gas is not given in the problem at certain points in the cycle can be expressed in terms of someknown quantities using the ideal gas law Thus use this law to find the temperature at points 2 and 3 in the pV diagramand then compute Note that you wonrsquot need to know how many moles are in the gas because when you calculate theamount of heat transferred in the process the number of moles will cancel out
ANSWER
Hint 9 Compute the heat input during process
What amount of heat is delivered to the engine during process
Express your answer in joules
Hint 1 Heat transfer during isothermal processes
Since in an isothermal process the thermal energy of the gas does not change is zero From the first law of thermodynamics itfollows that the heat transfer during an isothermal process is equal to the work done by the syst em or In other words all of the heat input during an isothermal process is converted into work done by the gas
ANSWER
ANSWER
Correct
If you prefer to express the effic iency as a percentage in this case you would say that the engine is 206 effic ient
Assess
Part C
2 rarr 3
Q
2 3
2 rarr 3
Δ T
Q = 983150 Δ T C
V
983150 C
V
= C
V
3 R
2
R
T
983152 V =
983150 R T
Δ T = minus T
3
T
2
983150
=Q
2 3
J
3 rarr 1
Q
3 1
3 rarr 1
Δ E
t h
Q = W
s
=Q
3 1
J
= 0206η
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 618
Here is a table of energy transfers for this problem with some entries missing
Process (J) (J) (J)
What must be the heat input in process to satis fy the condition that
Express your answer in joules
Hint 1 How to approach the problem
To compute the heat transfer occurring during the isobaric process you can either use the generic expression for heat transfer in isobaric
processes where is the heat capacity at constant pressure or you can apply directly the first law of thermodynamics
to the process To do that youll need to know the change in thermal energy of the gas during the isobaric processRead from the table the changes in thermal energy during the isochoric and the isothermal processes and then calculate for the isobaricprocess using the fact that for a complete cyc le
ANSWER
Correct
You can easily verify that your results make sense The net heat transfer per cyc le is which is equal to the
work done by the engine in one cyc le By the first law of thermodynamics it follows that which is what we would expect foa cyclic process
An Air Conditioner Refrigerator or Heat Pump
The typical operation cycle of a common refrigerator is shown schematically in the figure Boththe condenser coils to the left and the evaporator coils to the right contain a fluid (the workingsubstance) called refrigerant which is typically in vapor-liquid phase equilibrium The compressor takes in low-pressure low-temperature vapor and compresses it adiabatically to high-pressurehigh-temperature vapor which then reaches the condenser Here the refrigerant is at a higher temperature than that of the air surrounding the condenser coils and it releases heat by
undergoing a phase change The refrigerant leaves the condenser coils as a high-pressure high-temperature liquid and expands adiabatically at a controlled rate in the expansion valve As thefluid expands it cools down Thus when it enters the evaporator coils the refrigerant is at alower temperature than its surroundings and it absorbs heat The air surrounding the evaporator cools down and most of the refrigerant in the evaporator coils vaporizes It then reaches thecompressor as a low-pressure low-temperature vapor and a new cycle begins
Part A
Air conditioners operate on the same principle as refrigerators Consider an air conditioner that has 700 of refrigerant flowing through it s circuiteach cycle The refrigerant enters the evaporator coils in phase equilibrium with 540 of its mass as liquid and the rest as vapor It flows throughthe evaporator at a constant pressure and when it reaches the compressor 95 of its mass is vapor In each cycle how much heat is absorbed
by the refrigerant while it is in the evaporator The heat of vaporization of the refrigerant is 150times10 5
Express your answer numerically in joules
Hint 1 How to approach the problem
To transform a given mass of liquid refrigerant to vapor requires the addition of a certain quantity of heat that depends on the heat of vaporization of the refrigerant and the mass of refrigerant transformed to vapor Since the refrigerant is in phase equilibrium when it enters theevaporator it is already at boiling temperature and all the absorbed heat is used in the liquid-vapor phase change Note that the pressure is keptconstant in the evaporator Also a small fraction of refrigerant is still liquid when it leaves the evaporator so the refrigerant must still be inphase equilibrium at that point and no change in temperature has occurred in the evaporator
