Chapter_20 - Ch 20: Second Law of Thermodynamics 6)...

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Ch 20: Second Law of Thermodynamics 1) Second law: Heat flows from hot to cold. Focus: Heat Engines 2) Heat engine: Converts thermal energy to mechanical energy in a cyclic process. T Hot T Cold Engine W Q h Q c Hot reservoir Cold reservoir Energy conservation: Q h = Q c + W c h W Q Q = Take in heat Q h and expell heat Q c Do work W For each cycle: D U = 0
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Ch 20: Second Law of Thermodynamics 3) Work in a cycle: P V A B W = Area enclosed in Pressure P –Volume V cycle = PdV W A B B A W W W + = i.e., Area under upper curve Area under lower curve - 4) Engine efficiency: h c h c h h Q Q 1 Q Q Q Q = = = W e Example: Q h from burning fuel, Q c lost, W output.
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Ch 20: Second Law of Thermodynamics T Hot T Cold Pump W Q h Q c Hot reservoir Cold reservoir c h W Q Q = 5) Heat pump/refrigerator: Work is input to pump heat from cold to hot. Take in heat Q c and expell heat Q h Work input W Energy conservation: Q h = Q c + W Same result as engine! Coefficient of performance: W CP h Q = Heat pump: Heat moved into house for electric motor work input W CP c Q = Refrigerator: Heat removed from freezer for electric motor work input * Note direction of arrows compared to engine.
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Unformatted text preview: Ch 20: Second Law of Thermodynamics 6) Reversible process: Carried out infinitely slowlyseries of equilibrium states. Real processes are irreversible , carried out at finite speed, but can approach reversible. 7) Carnot cycle: Highest possible engine efficiency e : P V A B C D Hot T h isothermal expansion Q h input B A C B Adiabatic expansion Q = 0 A D Adiabatic compresssion Q = 0 D C Cold T c isothermal compression Q c output h c h c T T Q Q = * Key result: h c T T 1 e = Ch 20: Second Law of Thermodynamics 8) Entropy: S D S = 0 for adiabatic process , since D Q = 0. For isothermal process: D U = 0 = Q - W T T S W Q = = Measure of disorder, S is a system property. Adding heat increases disorder in the system. For Carnot cycle: T Q T Q c c h h = = S ( follows from sec 7 ) T S Q = For reversible process...
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Chapter_20 - Ch 20: Second Law of Thermodynamics 6)...

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