SM_chapter18 - Heat Engines Entropy and the Second Law of...

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491 Heat Engines, Entropy, and the Second Law of Thermodynamics CHAPTER OUTLINE 18.1 Heat Engines and the Second Law of Thermodynamics 18.2 Reversible and Irreversible Processes 18.3 The Carnot Engine 18.4 Heat Pumps and Refrigerators 18.5 An Alternative Statement of the Second Law 18.6 Entropy 18.7 Entropy and the Second Law of Thermodynamics 18.8 Entropy Changes in Irreversible Processes 18.9 Context Connection The Atmosphere as a Heat Engine ANSWERS TO QUESTIONS Q18.1 First, the efficiency of the automobile engine cannot 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. Q18.2 No. Any heat engine takes in energy by heat and must also put out energy by heat. The energy that is dumped as exhaust into the low- temperature sink will always be thermal pollution in the outside environment. So-called “steady growth” in human energy use cannot continue. Q18.3 A higher steam temperature means that more energy can be extracted from the steam. For a constant temperature heat sink at T c , and steam at T h , the efficiency of the power plant is determined by T T T T T h c h c h = 1 and is maximized for a high T h . Q18.4 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. Q18.5 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. Clearly much of the energy goes into heat, which, through the cooling system, is transferred 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 put into the atmosphere.
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492 Heat Engines, Entropy, and the Second Law of Thermodynamics Q18.6 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%.
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