W
s
Q Δ E
t h
1 rarr 2 minus 1 2 0
2 rarr 3 0 1 8 0 1 8 0
3 rarr 1 1 9 8 1 9 8 0
Q
1 2
1 rarr 2 ( Δ = 0 E
t h
)
n e t
Q
1 2
Q = 983150 Δ T C
983152
C
p
Δ = Q minus E
t h
W
s
1 rarr 2
Δ E
t h
Δ = 0 E
t h
= -300Q
1 2
J
= + + = 7 8 J Q
n e t
Q
1 2
Q
2 3
Q
3 1
W
o u t
Δ = 0 0 J E
t h
k g
Q
c
J k g
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 2 Find the percentage of refrigerant transformed to vapor
Consider a refrigerator where in each cycle 540 of refrigerant enters the evaporator as liquid in liquid-vapor phase equilibrium and 95leaves as vapor Assume that the pressure of the refrigerant is kept constant while it is in the evaporator What is the percentage of the
total mass of refrigerant transformed to vapor in the evaporator per cycle
ANSWER
ANSWER
Correct
Part B
In each cycle the change in internal energy of the refrigerant when it leaves the compresser is 120times105 What is the work done by the motorthe compressor
Express your answer in joules
Hint 1 Adiabatic compression
Recall that the compressor performs an adiabatic compression that is no heat exchange occurs while the refrigerant is in the compressorThen use the first law of thermodynamics to determine
ANSWER
Correct
Part C
If the direction of the refrigerant flow is inverted in an air conditioner the air conditioning unit turns into a heat pump and it can be used for heatingrather than cooling In this case the coils where the refrigerant would condense in the air conditioner become the evaporator coils when the unit isoperated as a heat pump and vice versa the evaporator coils of the air conditioner become the condenser coils in the heat pump Suppose youoperate the air conditioner described in Parts A and B as a heat pump to heat your bedroom In each cycle what is the amount of heat released
into the room You may assume that the energy changes and work done during the expansion process are negligible compared to those for other processes during the cycle
Express your answer numerically in joules
Hint 1 How to approach the problem
When the air conditioner is operated in the cooling mode in each cycle it absorbs an amount of heat from the air inside the room as you
calculated in Part A and gives off a certain amount of heat to the outside When it is operated in the heating mode instead the direction of the flow of the refrigerant inside the coil circuit is simply inverted but the operation cycle remains the same In the heating mode then thesame quantity is now absorbed from the air outside the room and the amount of heat is released to the air inside the room Note that
the compressor does the same amount of work regardless of the mode of operation and use the first law to determine
Hint 2 Find the right expresssion for the first law of thermodynamics
Suppose you have an ideal sys tem that absorbs heat from a cold reservoir and releases heat to a hot reservoir The net input of work
required by this system for operation is Which of the following expressions is correct
Hint 1 Cyclic processes
Remember that the net internal energy change in a cyclic process is zero since the system has the same temperature when it returns tothe starting point
983149
=983149
= 515times105 Q
c
J
J W
W
= 120times105 W J
Q
h
Q
c
Q
h
Q
c
Q
h
Q
h
Q
c
Q
h
W
i n
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 818
ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 918
Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1118
Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1218
Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
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Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
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- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
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Here is a table of energy transfers for this problem with some entries missing
Process (J) (J) (J)
What must be the heat input in process to satis fy the condition that
Express your answer in joules
Hint 1 How to approach the problem
To compute the heat transfer occurring during the isobaric process you can either use the generic expression for heat transfer in isobaric
processes where is the heat capacity at constant pressure or you can apply directly the first law of thermodynamics
to the process To do that youll need to know the change in thermal energy of the gas during the isobaric processRead from the table the changes in thermal energy during the isochoric and the isothermal processes and then calculate for the isobaricprocess using the fact that for a complete cyc le
ANSWER
Correct
You can easily verify that your results make sense The net heat transfer per cyc le is which is equal to the
