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# Lecture20 - PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 20...

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PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 20

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Heat Engine Refrigerator, Heat Pump Last Lecture Q hot engine Q cold W Q hot fridge Q cold W e = W Q hot < e Carnot = 1 " T cold T hot W = Q hot " Q cold COP = Q c , h W < COP Carnot = T c , h T hot " T cold
Entropy Measure of Disorder of the system (randomness, ignorance) Entropy: S = k B log(N) N = # of possible arrangements for fixed E and Q Number of ways for 12 molecules to arrange themselves in two halves of container. S is greater if molecules spread evenly in both halves.

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On a macroscopic level, one finds that adding heat raises entropy: Temperature in Kelvin! ! S = Q / T 2 nd Law of Thermodynamics (version 2) The Total Entropy of the Universe can never decrease. (but entropy of system can increase or decrease)
Why does Q flow from hot to cold? Consider two systems, one with T A and one with T B Allow Q > 0 to flow from T A to T B Entropy changes by: ! S = Q/T B - Q/T A This can only occur if ! S > 0, requiring T A > T B . System will achieve more randomness by exchanging heat until T B = T A

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Carnot Engine Carnot cycle is most efficient possible, because the total entropy change is zero. It is a “reversible process”.
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Lecture20 - PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 20...

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