P1259014505crnus

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Heat Engines and Refregerators Readings
Chapter 19
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Heat Engine
Heat engine is the device which transform heat
into work.
Hot reservoir.
Cold reservoir.
Heat transfer from hot to cold. How can we
produce the work during this heat transfer?
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Example Work during heat transfer.
This is not enough. We want to do it many times.
So the gas should be return the initial state.
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Example Work during heat transfer.
To return (gas) system to the initial state we
need to transfer some heat to the cold reservoir.

Heat engine any CLOSED-CYCLE device that extract
heat from hot reservoir, does useful work, and
exhausts heat to a cold reservoir.
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Heat engine Work during heat transfer.
Work done by the heat engine (gas) is minus the
work done by external forces.
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Heat engine Ideal gas.
Heat engine is determined by the pV (closed)
cycle. Different cycles different heat engines.

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Heat engine Thermal efficiency
The efficiency of heat to work transfer is
characterized by thermal efficiency, defined as
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Heat engine Thermal efficiency
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Heat engine Ideal gas.
Thermal efficiency depends on the type of the
cycle.
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Heat engine Ideal gas Thermal efficiency

• Thermal efficiency depends on the type of the
  cycle.
• To find thermal efficiency you need to
• Method A
• find the work done by the system It is equal to
  the area enclosed by the cycle on a pV diagram
• find the heat transferred from the hot reservoir
  (positive heat during elementary subprocess).
• Method B
• find the heat transferred during all elementary
  subbprocesses. Then
• sum of all positive heat is the net heat
  transferred from the hot reservoir.
• sum of all negative heat is the net heat
  transferred to cold reservoir.

Elementary subprocess
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Example Thermal efficiency
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Example Thermal efficiency
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Thermal efficiency
What type of process has the maximum thermal
efficiency? Carnot Cycle It consists of two
adiabatic processes and two isothermal processes.
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Thermal efficiency
For any type of heat engine
Where is the temperature of cold
reservoir (the lowest temperature of the gas
during the cycle) is the temperature
of the hot reservoir (the highest temperature of
the gas during the cycle). The equality is only
for Carnot cycle.
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Example What is the maximum thermal efficiency
if
Where the temperature is in Kelvin then
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Refrigerators Heat transfer from cold reservoir
to hot reservoir.
Refrigerator is an any closed-cycle device that
uses external work to remove heat from
a cold reservoir and exhaust heat to a
hot reservoir. is a positive heat
transfer to the gas during the cycle, and
is the (minus) negative heat transfer,
is the work done by external forces (this work is
positive).
Coefficient of performance
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Refrigerators Coefficient of performance.
For any refrigerator
The equality is for Carnot cycle
Example What is the maximum coefficient of
performance if