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chap2 - Chapter 2 Thermodynamic Concepts and Processes c...

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Chapter 2 Thermodynamic Concepts and Processes c circlecopyrt 2009 by Harvey Gould and Jan Tobochnik 1 August 2009 We introduce the concepts of temperature, energy, work, heating, entropy, engines, and the laws of thermodynamics and related macroscopic concepts. 2.1 Introduction In this chapter we will discuss ways of thinking about macroscopic systems and introduce the basic concepts of thermodynamics. Because these ways of thinking are very different from the ways that we think about microscopic systems, most students of thermodynamics initially find it difficult to apply the abstract principles of thermodynamics to concrete problems. However, the study of thermodynamics has many rewards as was appreciated by Einstein: A theory is the more impressive the greater the simplicity of its premises, the more different kinds of things it relates, and the more extended its area of applicability. Therefore the deep impression that classical thermodynamics made to me. It is the only physical theory of universal content which I am convinced will never be overthrown, within the framework of applicability of its basic concepts. 1 The essence of thermodynamics can be summarized by two laws: 2 (1) Energy is conserved and (2) entropy increases. These statements of the laws are deceptively simple. What is energy? You are probably familiar with the concept of energy from other courses, but can you define it? 1 A. Einstein, Autobiographical Notes , Open Court Publishing Company (1991). 2 The nature of thermodynamics is summarized in the song, “First and Second Law,” by Michael Flanders and Donald Swann. 31
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CHAPTER 2. THERMODYNAMIC CONCEPTS 32 Abstract concepts such as energy and entropy are not easily defined nor understood. However, as you apply these concepts in a variety of contexts, you will gradually come to understand them. Because thermodynamics describes the macroscopic properties of macroscopic systems without appeal to arguments based on the nature of their microscopic constituents, the concepts of energy and entropy in this context are very abstract. So why bother introducing thermodynamics as a subject in its own right, when we could more easily introduce energy and entropy from microscopic considerations? Besides the intellectual challenge, an important reason is that the way of thinking required by thermodynamics can be applied in other contexts where the microscopic properties of the system are poorly understood or very complex. However, there is no need to forget the general considerations that we discussed in Chapter 1 . And you are also encouraged to read ahead, especially in Chapter 4 where the nature of entropy is introduced from first principles. 2.2 The System The first step in applying thermodynamics is to select the appropriate part of the universe of interest. This part of the universe is called the system . In this context the term system is simply anything that we wish to consider. The system is defined by a closed surface called the boundary (see Figure 2.1 ). The boundary may be real or imaginary and may or may not be fixed in shape
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