Instructors_Guide_Ch19 - 19 Heat Engines and Refrigerators...

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19 Heat Engines and Refrigerators Recommended class days: 3 minimum, 4 preferred Background Information Thermodynamics, properly presented, is a grand subject. It is a sweeping, “big picture” view of energy. Unfortunately, this big picture is barely glimpsed in the standard presentation, which becomes focused on myriad details and tedious calculations. Indeed, thermodynamics really does require attention to details and equations. There are many different processes, many different parameters, and many different relationships between the parameters—some general, others applicable only to specific processes. There are 20 or so distinct and important equations in this chapter (many in Table 19.1), depending on how you define “important,” plus many other minor equations. It takes practice and experience—and the guidance of the instructor—for students to have any hope of seeing the big picture without drowning in this sea of details. However, there is a systematic procedure to follow when analyzing heat engines, a procedure given in Problem-Solving Strategy 19.1. By following this strategy in your examples, and empha- sizing it, you can help students distinguish what thermodynamics is all about from the details of the calculations. Student Learning Objectives • To understand the thermodynamics of the four basic processes of an ideal gas. • To understand the physics of simple heat engines and refrigerators. • To recognize that thermodynamics has practical applications to real devices. • To learn that there is a limit to the efficiency of a heat engine. Pedagogical Approach In order to avoid the complications of phase changes, this chapter considers only heat engines and refrigerators using an ideal gas as the working substance. Students are now familiar with the four basic gas processes, and they should be able to visualize how these come about. Table 19.l summarizes the information. Even so, students will need additional practice with ideal-gas calculations before becoming adept. The presentation of information is straightforward. Care has been taken to: • Emphasize that thermodynamics is about the transfer and transformation of energy. • Use a variety of gas processes so that students can recognize the similarities and differences. • Use realistic examples. This chapter finally introduces the work done by the system, distinguishing it with the symbol W s . The net work done by a heat engine during one full cycle is called W out and the work done on a 19-1
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19-2 Instructor’s Guide refrigerator during one full cycle is W in . It is important to distinguish between these because work really is used in these three different contexts. Experienced physicists can get away with the single symbol W because they recognize, from the context, how W is being used. Students, however, will get these three ideas hopelessly confused unless the notation is clear. The heat engine embodies the central concepts of thermodynamics—transferring and trans-
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Instructors_Guide_Ch19 - 19 Heat Engines and Refrigerators...

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