work done by the engine in one cyc le By the first law of thermodynamics it follows that which is what we would expect foa cyclic process
An Air Conditioner Refrigerator or Heat Pump
The typical operation cycle of a common refrigerator is shown schematically in the figure Boththe condenser coils to the left and the evaporator coils to the right contain a fluid (the workingsubstance) called refrigerant which is typically in vapor-liquid phase equilibrium The compressor takes in low-pressure low-temperature vapor and compresses it adiabatically to high-pressurehigh-temperature vapor which then reaches the condenser Here the refrigerant is at a higher temperature than that of the air surrounding the condenser coils and it releases heat by
undergoing a phase change The refrigerant leaves the condenser coils as a high-pressure high-temperature liquid and expands adiabatically at a controlled rate in the expansion valve As thefluid expands it cools down Thus when it enters the evaporator coils the refrigerant is at alower temperature than its surroundings and it absorbs heat The air surrounding the evaporator cools down and most of the refrigerant in the evaporator coils vaporizes It then reaches thecompressor as a low-pressure low-temperature vapor and a new cycle begins
Part A
Air conditioners operate on the same principle as refrigerators Consider an air conditioner that has 700 of refrigerant flowing through it s circuiteach cycle The refrigerant enters the evaporator coils in phase equilibrium with 540 of its mass as liquid and the rest as vapor It flows throughthe evaporator at a constant pressure and when it reaches the compressor 95 of its mass is vapor In each cycle how much heat is absorbed
by the refrigerant while it is in the evaporator The heat of vaporization of the refrigerant is 150times10 5
Express your answer numerically in joules
Hint 1 How to approach the problem
To transform a given mass of liquid refrigerant to vapor requires the addition of a certain quantity of heat that depends on the heat of vaporization of the refrigerant and the mass of refrigerant transformed to vapor Since the refrigerant is in phase equilibrium when it enters theevaporator it is already at boiling temperature and all the absorbed heat is used in the liquid-vapor phase change Note that the pressure is keptconstant in the evaporator Also a small fraction of refrigerant is still liquid when it leaves the evaporator so the refrigerant must still be inphase equilibrium at that point and no change in temperature has occurred in the evaporator
W
s
Q Δ E
t h
1 rarr 2 minus 1 2 0
2 rarr 3 0 1 8 0 1 8 0
3 rarr 1 1 9 8 1 9 8 0
Q
1 2
1 rarr 2 ( Δ = 0 E
t h
)
n e t
Q
1 2
Q = 983150 Δ T C
983152
C
p
Δ = Q minus E
t h
W
s
1 rarr 2
Δ E
t h
Δ = 0 E
t h
= -300Q
1 2
J
= + + = 7 8 J Q
n e t
Q
1 2
Q
2 3
Q
3 1
W
o u t
Δ = 0 0 J E
t h
k g
Q
c
J k g
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Hint 2 Find the percentage of refrigerant transformed to vapor
Consider a refrigerator where in each cycle 540 of refrigerant enters the evaporator as liquid in liquid-vapor phase equilibrium and 95leaves as vapor Assume that the pressure of the refrigerant is kept constant while it is in the evaporator What is the percentage of the
total mass of refrigerant transformed to vapor in the evaporator per cycle
ANSWER
ANSWER
Correct
Part B
In each cycle the change in internal energy of the refrigerant when it leaves the compresser is 120times105 What is the work done by the motorthe compressor
Express your answer in joules
Hint 1 Adiabatic compression
Recall that the compressor performs an adiabatic compression that is no heat exchange occurs while the refrigerant is in the compressorThen use the first law of thermodynamics to determine
ANSWER
Correct
Part C
If the direction of the refrigerant flow is inverted in an air conditioner the air conditioning unit turns into a heat pump and it can be used for heatingrather than cooling In this case the coils where the refrigerant would condense in the air conditioner become the evaporator coils when the unit isoperated as a heat pump and vice versa the evaporator coils of the air conditioner become the condenser coils in the heat pump Suppose youoperate the air conditioner described in Parts A and B as a heat pump to heat your bedroom In each cycle what is the amount of heat released
into the room You may assume that the energy changes and work done during the expansion process are negligible compared to those for other processes during the cycle
Express your answer numerically in joules
Hint 1 How to approach the problem
When the air conditioner is operated in the cooling mode in each cycle it absorbs an amount of heat from the air inside the room as you
