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Lect07 - Physics 213 Sept 14 2006 Physics 213 Sept 14 2006...

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Physics II: Heat & Electromagnetism today 2nd Law of Thermodynamics (see last lecture’s notes) Entropy Electric charge and the structure of matter Conductors and insulators Induced charges Physics 213 Sept. 14, 2006 7.1 … and a note on heat pumps Consider T in ~ 68° F and T out ~ 41° F: K Carnot = T H /(T H -T C ) = 293K/15K ~ 20 K Actual ~ 4 Method: Natural Gas Furnace Electric Heat Heat Pump Heating Efficiency or COP 90% 100% K~4 Fuel used per 1000 J of Heat 1kJ/0.9=1110J Electricity used per 1000 J of Heat 20 J 1000 J 1000 J/4=250 J Electric Power Plant Efficiency 0.4 0.4 0.4 Energy used to make Electricity 20 J/0.4=50 J 1000 J/ 0.4=2500 J 250 J/0.4=625 J Energy resources wasted 110J +30J=140J 1500 J 375 J What is consumed, What you pay $ for 1160 J 2500 J 625 J Physics 213 Sept. 14, 2006 7.2 Entropy: • quantitative measure of disorder • a state variable A gas increases its disorder if it expands (V ! V +dV. Keep its internal energy constant (isothermal) -a measure of disorder: dV/V -1st law: dU=0 dQ = dW = p dV -Ideal Gas: V = nRT/p dS = dQ rev T " S = Q rev T
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