10_1425_web_Lec_36_Entropy

10_1425_web_Lec_36_Entropy - Entropy Physics 1425 Lecture...

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Entropy Physics 1425 Lecture 36 Michael Fowler, UVa
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First and Second Laws of Thermodynamics A quick review…. First Law: total energy conserved in any process : joules in = joules out Second Law : heat only flows one way, and we can’t turn heat into just work, etc… This Second Law sounds a bit vague compared with the First Law! Can it be stated more precisely, more numerically , like the First Law?
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Back to the Carnot Cycle… But this time think of it as showing two different routes from a to c : abc and adc . We know that the total change in a state variable, P , V or T , from a to c doesn’t depend on which path we take. BUT the total heat flow into the gas DOES depend on path! The “amount of heat” in the gas is meaningless. a b d c
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Back to the Carnot Cycle… However, the heat input Q H along abc is related to the heat input Q L along adc : Clausius defined “en t ropy” by: the entropy change along a half Carnot cycle is This doesn’t depend on path! a b d c C H HC Q Q TT = Q S T ∆=
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Different Reversible Paths from a to c Now suppose we follow the path aefgc , where efgd is a little Carnot cycle. Then the entropy change efg is the same as edg , so the total entropy change along this path from a to c is the same as adc . By adding lots of little Carnot zigzags like this, we can get from any reversible path from a to c to any other reversible path, and define the entropy change from a to c unambiguously as: z a d c e b f g . revers path dQ S T ∆=
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What about Irreversible Paths? Suppose we have an insulated container with a partition down the middle , ideal gas on one side, vacuum on the other. The partition is suddenly removed—gas fills the whole space. What happens to the gas temperature?
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10_1425_web_Lec_36_Entropy - Entropy Physics 1425 Lecture...

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