calculated in Part A and gives off a certain amount of heat to the outside When it is operated in the heating mode instead the direction of the flow of the refrigerant inside the coil circuit is simply inverted but the operation cycle remains the same In the heating mode then thesame quantity is now absorbed from the air outside the room and the amount of heat is released to the air inside the room Note that
the compressor does the same amount of work regardless of the mode of operation and use the first law to determine
Hint 2 Find the right expresssion for the first law of thermodynamics
Suppose you have an ideal sys tem that absorbs heat from a cold reservoir and releases heat to a hot reservoir The net input of work
required by this system for operation is Which of the following expressions is correct
Hint 1 Cyclic processes
Remember that the net internal energy change in a cyclic process is zero since the system has the same temperature when it returns tothe starting point
983149
=983149
= 515times105 Q
c
J
J W
W
= 120times105 W J
Q
h
Q
c
Q
h
Q
c
Q
h
Q
h
Q
c
Q
h
W
i n
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ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
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Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
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Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
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Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
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Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 718
Hint 2 Find the percentage of refrigerant transformed to vapor
Consider a refrigerator where in each cycle 540 of refrigerant enters the evaporator as liquid in liquid-vapor phase equilibrium and 95leaves as vapor Assume that the pressure of the refrigerant is kept constant while it is in the evaporator What is the percentage of the
total mass of refrigerant transformed to vapor in the evaporator per cycle
ANSWER
ANSWER
Correct
Part B
In each cycle the change in internal energy of the refrigerant when it leaves the compresser is 120times105 What is the work done by the motorthe compressor
Express your answer in joules
Hint 1 Adiabatic compression
Recall that the compressor performs an adiabatic compression that is no heat exchange occurs while the refrigerant is in the compressorThen use the first law of thermodynamics to determine
ANSWER
Correct
Part C
If the direction of the refrigerant flow is inverted in an air conditioner the air conditioning unit turns into a heat pump and it can be used for heatingrather than cooling In this case the coils where the refrigerant would condense in the air conditioner become the evaporator coils when the unit isoperated as a heat pump and vice versa the evaporator coils of the air conditioner become the condenser coils in the heat pump Suppose youoperate the air conditioner described in Parts A and B as a heat pump to heat your bedroom In each cycle what is the amount of heat released
into the room You may assume that the energy changes and work done during the expansion process are negligible compared to those for other processes during the cycle
Express your answer numerically in joules
Hint 1 How to approach the problem
When the air conditioner is operated in the cooling mode in each cycle it absorbs an amount of heat from the air inside the room as you
calculated in Part A and gives off a certain amount of heat to the outside When it is operated in the heating mode instead the direction of the flow of the refrigerant inside the coil circuit is simply inverted but the operation cycle remains the same In the heating mode then thesame quantity is now absorbed from the air outside the room and the amount of heat is released to the air inside the room Note that
the compressor does the same amount of work regardless of the mode of operation and use the first law to determine
Hint 2 Find the right expresssion for the first law of thermodynamics
Suppose you have an ideal sys tem that absorbs heat from a cold reservoir and releases heat to a hot reservoir The net input of work
required by this system for operation is Which of the following expressions is correct
Hint 1 Cyclic processes
Remember that the net internal energy change in a cyclic process is zero since the system has the same temperature when it returns tothe starting point
983149
=983149
= 515times105 Q
c
J
J W
W
= 120times105 W J
Q
h
Q
c
Q
h
Q
c
Q
h
Q
h
Q
c
Q
h
W
i n
7242019 Mastering Physics HW 4 Ch 18 19
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ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
7242019 Mastering Physics HW 4 Ch 18 19
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Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
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Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
