22 - Heat Engines Entropy and the Second Law of Thermodynamics

22 - Heat Engines Entropy and the Second Law of Thermodynamics

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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 Chapter 22 ± This cutaway image of an automobile engine shows two pistons that have work done on them by an explosive mixture of air and fuel, ultimately leading to the motion of the automobile. This apparatus can be modeled as a heat engine, which we study in this chapter. (Courtesy of Ford Motor Company) 667
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668 T he first law of thermodynamics, which we studied in Chapter 20, is a statement of conservation of energy. This law states that a change in internal energy in a system can occur as a result of energy transfer by heat or by work, or by both. As was stated in Chapter 20, the law makes no distinction between the results of heat and the results of work—either heat or work can cause a change in internal energy. However, there is an important distinction between heat and work that is not evident from the first law. One manifestation of this distinction is that it is impossible to design a device that, operat- ing in a cyclic fashion, takes in energy by heat and expels an equal amount of energy by work. A cyclic device that takes in energy by heat and expels a fraction of this energy by heat engine . Although the first law of thermodynamics is very important, it makes no distinc- tion between processes that occur spontaneously and those that do not. However, only certain types of energy-conversion and energy-transfer processes actually take second law of thermodynamics , the major topic in this chapter, establishes which processes do and which do not occur. The following are examples of processes that do not violate the principle of conservation of energy if they pro- ceed in either direction, but are observed to proceed in only one direction, governed When two objects at different temperatures are placed in thermal contact with each other, the net transfer of energy by heat is always from the warmer object to the cooler object, never from the cooler to the warmer. A rubber ball dropped to the ground bounces several times and eventually comes to rest, but a ball lying on the ground never gathers internal energy from the ground and begins bouncing on its own. An oscillating pendulum eventually comes to rest because of collisions with air mol- ecules and friction at the point of suspension. The mechanical energy of the system is converted to internal energy in the air, the pendulum, and the suspension; the reverse conversion of energy never occurs. —that is, they are processes that occur naturally in one direction only. No irreversible process has ever been observed to run backward—if it were to do so, it would violate the second law of thermodynamics.
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22 - Heat Engines Entropy and the Second Law of Thermodynamics

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