Lecture_37_PPT

# Lecture_37_PPT - Chapter 22 Entropy and the Second Law of...

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Chapter 22 Entropy and the Second Law of Thermodynamics

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Heat Engine z A heat engine is a device that takes in energy by heat and, operating in a cyclic process, expels a fraction of that energy by means of work z A heat engine carries some working substance through a cyclical process z JAVA
Heat Engine z The working substance absorbs energy by heat from a high temperature energy reservoir ( Q h ) z Work is done by the engine ( W eng ) z Energy is expelled as heat to a lower temperature reservoir ( Q c )

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Heat Engine z Since it is a cyclical process, its initial and final internal energies are the same Δ E int = 0 z Therefore, Q net = W eng z The work done by the engine equals the net energy absorbed by the engine
Heat Engine, Thermal efficiency eng 1 hc c hh h W QQ Q e Q ≡= = z Thermal efficiency is defined as the ratio of the net work done by the engine during one cycle to the energy input at the higher temperature z We can think of the efficiency as the ratio of what you gain to what you give z Animation

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Heat Engine eng 1 hc c hh h W QQ Q e Q ≡= = z In practice, all heat engines expel only a fraction of the input energy by mechanical work z Therefore, their efficiency is always less than 100%
Heat Pump Process z Energy is extracted from the cold reservoir, Q C z Energy is transferred to the hot reservoir, Q h z Work must be done on the engine, W z Examples z A refrigerator is a common type of heat pump z An air conditioner is another example of a heat pump

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COP, Cooling Mode z In cooling mode , you “gain” energy removed from a cold temperature reservoir z A good refrigerator should have a high COP z Typical values are 5 or 6 COP c Q W =
Cyclic Processes 1 2 3 Q h = Q 31 = 3/2 ( P i -P f ) V i Q c = Q 23 = 5/2 P f ( V i -V f ) e = 1 – Q c / Q h P i V i 5/3 = P f V f 5/3

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Lecture_37_PPT - Chapter 22 Entropy and the Second Law of...

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