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Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
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Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 818
ANSWER
ANSWER
All attempts used correct answer withheld by instructor
Heat Pumps and Refrigerators
Learning Goal
To understand that a heat engine run backward is a heat pump that can be used as a refrigerator
By now you should be familiar with heat engines--devices theoretical or actual designed to convert heat into work You should understand the following
1 Heat engines must be cy clical that is they must return to their original state some time aft er having absorbed some heat and done somework)
2 Heat engines cannot convert heat into work without generating some waste heat in the process
The second characteristic is a rigorous result even for a perfect engine and follows from thermodynamics A perfect heat engine is reversible another result of the laws of thermodynamics
If a heat engine is run backward (ie with every input and output reversed) it becomes a heat pump (as pictured schematic ally ) Work must be puinto a heat pump and it then pumps heat from a colder temperature to a hotter temperature
that is against the usual direction of heat flow (which explains why it is called a heatpump)
The heat coming out the hot side of a heat pump or the heat going in to the cold side of
a refrigerator is more than the work put in in fact it can be many times larger For this reason theratio of the heat to the work in heat pumps and refrigerators is called the coefficient of
performance In a refrigerator this is the ratio of heat removed from the cold side to workput in
In a heat pump the coeffic ient of performance is the ratio of heat exiting the hot side to the
work put in
Take and to be the magnitudes of the heat emitted and absorbed respectively
Part A
What is the relationship of to the work done by the sys tem
Express in terms of and other quantities given in the introduction
Hint 1 Note the differences in wording
Recall that is the work done by the syst em is the work done on the system
ANSWER
Correct
= minus Q
c
W
i n
Q
h
= minus Q
h
Q
c
W
i n
= + Q
h
Q
c
W
i n
=Q
h
4
J
W
i n
T
c
T
h
Q
h
Q
c
K Q
c
= K
f r i g
Q
c
W
i n
Q
h
= K
p u m p
Q
h
W
i n
Q
h
Q
c
W
i n
W
W
i n
W
W W
i n
=W
i n
minus W
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 918
Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1118
Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1218
Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 918
Part B
Find the heat pumped out by the ideal heat pump
Express in terms of and
Hint 1 Conservation of energy and the first law
Apply conservation of energy If you think in t erms of the first law of thermodynamics remember the sign conventions for heat and work andnote that the internal energy does not change in an engine after one cycle
ANSWER
Correct
Part C
A heat pump is used t o heat a house in winter t he inside radiators are at and the outside heat ex changer is at If it is a perfect (ie Carnotcyc le) heat pump what is its coeffic ient of performance
Give your answer in terms of and
Hint 1 Heat pump efficiency in terms of and
What is the efficiency of a heat pump in terms of the heats in and out Use the expression for the efficiency of the heat pump and the
expression that you found involving the work done on the sys tem and the outside heats and
Give your answer in terms of and
ANSWER
Hint 2 Relation between and in a Carnot cycle
Recall that in a Carnot cycle
ANSWER
Correct
Part D
The heat pump is designed to move heat This is only possible if certain relationships between the heats and temperatures at the hot and cold sideshold true Indicate the statement that must apply for the heat pump to work
ANSWER
Q
h
Q
h
Q
c
W
i n
=Q
h
+ W
i n
Q
c
T
h
T
c
K
p u m p
T
h
T
c
Q
h
Q
c
K
p u m p
W
i n
Q
h
Q
c
Q
h
Q
c
=K
p u m p
Q
h
Q
c
=
Q
h
Q
c
T
h
T
c
=K
p u m p
T
983144
minus T
983144
T
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1118
Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1218
Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1018
Correct
Part E
Assume that you heat your home with a heat pump whose heat exchanger is at and which maintains the baseboard radiators at If it would cost $1000 to heat the house for one winter with ideal electric heaters (which have a coefficient of performance of 1) how
much would it cost if the actual coefficient of performance of the heat pump were 75 of that allowed by thermodynamics
Express the cost in dollars
Hint 1 Money heat and the efficiency
