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C04 - 4 Introduction EVALUATING PROPERTIES To apply the...

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EVALUATING PROPERTIES 4 Introduction… To apply the energy balance to a system of interest requires knowledge of the properties of the system and how the properties are related. The objective of this chapter is to introduce property relations relevant to engineering thermodynamics. As part of the presentation, several examples are provided that illustrate the use of the closed system energy balance introduced in Chap. 3 together with the property relations considered in this chapter. Fixing the State The state of a closed system at equilibrium is its condition as described by the values of its thermodynamic properties. From observation of many systems, it is known that not all of these properties are independent of one another, and the state can be uniquely determined by giv- ing the values of the independent properties. Values for all other thermodynamic properties can be determined once this independent subset is specified. A general rule known as the state principle has been developed as a guide in determining the number of independent properties required to fix the state of a system. For most applications, we are interested in what the state principle says about the intensive states of systems. Of particular interest are systems of commonly encountered substances, such as water or a uniform mixture of nonreacting gases. These systems are classed as simple com- pressible systems. Experience shows that the simple compressible systems model is useful for a wide range of engineering applications. For such systems, the state principle indicates that the number of independent intensive properties is two . For Example… in the case of a gas, temperature and another intensive property such as specific volume might be selected as the two independent properties. The state principle then affirms that pressure, specific internal energy, and all other pertinent intensive properties could be determined as functions of T and v : p p ( T , v ), u u ( T , v ), and so on. The func- tional relations would be developed using experimental data and would depend explicitly on the particular chemical identity of the substances making up the system. Intensive properties such as velocity and elevation that are assigned values relative to da- tums outside the system are excluded from present considerations. Also, as suggested by the name, changes in volume can have a significant influence on the energy of simple com- pressible systems. The only mode of energy transfer by work that can occur as a simple compressible system undergoes quasiequilibrium processes, is associated with volume change and is given by p dV . 4.1 59 chapter objective state principle simple compressible systems

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60 Chapter 4. Evaluating Properties Evaluating Properties: General Considerations This part of the chapter is concerned generally with the thermodynamic properties of sim- ple compressible systems consisting of pure substances. A pure substance is one of uniform and invariable chemical composition. Property relations for systems in which composition
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