Ch1b09Lecture13

# Ch1b09Lecture13 - MidTerm Review 7 p.m. Thursday evening in...

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1 MidTerm Review 7 p.m. Thursday evening in here Midterm covers up to but not including the Carnot Cycle

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2 Entropy & the 2 nd Law
3 Entropy & the 2 nd Law 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

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4 = = 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 Volume A B C D -work net = A B D C V V V V 0 = + l b h f T q T q These relationships further imply that f denotes forward pathway A Æ B Æ C b denotes return pathway C Æ D Æ A Show this to yourself! For the isothermal pathways:
5 Entropy & the 2 nd Law 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?

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6 Entropy & the 2 nd Law P Volume A B C D -work net Approximate the loop with 1 Carnot Cycle (not a very good approximation) Recall that state functions don’t 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.
7 A general cycle can be divided into many small Carnot cycles. The match is exact in the limit of infinitesimally small cycles. Paths cancel in the interior of the collection of cycles Only the perimeter, an increasingly good approximation to the true cycle as the number of cycles increases, survives. Entropy & the 2 nd Law

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8 Entropy & the 2 nd Law 0 = + l b h f T q T q f denotes forward pathway A Æ B Æ C b denotes return pathway C Æ D Æ A The implication is that q/T is a state function f rev i dq S T ∆≡ More generally, Clausius defined entropy as: Units J·K -1 No matter how we traverse this loop, this equation holds. Thus: ‘i , f’ here mean initial and final
9 Because the entropy change around every individual cycle is zero, the integral of the entropy around the perimeter is zero too.

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## This note was uploaded on 09/25/2010 for the course CH 104 taught by Professor Bopanna during the Fall '09 term at UMBC.

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Ch1b09Lecture13 - MidTerm Review 7 p.m. Thursday evening in...

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