The amount of money one has to pay for the heat is directly proportional to the work done to generate the heat Thus the more efficient theheat generation the less work needs to be done and the lower the heating billYou are given that the cost of is $1000 You also have an equation for in terms of the temperatures
Set this equal to and solve for the monetary value of the amount of external energy input the pump requires You can measure
energies in units of currency for this calculation
Hint 2 Units of and
Keep in mind that when calculating an efficiency of a thermodynamic device you need to use temperature in kelvins That is
ANSWER
Correct
This savings is accompanied by more initial capital costs both for the heat pump and for the generous area of baseboard heaters needed totransfer enough heat to the house without raising which would reduce the coeffic ient of performance An additional problem is icing of theoutside heat exchanger which is very difficult to avoid if the outside air is humid and not much above zero degrees Celsius Therefore heatpumps are most useful in temperate climates or where the heat can be obtained from a groundwater that is abundant or flowing (eg an
underground stream)
Six New Heat Engines Conceptual Question
As part of your job at the patent office you are asked to evaluate t he s ix designs s hown in the figure for innovativ e new heat engines
Part A
Which of the designs violate(s) the first law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ACD)
= C T
c
2
∘
= C T
h
4 7
∘
Q
h
K
p u m p
= 7 5 K
a c t u a l
o f = K
p u m p
3
4
T
h
minus T
h
T
c
Q
h
W
i n
W
i n
T
h
T
c
C = 2 7 3 K 0
∘
Cost = 1875 dollars
T
h
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1118
Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1218
Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1118
Hint 1 The first law of thermodynamics applied to a heat engine
By conservation of energy the heat energy input to an engine must equal the sum of the work output and heat energy output
ANSWER
Correct
Part B
Which of the remaining designs violate(s) the second law of thermodynamics
Give the letter(s) of the design(s) in alphabetical order without commas or spaces (eg ABD)
Hint 1 The second law of thermodynamics applied to a heat engine
By the second law of thermodynamics a heat engine operating between two reservoirs and has a maximum thermal effic iencygiven by
An engine with thermal efficiency greater than t hat given by the above equation v iolates the second law of thermodynamics
ANSWER
Correct
Part C
Which of the remaining designs has the highest thermal efficiency
ANSWER
Correct
Carnot Cycle
After Count Rumford (Benjamin Thompson) and James Prescott Joule had shown t he equivalence of mechanical energy and heat it was natural that
engineers believed it possible to make a heat engine (eg a steam engine) that would convert heat completely into mechanical energy Sadi Carnotconsidered a hypothetical piston engine that contained moles of an ideal gas showing first that it was reversible and most importantly thatmdashregardlesof the specific heat of the gasmdashit had limited effic iency defined as where is the net work done by the engine and is the quantity of
heat put into the engine at a (high) temperature Furthermore he showed that the engine must necessarily put an amount of heat back into a hea
reservoir at a lower temperature
The cycle associated with a Carnot engine is known as a Carnot cycle A pV plot of the Carnot cycle is shown in the figure The working gas first expandisothermally from state A to state B absorbing heat from a reservoir at temperature The gas then expands adiabatically until it reaches a
temperature in state C The gas is compressed isothermally to state D giving off heat Finally the gas is adiabatically compressed to state A i
original state
CF
T
h o t
T
c o l d
983141
m a x
= 1 minus 983141
m a x
T
c o l d
T
h o t
BD
device A
device E
983150
983141 = W Q
h
W Q
h
T
h
Q
c
T
c
Q
h
T
h
T
c
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1218
Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1218
Part A
Which of the following statements are true
Check all that apply
Hint 1 Heat flow in an adiabatic process
An adiabatic process is one in which heat does not flow into or out of the gas
ANSWER
Correct
Part B
Find the total work done by the gas after it completes a single Carnot cycle
Express the work in terms of any or all of the quantities and
Hint 1 How to approach the problem
Find the total amount of heat added during the entire cycle and the change in internal energy of the gas over the entire cycle Then apply thefirst law of thermodynamics
Hint 2 Compute the change in internal energy
What is the net change in the gass internal energy after one complete cycle
ANSWER
ANSWER
Correct
For the gas to do positive work the cycle must be traversed in a clockwise manner
Positive heat is added to the gas as it proceeds from state C to state D
