The Gas Giants Astronomy 311 Professor Lee Carkner Lecture 16.
First Law of Thermodynamics Physics 202 Professor Lee Carkner Lecture 11.
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Transcript of First Law of Thermodynamics Physics 202 Professor Lee Carkner Lecture 11.
First Law of Thermodynamics
Physics 202Professor Lee
CarknerLecture 11
PAL #10 Temperature How does the Galileo thermometer work?
The water in the tube changes density with T
Water gets less dense as T rises and so the balls fall (64 to 80) Heat Al until it floats
Need = 1000 kg/m3 so V = m/ = 2700/1000 = 2.7 V = VDT, T = V/V = 1.7/((1)(6.9X10-6)) = 24658 C Al would melt before getting to this temperature
Heat Capacity
Put a wooden spoon in a pot of water and boil it
Heat capacity (unit energy per degree temp) is given by:Q = C (Tf - Ti) = C T
Where: Q = heat (J)
C is a property of a specific object
Specific Heat
C=cm Where m is the mass and c is the specific
heat Each type of substance has a c (e.g. water,
iron, etc.)
Q = cm(Tf - Ti) Amount of heat (Q) needed to change the
temperature of m kg of a certain type of substance from Ti to Tf
Molar Specific Heat
One mole = 6.02 X 1023 atoms or molecules
Q = cmol (# of moles) (Tf - Ti) Used for gas
Heat of Transformation
Boiling water stays at 100 deg C, the added energy goes into transforming the water from liquid to gas
Q = L m Where L is the heat of transformation
Vaporization and Fusion For the phase change from liquid to gas
the heat of vaporization, LV is needed For the phase change from solid to liquid
the heat of fusion, LF is needed
The total heat necessary to change temperature and state is the sum of heats required for each
Phase Curve for Water
Calorimetry1) Write out equation adding all heats in symbolic form and set
equal to zero e.g., Q1 + Q2 + Q3 = 0 or m1c1T1 + m2L2+ m3c3T3 =0
2) Substitute numbers, use correct values for c and L
3) Make sure units for m, c, L and T match
4) Always use T = Tf-Ti (don’t add your own sign!) Temperature decrease results in a negative loss of heat
5) For Lm, make positive if heat is absorbed and negative if heat is given off
Solve for required variable
Work and Internal Energy
No heat can travel in or out
If weight is removed from the piston head, the remaining weight will rise
Where does the energy come from?
It must come from the internal energy of the gas Internal energy depends on
temperature
Work and Heat
The thermal reservoir can add or subtract heat from the system
What happens to the internal energy of the system as heat is applied or work is done?
The First Law of Thermodynamics
This conservation of energy is called the First Law of Thermodynamics
Eint = Q - W
Sign convention: If work is done by the system W is positive, if work is
done on the system W is negative
Work, Pressure and Volume How does work change the system?
The amount of work done in moving something a
small distance ds is:
However, F = pA and dV = A ds
The total work is dW integrated over the volume change
W = dW = p dV (integrated from Vi to Vf) Work is the area under the curve on a p-V diagram
p-V Diagrams
The p-V Curve Pressure must be non-zero in order for work to
be done
If the volume increases, work is done by the system and the work is positive
No volume change, no work If the process is cyclical and returns to the same
point by two different paths the area between the paths is equal to the work (and also equal to the heat)
Adiabatic
Q=0 so Eint = -W
Consider a thermally isolated (insulated) piston with weight on top
By changing the weight, the gas is compressed or expands
Constant Volume
Eint = Q If any heat is applied to the system
it goes directly into internal energy
The gas cannot expand
Cyclical Process
The final pressure, volume and internal energy are the same as the initial
Eint = 0 so Q=W There are many different ways to
produce a cyclical process
Next Time
Read: 18.11-18.12
Which of the following places the temperatures in the correct rank, highest to lowest?
a) 50 X, 50 W, 50 Y b) 50 X, 50 Y, 50 W c) 50 W, 50 X, 50 Y d) 50 Y, 50 W, 50 X e) 50 Y, 50 X, 50 W
Consider 3 aluminum plates, plate A has area of 1X1 m, plate B has area of 2X2 m and plate C has area of 3X3 m. If the temperature of the plates is raised by 10 degrees, which plates has the greatest increase in area?
a) Ab) Bc) Cd) All tiee) You cannot tell with out knowing the
thickness