C03 - 3 Introduction U SING ENERGY AND THE FIRST LAW OF...

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USING ENERGY AND THE FIRST LAW OF THERMODYNAMICS 3 Introduction… Energy is a fundamental concept of thermod l ynamics and one of the most significant aspects of engineering analysis. In this chapter we discuss energy and develop equations for applying the principle of conservation of energy. The current presentation is limited to closed systems. In Chap. 5 the discussion is extended to control volumes. Energy is a familiar notion, and you already know a great deal about it. In the present chapter several important aspects of the energy concept are developed. Some of these we have encountered in Chap. 1. A basic idea is that energy can be stored within systems in various forms. Energy also can be converted from one form to another and transferred between systems. For closed systems, energy can be transferred by work and heat transfer. The total amount of energy is conserved in all transformations and transfers. The objective of this chapter is to organize these ideas about energy into forms suitable for engineering analysis. The presentation begins with a review of energy concepts from mechanics. The thermodynamic concept of energy is then intro- duced as an extension of the concept of energy in mechanics. Reviewing Mechanical Concepts of Energy Building on the contributions of Galileo and others, Newton formulated a general descrip- tion of the motions of objects under the influence of applied forces. Newton’s laws of motion, which provide the basis for classical mechanics, led to the concepts of work, kinetic energy, and potential energy, and these led eventually to a broadened concept of energy. In the pres- ent section, we review mechanical concepts of energy. 3.1.1 Kinetic and Potential Energy Consider a body of mass m that moves from a position where the magnitude of its ve- locity is V 1 and its elevation is z 1 to another where its velocity is V 2 and elevation is z 2 , each relative to a specified coordinate frame such as the surface of the earth. The quan- tity 1 / 2 m V 2 is the kinetic energy, KE, of the body. The change in kinetic energy, ±²³ ,of the body is (3.1) Kinetic energy can be assigned a value knowing only the mass of the body and the mag- nitude of its instantaneous velocity relative to a specified coordinate frame, without regard ¢ KE ´ KE 2 µ KE 1 ´ 1 2 m 1 V 2 2 µ V 1 2 2 3.1 31 chapter objective z m g 1 2 kinetic energy
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32 Chapter 3. Using Energy and the First Law of Thermodynamics for how this velocity was attained. Hence, kinetic energy is a property of the body. Since kinetic energy is associated with the body as a whole, it is an extensive property. The quantity mgz is the gravitational potential energy, PE. The change in gravitational potential energy, ± PE, is (3.2) Potential energy is associated with the force of gravity ( Sec. 2.3 ) and is therefore an attrib- ute of a system consisting of the body and the earth together. However, evaluating the force of gravity as mg enables the gravitational potential energy to be determined for a specified value of g knowing only the mass of the body and its elevation. With this view, potential en-
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C03 - 3 Introduction U SING ENERGY AND THE FIRST LAW OF...

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