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Unformatted text preview: Lecture 15 1 Applications of thermo are manifold. Let’s return to our example from Lecture 13: As we complete the reversible cycle cyc cyc dU q w = = + ∫ Ñ The area inside the lines is the work done by the system on the surroundings over the entire cycle. cyc cyc a c cyc q w q q w = + = + + Recall that b and d are adiabatic processes, so q b =q d =0. ( 29 2 1 b S 0 reversible adiabatic a a c c d q T S q T S S = ∆ = ∆ ∆ = ∆ =- a c dS S S = = ∆ + ∆ ∫ Ñ More Entropy (20.6-8) Lecture 15 2 2 , We define maximum efficiency, , as cyc rev T w q ε- Why - ? 0 for this system cyc cyc w w < 2 , Why ? rev T q 1 1 2 2 2 1 2 1 1 1 a c c c a a a q q q T S T q q T S T T T T ε ε + ∆ = = + = + = - ∆- = Doesn’t depend on any details: type of cycle, materials, etc. 1 2 For a steam engine: T 373 (Boiling point of water at 1 atm) T 500 (hotter!) 127 = 0.254 25.4% efficiency 500 K K ε = = = = How can we improve the efficiency? Lower T 1 (this is why efficiency is determined relative to q rev,T2 !) Lecture 15...
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