630_Physics ProblemsTechnical Physics

630_Physics ProblemsTechnical Physics - 632 Heat Engines...

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632 Heat Engines, Entropy, and the Second Law of Thermodynamics Q22.7 Suppose the ambient temperature is 20°C. A gas can be heated to the temperature of the bottom of the pond, and allowed to cool as it blows through a turbine. The Carnot efficiency of such an engine is about e T T c h == = 80 373 22%. Q22.8 No, because the work done to run the heat pump represents energy transferred into the house by heat. Q22.9 A slice of hot pizza cools off. Road friction brings a skidding car to a stop. A cup falls to the floor and shatters. Your cat dies. Any process is irreversible if it looks funny or frightening when shown in a videotape running backwards. The free flight of a projectile is nearly reversible. Q22.10 Below the frost line, the winter temperature is much higher than the air or surface temperature. The earth is a huge reservoir of internal energy, but digging a lot of deep trenches is much more expensive than setting a heat-exchanger out on a concrete pad. A heat pump can have a much
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