The net work done by the gas is proportional to the area inside the closed curve
The heat transferred as the gas proceeds from state B to state C is greater than the heat transferred as the gas proceeds from state D tostate A
W
Q
h
T
h
Q
c
T
c
983140 Q =
983140 U +
983140 W
U
change in =U
=W
minus Q
983144
Q
c
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1318
Part C
Suppose there are moles of the ideal gas and the volumes of the gas in states A and B are respectively and Find the heat absorb
by the gas as it expands from state A to state B
Express the heat absorbed by the gas in terms of the temperature of the hot reservoir and the gas constant
Hint 1 General method of finding
First find the work done by the gas as it expands from state A to state B Then use the first law of thermodynamics to relate to
both and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change inthe gass internal energy
Hint 2 Find the work done by the gas
What is the work done by the gas as it expands from state A to state B
Express the work in terms of the temperature of the hot reservoir and the gas constant
Hint 1 How to find the work done by the gas
To find the net work done by the gas as it proceeds from state A to state B you need to integrate from state A tostate B
Hint 2 Express in terms of
Use the ideal gas equation of state to find an expression for the pressure in terms of and
ANSWER
Hint 3 Express the integral over
Given that and that
what is the integral
Express your answer in terms of and
ANSWER
ANSWER
Hint 3 Relation between and
Because the process is isothermal the internal energy of the gas does not change Therefore the work done by the gas will equal the net heatflow into the gas
ANSWER
Correct
983150 V
A
V
B
Q
h
983150 V
A
V
B
T
h
R
Q
h
W
A B
W
A B
Q
h
W
A B
983150 V
A
V
B
T
h
R
W
A B
983140 W =
983152 983140 V
983152 V
983152 983150 R T
h
V
= 983152 ( V )
983140 V
983152 ( V ) prop 1 V
= 983152 ( V ) 983140 V
W
A B
int
V
B
V
A
983140 V V int
V
B
V
A
V
A
V
B
=int
V
B
V
A
983140 V
V
=W
A B
Q
h
W
A B
= Q
h
W
A B
=Q
h
983150 R
( l n
( 983081 983081
T
983144
V
B
V
A
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1418
Part D
The volume of the gas in state C is and its volume in state D is Find the magnitude of the heat that flows out of the gas as it proceed
from state C to state D
Express your answer in terms of (the temperature of the cold reservoir) and
Hint 1 How to approach the problem
First find the work done by the gas as it expands from state C to state D Then use the first law of thermodynamics to relate toboth and the change in the gass internal energy Finally recall that this process is isothermal what does this tell you about the change in
the gass internal energy
ANSWER
Correct
Observe that the three parts together imply that This is because BC and DA are adiabatic processes So using the first law whereas So or This is
a general result Any two adiabatic processes operating between the same two temperatures result in the same amount of work regardless of thepressure and volume differences
Part E
Now by considering the adiabatic processes (from B to C and from D to A) find the ratio in terms of and
Hint 1 How to approach the problem
Suppose the ratio of the gass specific heats is denoted by Along adiabatic curves you know that where is some
constant Rewrite this equation in terms of and instead of and Then use this new equation to relate the temperature and volume atthe end points of the two adiabatic legs of the Carnot cyc le This will give you two equations that you can solve for
Hint 2 Rewrite in terms of and
Use the ideal gas equation of state to eliminate from the expression
Express your answer in terms of and the temperature
ANSWER
Hint 3 Express and in terms of and
States B and C are connected by an adiabatic expansion Use the result found in the previous hint to find an expression for
Express your answer in terms of and
ANSWER
Hint 4 Express and in terms of and
States D and A are connected by an adiabatic expansion Use the result found in Hint 2 to find an expression for
Express your answer in terms of and
ANSWER
V
C
V
D
Q
c
983150 V
C
V
D
T
c
R
W
C D
W
C D
Q
c
=Q
c
983150 R
( l n
( 983081 983081
T
c
V
C
V
D
+ = 0 W
B C
W
D A
= minus Δ = minus 983150 ( minus ) W
B C
U
B C
C
V
T
h
T
c
= minus Δ = minus 983150 ( minus ) W
D A
U
D A
C
V
T
c
T
h
= minus W
B C
W
D A
+ = 0 W
B C
W
D A
V
C
V
D
V
A
V
B
γ = C
983152
C
983158
983152 = C V
γ
C
T V 983152 V
V
C
V
D
983152 V
γ
T V
983152 983152 V
γ
γ 983150 R T
= 983152 V
γ
T
h
V
B
T
c
V
C
T
h
V
( γ minus 1 )
B
T
c
V
C
γ
=T
h
V
( γ minus 1 )
B
T
h
V
A
T
c
V
D
T
h
V
( γ minus 1 )
A
T
c
V
D
γ
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1518
Hint 5 Solving for in terms of and
