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SM_chapter22 - 22 Heat Engines Entropy and the Second Law...

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22 Heat Engines, Entropy, and the Second Law of Thermodynamics CHAPTER OUTLINE 22.1 Heat Engines and the Second Law of Thermodynamics 22.2 Heat Pumps and Refrigerators 22.3 Reversible and Irreversible Processes 22.4 The Carnot Engine 22.5 Gasoline and Diesel Engines 22.6 Entropy 22.7 Entropy Changes in Irreversible Processes 22.8 Entropy on a Microscopic Scale Q22.3 A higher steam temperature means that more energy can be extracted from the steam. For a con- stant temperature heat sink at T c , and steam at T h , the efficiency of the power plant goes as T T T T T h c h c h = 1 and is maximized for a high T h . Q22.4 No. The first law of thermodynamics is a statement about energy conservation, while the second is a statement about stable thermal equilibrium. They are by no means mutually exclusive. For the particular case of a cycling heat engine, the first law implies Q W Q h eng c = + , and the second law implies Q c > 0. Q22.5 Take an automobile as an example. According to the first law or the idea of energy conservation, it must take in all the energy it puts out. Its energy source is chemical energy in gasoline. During the combustion process, some of that energy goes into moving the pistons and eventually into the mechanical motion of the car. The chemical energy turning into internal energy can be mod- eled as energy input by heat. The second law says that not all of the energy input can become output mechanical energy. Much of the input energy must and does become energy output by heat, which, through the cooling system, is dissipated into the atmosphere. Moreover, there are numerous places where friction, both mechanical and fluid, turns mechanical energy into heat. In even the most efficient internal combustion engine cars, less than 30% of the energy from the fuel actually goes into moving the car. The rest ends up as useless heat in the atmosphere. 571 ANSWERS TO QUESTIONS Q22.1 First, the efficiency of the automobile engine can- not exceed the Carnot efficiency: it is limited by the temperature of burning fuel and the temperature of the environment into which the exhaust is dumped. Second, the engine block cannot be allowed to go over a certain temperature. Third, any practical engine has friction, incomplete burning of fuel, and limits set by timing and energy transfer by heat. *Q22.2 For any cyclic process the total input energy must be equal to the total output energy. This is a consequence of the first law of thermodynamics. It is satisfied by processes ii, iv, v, vi, vii but not by processes i, iii, viii. The second law says that a cyclic process that takes in energy by heat must put out some of the energy by heat. This is not satisfied for processes v, vii, and viii. Thus the answers are (i) b (ii) a (iii) b (iv) a (v) c (vi) a (vii) c (viii) d. 13794_22_ch22_p571-600.indd 571 13794_22_ch22_p571-600.indd 571 1/8/07 7:53:24 PM 1/8/07 7:53:24 PM
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*Q22.6 Answer (b). In the reversible adiabatic expansion OA , the gas does work against a piston, takes in no energy by heat, and so drops in internal energy and in temperature. In the free adiabatic expansion OB
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