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Instructors_Guide_Ch11

# Instructors_Guide_Ch11 - 11 Work Recommended class days 2...

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11 Work Recommended class days: 2 minimum Background Information Chapter 10 found that energy conservation was a useful idea, but its use was restricted to situations without friction or other dissipative forces. The goal of Chapter 11 is to expand those initial ideas into a more complete understanding of work and energy. The chapter opens by posing three important questions: • How many kinds of energy are there? • Under what conditions is energy conserved? • How does a system gain or lose energy? Addressing these questions puts the concept of energy on a firmer foundation. The chapter begins with the basic energy model shown in the figure. In particular, • The idea of thermal energy E th is introduced early, then discussed in more detail later in the chapter. • Energy can be transformed within the system without changing the value of E sys . • Energy can be transferred between the system and the environment. For now, we’re only concerned with the mechanical transfer of energy due to forces acting on the system. The mechanical transfer of energy is defined as work . This approach defines work by its function, not by how it is calculated. Our ultimate goal, which will not be reached until thermodynamics, is for students to understand energy as a quantity that can be either transferred to or from the system or transformed within the system. Hence it becomes important to distinguish clearly between these two concepts early in the teaching of energy. In fact, looking ahead to thermodynamics, we often refer to the first law of thermodynamics as the ultimate statement of energy conservation, yet the usual presentation doesn’t allow students to see any connection between thermodynamics and the “law of conservation of energy” they learned in mechanics. To make this connection, we need to introduce the ideas in such a way that the concepts of energy and work can be gradually and coherently generalized until we reach the first law of thermodynamics. As an example, nearly all textbooks state the “law of conservation of energy” in a form equivalent to K + U + E th = 0 where the change in thermal energy is related to the work done by nonconservative forces: E th = - W nc . This statement, as usually presented, is not true. It hasn’t properly distinguished transfer of energy from transformation of energy. 11-1

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11-2 Instructor’s Guide Simple examples are easily found. If you push a block across a table at constant speed, then K = 0, U = 0, and E th 0. If you pick up a book from the floor and place it on a shelf, then K = 0, U 0, and E th = 0. In neither case is the above sum equal to zero. The difficulty is that “the push” and “the lift” are not nonconservative forces associated with thermal energy. They are external forces that act on the system and transfer energy to the system. The above statement doesn’t allow for energy transfers.
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Instructors_Guide_Ch11 - 11 Work Recommended class days 2...

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