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S E C T I O N 2 2 . 1 • Heat Engines and the Second Law of Thermodynamics 669 22.1 Heat Engines and the Second Law of Thermodynamics A heat engine is a device that takes in energy by heat 2 and, operating in a cyclic process, expels a fraction of that energy by means of work. For instance, in a typical A heat engine carries some working substance through a cyclic process during which (1) the working substance absorbs energy by heat from a high-temperature en- It is useful to represent a heat engine schematically as in Figure 22.2. The engine absorbs a quantity of energy !Q h ! from the hot reservoir. For this discussion of heat en- gines, we will use absolute values to make all energy transfers positive and will indicate eng (so that negative work W ! " W eng is done on the engine), and then gives up a quantity of energy !Q c ! to the cold reservoir. Because the working substance goes 2 We will use heat as our model for energy transfer into a heat engine. Other methods of energy transfer are also possible in the model of a heat engine, however. For example, the Earth’s atmosphere Figure 22.1 This steam-driven locomotive runs from Durango to Silverton, Colorado. It obtains its energy by burning wood or coal. The generated energy vaporizes water into steam, which powers the locomotive. (This locomotive must take on water from tanks located along the route to replace steam lost through the funnel.) Modern locomotives use diesel fuel instead of wood or coal. Whether old- fashioned or modern, such locomotives can be modeled as heat engines, which extract energy from a burning fuel and convert a fraction of it to mechanical energy. © Phil Degginger/Stone/Getty Hot reservoir at T h Q h Q c W eng Cold reservoir at T c Engine Active Figure 22.2 Schematic representation of a heat engine. The engine does work W eng . The arrow at the top represents energy Q h # 0 entering the engine. At the bottom, Q c $ 0 represents energy leaving the engine. At the Active Figures link at http://www.pse6.com, you can select the efficiency of the engine and observe the transfer of energy. |