13 - Second Law

13 - Second Law - 122 123 Entropy & the 2 nd...

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Unformatted text preview: 122 123 Entropy & the 2 nd Law 124 Entropy & the 2 nd Law P Volume A B C D-work net P Volume A B C D-work net Recall the efficiency of the Carnot Engine A B H A B L H AB net V V nRT V V T T nR q w eff ln ln H L AB net T T q w eff 1 It is impossible to make a 100% efficient engine This provided the origins of the 2 nd Law of Thermodynamics 125 C D L CD CD V V nRT q w ln A B H AB AB V V nRT q w ln Entropy & the 2 nd Law P Volume A B C D-work net P Volume A B C D-work net A B D C V V V V For the isothermal pathways: Recall that the above equations imply: 126 C D L CD CD V V nRT q w ln A B H AB AB V V nRT q w ln Entropy & the 2 nd Law P Volume A B C D-work net P Volume A B C D-work net A B D C V V V V l b h f T q T q These relationships further imply that f denotes forward pathway A B C b denotes return pathway C D A Show this to yourself! For the isothermal pathways: 127 Entropy & the 2 nd Law P Volume A B C D-work net P Volume A B C D-work net Consider the loop drawn below and the Carnot cycle drawn above Can we hypothesize reversible adiabatic and isothermal pathways around this loop? 128 Entropy & the 2 nd Law P Volume A B C D-work net P Volume A B C D-work net Approximate the loop with 1 Carnot Cycle (not a very good approximation) Recall that state functions dont care about the path taken around the loop In a thermodynamic cycle, the overall change in a state function (from the initial state to the final state and then back to the initial state again) is zero. 129 A general cycle can be divided into many small Carnot cycles....
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13 - Second Law - 122 123 Entropy & the 2 nd...

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