ch22 - The Big Ideas-Chapter 22(Serway and Beichner Physics...

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The Big Ideas—Chapter 22 (Serway and Beichner, Physics for Scientists and Engineers, 5 th Edition) AJM:3/4/01 1 Section 1 A cyclic process may be exploited to construct a “heat engine”—a device that trades heat from a hot reservoir for work at the cost of some exhaust or waste heat—or a “heat pump” (or “refrigerator”)—a device that causes heat to flow from cold reservoirs to hot reservoirs with the necessary expenditure of mechanical energy. In both cases we distinguish between the heat to the system from the “hot” reservoir— Q h —and the heat from the system to the “cold” reservoir— Q c . The first law of thermodynamics connects these heats with the net work done per cycle. A “perfect heat engine” is one for which there is no waste heat, that is, is uses all heat from the hot reservoir to do work . A “perfect heat pump” is one that requires no work input, that is, it simply causes heat to flow from the cold reservoir to the hot reservoir. Although both perfect heat engines and perfect refrigerators are completely compatible with the first law of thermodynamics, they are not allowed by nature. These observations form the basis for the Kelvin-Planck and Clausius statements of the “Second Law of Thermodynamics.” We define the efficiency of a heat engine as the ratio of the work done per cycle to the heat from the hot reservoir per cycle usually expressed as a percentage. The second law of thermodynamics says that the efficiency is always less than 100%. W = Q h - Q c Perfect heat engine: Q c = 0 Perfect heat pump: W = 0 2 nd Law of Thermo Kelvin-Planck: No such thing as a perfect heat engine.
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