Combine the results of the previous two hints eliminating the temperatures and (One way to do this is to divide one equation by theother) This should allow you to solve for in terms of and
ANSWER
Correct
Part F
Using your expressions for and (found in Parts C and D) and your result from Part E find a simplified expression for
No volume variables should appear in your expression nor should any constants (eg or )
ANSWER
Correct
Part G
The efficiency of any engine is by definition Carnot proved that no engine can have an effic iency greater than that of a Carnot engine
Find the effic iency of a Carnot engine
Express the efficiency in terms of and
Hint 1 Express the efficiency in terms of and
Using your result from Part B find the efficiency (of any engine) in terms of the engines heat input and output
Express your answer in terms of and
ANSWER
ANSWER
Correct
Because is generally fixed (eg the cold reservoir for power plants is often a river or a lake) engineers trying to increase effic iency havealways sought to raise the upper temperature This explains why (historically ) there were some spectacular explosions of boilers used for steam power
Heat Engines Introduced
Learning Goal
=T
h
V
( γ minus 1 )
A
V
C
V
D
V
A
V
B
T
h
T
c
V
C
V
D
V
A
V
B
= V
C
V
D
V
B
V
A
Q
h
Q
c
Q
c
Q
h
983150 R
= Q
c
Q
h
T
c
T
983144
983141 = W Q
h
983141
C a r n o t
T
h
T
c
Q
h
Q
c
983141
Q
h
Q
c
=983141
=983141
C a r n o t
minus T
983144
T
c
T
983144
T
c
T
h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1618
To understand what a heat engine is and its theoretical limitations
Ever since Hero demonstrated a crude steam turbine in ancient Greece humans have dreamed of converting heat into work If a fire can boil a pot andmake the lid jump up and down why cant heat be made to do useful work
A heat engine is a devic e designed to convert heat into work The heat engines we will study will be c yclic The working s ubstance eventually returns to ioriginal state sometime after having absorbed a quantity of heat and done some work A cyclic heat engine cannot convert heat into work withoutgenerating some waste heat in the process Although by no means intuitively obvious this is an important fact of nature since it dramatically affects thetechnology of energy generation If it were possible to convert heat into work without any waste heat then one would be able to build refrigerators that aremore than 100 efficient
Consequently the impossible heat engine pictured schematically here cannot exist even intheory Engineers tried hard for many years to make such a device but Sadi Carnot proved in
1824 that it was impossible
The next figure shows an ideal heatengine one that obeys the laws of thermodynamics It takes in heat
at a temperature and does work In the process of doing this it generateswaste heat at a cooler temperature
Take and to be the magnitudes
of the heat absorbed and emittedrespectively therefore both quantitiesare positive
Part A
A heat engine is designed to do work This is possible only if certain relationships between t he heats and temperatures at the input and output holdtrue Which of the following sets of statements must apply for the heat engine to do work
ANSWER
Correct
Part B
Find the work done by the ideal heat engine
Express in terms of and
ANSWER
Correct
Part C
The thermal efficiency texttipee of a heat engine is defined as follows e = WQ_rm h
Express the efficiency in terms of texttipQ_rm hQ_h and texttipQ_rm cQ_c
ANSWER
Q
h
T
h
W
Q
c
T
c
Q
h
Q
c
and
and
and
and
lt Q
h
Q
c
lt T
h
T
c
gt Q
h
Q
c
lt T
h
T
c
lt Q
h
Q
c
gt T
h
T
c
gt Q
h
Q
c
gt T
h
T
c
W
W Q
h
Q
c
= Q_h-Q_cW
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1718
Correct
Problem 1910
The cycle of the figure consists of three processes
Part A
Make a chart with rows labeled A - C and columns labeled Delta E_rm th W_rm s and Q Fill each box in the chart with + - or 0 to indicatewhether the quantity increases decreases or stays the same during that process
ANSWER
Correct
Problem 1963
A heat engine with 0100 rm mol of a monatomic ideal gas initially fills a 3000 rm cm^3 cylinder at 600 rm K The gas goes through the followinclosed cycle
texttipee = largefracQ_h-Q_cQ_h
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points
7242019 Mastering Physics HW 4 Ch 18 19
httpslidepdfcomreaderfullmastering-physics-hw-4-ch-18-19 1818
- Isothermal expansion to 4000 rm cm^3- Isochoric cooling to 400 rm K - Isothermal compression to 3000 rm cm^3- Isochoric heating to 600 rm K
Part A
How much work does this engine do per cycle
Express your answer with the appropriate units
ANSWER
Completed correct answer withheld by instructor
Part B
What is its thermal efficiency
Express your answer with the appropriate units
ANSWER
Score Summary
Your score on this assignment is 815You received 733 out of a possible total of 9 points