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Transcript of sdfas ffka sab bkjqw98urnkasf98y kjnvakf8auhfasfnmsg
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Thermodynamics
Temperature, Heat, Work
Heat Engines
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Introduction
In mechanics we deal with quantities suchas mass, position, velocity, acceleration,
energy, momentum, etc. Question: What happens to the energy of
a ball when we drop it on the floor?
Answer: It goes into heat energy.
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The answer is a bit longer.
In Thermodynamics we deal withquantities which describe our system,
usually (but not always) a gas. Volume, Temperature, Pressure, Heat
Energy, Work.
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We all know about Volume.
Pressure:
Area
ForcePressure
Demonstrations: Balloons, Bed of Nails, Magdeburg hemispheres.
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Temperature and Heat
Everyone has a qualitative understandingof temperature, but it is not very exact.
Question: Why can you put your hand in a400 F oven and not get instantly burned,but if you touch the metal rack, you do?
Answer: Even though the air and the rack are
at the same temperature, they have verydifferent energy contents and they conductheat at different rates (have different specificheats).
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Heat
Heat is the randommotion of the particlesin the gas, i.e. a
“degraded” from of kinetic energy.
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The higher the temperature, the faster theparticles (atoms/molecules) are moving,i.e. more Kinetic Energy.
We will take heat to mean the thermalenergy in a body OR the thermal energy
transferred into/out of a body
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Work Done by a Gas
Work=(Force)x(distance)
=Fy
Force=(Pressure)x(Area) W=P(A y)
=P V
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First Law of Thermodynamics
Conservation of energy When heat is added into a system it can
either 1) change the internal energy of the
system (i.e. make it hotter) or 2) go intodoing work.
Q=W +U.
Note: For our purposes, Internal Energy is the part of the energy that depends on Temperature (ie,U is heat).
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Second Law: Heat Engines
If we can create an “engine” thatoperates in a cycle, we return to ourstarting point each time and thereforehave the same internal energy. Thus,for a complete cycle
Q=W
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Model Heat Engine
Qhot= W+Qcold
or
Qhot-Qcold=W
(what goes in mustcome out)
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Second Law of Thermodynamics
(What can actually happen) Heat does not voluntarily flow from cold to
hot.
OR
All heat engines have an efficiency<100%. (Not all heat can be converted
into work.)
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Entropy
Entropyrefers to the amount of disorder
in a system
In all natural systems, entropy is favored Trees in a forest grow sporadically
Has anyone ever gotten a royal flush? Whatare the odds?
The greater entropy in the system, theless energy available to do work
Peewee vs professional football