Lecture 16

Lecture 16 - Chap 20:1-4 Second Law of Thermodynamics...

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May 29, 2008 Physics 40B Lecture 16 1 Chap. 20:1-4 Second Law of Thermodynamics Review p i V i " = p f V f T i V i # 1 = T f V f # 1 p i V i T i = p f V f T f (Ideal Gas Law) W = " 1 # -1 \$ % ( ) p f V f " p i V i ( ) Adiabatic ( Q = 0 ) Expansion (Compression) of Ideal Gas

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May 29, 2008 Physics 40B Lecture 16 2 Heat Engines: purpose is to convert heat energy to work Theoretical Model Engine T H T C High temperature heat reservoir Low temperature heat reservoir Q H (in) Q C (out) Q H --> Positive heat energy per cycle transferred into engine, Q C --> Positive heat energy per cycle exhausted from engine W --> Positive work done by engine per cycle on surroundings Complete Cycle: Δ U = 0, Q Net = W = W W = Q Net = Q H - Q C (Q’s are positive) Work W(out)
May 29, 2008 Physics 40B Lecture 16 3 Engine Thermal Efﬁciency V p Area = Work Note: W --> Q H , e --> 1 (100%) Q C --> 0, e --> 1 (100%) Efficiency: e = W Q H = Q Net Q H = Q H " Q C Q H = 1 " Q C Q H

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May 29, 2008 Physics 40B Lecture 16 4 Exmple Engine T H T C High temperature heat reservoir Low temperature heat reservoir Q H (in) = 200J Q C (out) = 152 J W = 48 J
May 29, 2008 Physics 40B Lecture 16 5 Second Law of Thermodynamics Kelvin-Planck Statement It is impossible to construct a heat engine that operates in a cycle and has no other effect than to absorb energy from a heat reservoir and perform an equal amount of work (that is, no engine can be 100% efﬁcient)

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May 29, 2008 Physics 40B Lecture 16 6 Reversibility (and Irreversibility) Examples of irreversible processes: Heat conduction from Hot to Cold: can’t bring system back to original state without permanent change to surroundings--must do work. Free expansion : must do work to compress gas isothermally to original volume. Any process involving friction. We can approximate a reversible process by slowly (
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Lecture 16 - Chap 20:1-4 Second Law of Thermodynamics...

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