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Instructors_Guide_Ch17

Instructors_Guide_Ch17 - 17 Work Heat and the First Law of...

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17 Work, Heat, and the First Law of Thermodynamics Recommended class days: 2 Background Information Chapter 16 introduced the state variables needed to characterize a macroscopic system. Now we begin the process of looking at how those state variables change as a consequence of energy transferred into or out of the system. Chapter 11 developed the work-kinetic energy theorem in the form K = W = W c + W nc + W ext where W c is the work done by conservative forces, W nc is the work done by nonconservative forces within the system (e.g., friction), and W ext is the work done by external forces. Dividing the forces into these three groups is important. Conservative forces are associated with potential energies via U = - W c , and the dissipative nonconservative forces are associated with increasing the thermal energy of the system via E th = - W nc . This leads to E mech + E th = W ext where E mech = K + U . The systems we want to study in thermodynamics are stationary containers of gases or liquids whose center-of-mass mechanical energy does not change. Thus E mech = 0 and E th = W ext Here W ext is the work done by piston rods or by other forces that compress or expand the gas. These are forces of a very different nature from the friction forces we’ve called nonconservative forces. Since all the forces in thermodynamics that do work are external forces, we can drop the subscript. Then, after noting that work is one way to transfer energy to the system, but not the only way, we can define heat Q as a nonmechanical energy transfer due to a temperature difference. Heat enters on an equal footing with work, with both being mechanisms for transferring energy. With heat included, we have E th = W + Q This is, of course, the first law of thermodynamics. We have continued to modify and enlarge the concept of energy, but there has been no discontinuity of ideas in moving from mechanics to thermodynamics. You should encourage students to review the development that has taken us from kinetic energy to the first law, and you can ask students to articulate what new ideas were added at each step. These ideas are summarized in graphical form in the “thermodynamics energy model” of Figure 17.12. I urge you to explicitly call this figure to your students’ attention. The major new concept is heat . It’s well known that students confuse heat with thermal energy. Most students think that heat is what a thermometer measures—not a surprising belief, considering the everyday use of the word heat . It is important at every opportunity to emphasize that heat and 17-1

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17-2 Instructor’s Guide work are energy that is transferred between the system and the environment. Neither heat nor work is a property of the system. Because temperature measures a property of the system, temperature must be measuring something other than heat. Our own language can mislead students if we’re not careful. I’ve caught myself saying, “I’m
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Instructors_Guide_Ch17 - 17 Work Heat and the First Law of...

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