Chapter 8 -INTRODUCTION TO THERMODYNAMICS 



Another common statement of the first law 

 of thermodynamics is that a perpetual motion 

 machine of the first class is impossible. To 

 understand the significance of this statement, it 

 is necessary to understand the classification of 

 perpetual motion machines. Although no per- 

 petual motion machine exists— or, indeed, has 

 ever been constructed— it is possible to conceive 

 of three different categories. Aperpetual motion 

 machine of the first class is one which would put 

 out more energy in the form of work than it ab- 

 sorbed in the form of heat. Since such a machine 

 would actually create energy, it would violate 

 the first law of thermodynamics and the principle 

 of the conservation of energy. Aperpetual motion 

 machine of the second class would permit the 

 reversal of irreversible processes and would 

 thus violate the second law of thermodynamics, 

 as discussed presently. A machine of the third 

 class would be one in which absolutely no fric- 

 tion existed. Interestingly enough, there are no 

 theoretical grounds for declaring that a machine 

 of the third class is completely impossible; how- 

 ever, such a machine would be entirely contrary 

 to our experience and would violate some of our 

 profoundest convictions about the nature of 

 energy and matter. 



THERMODYNAMIC SYSTEMS 



A thermodynamic system may be defined as 

 a bounded region which contains matter. The 

 boundaries may be fixed or they may vary in 

 shape, form, and location. The matter within a 

 system may be matter in any form— solid, 

 liquid, or gas— or in some combination of forms. 

 For some purposes, devices such as engines, 

 pumps, boilers, and so forth may be regarded 

 as being matter included within a thermodynamic 

 system; for other purposes, each such device 

 may be considered as a system in itself. A 

 thermodynamic system may be entirely real, 

 entirely imaginary, or a mixture of real and 

 imaginary, A thermodynamic system may be 

 capable of exchanging energy, in the form of 

 heat and/or work, with its environs; or it may 

 be an isolated system, in which case no heat can 

 flow to or from the system and no work can be 

 done on or by the system. 



If a thermodynamic system appears to be a 

 flexible thing, consider the further statement that 

 "... a system may be said to be whatever one 

 is talking about, and its environs are everythi..g 



else."^*^ Such flexibility of definition is entirely 

 reasonable for most purposes. When we must 

 account for energy, however, we will find it 

 necessary to rigidly define and limit the sys- 

 tem or systems under consideration. It is in 

 terms of energy accounting, then, that the con- 

 cept of a thermodynamic system is most useful. 

 A thermodynamic system requires a work- 

 ing substance to receive, store, transport, and 

 deliver energy. In most systems, the working 

 substance is a fluid— liquid, vapor, or gas.l^ 

 The state of a thermodynamic system is speci- 

 fied by giving the values of two or more prop- 

 erties. These properties, which are called state 

 variables or thermodynamic coordinates 

 common properties 



m- 



as pressure, 



volume, and mass, as well as 



elude such 

 temperature, 

 more complex properties such as enthalpy and 

 entropy (discussed later). Although some sys- 

 tems are adequately described by giving the value 

 of only two variables, many systems require the 

 specification of three or more variables. 



THERMODYNAMIC PROCESSES 



A thermodynamic process may be defined as 

 any physical occurrence during which an effect 

 is produced by the transformation or redistri- 

 bution of energy. The occurrence of a thermo- 

 dynamic process is evidenced by changes in some 

 or all of the state variables of the system. The 

 processes of most interest in engineering are 

 those involving heat and work. 



In connection with any process, it is usually 

 necessary to consider the physical character of 

 the process; the manner in which energy is 

 transformed or redistributed as the process 

 takes place; the kind and amount of energy that 

 is stored in the system before and after the 

 process, and the location of such energy; and the 

 changes which are brought about in the system 



13Kiefer, Kinney, and Stuart, Principles of Engineer- 

 ing Thermodynamics , 2nd ed., John Wiley & Sons, New 

 York, 1954 (p. 32), 



14 



Some writers use the term gas to indicate a gaseous 



substance that can be liquefied only by very large 

 changes in pressure or temperature, reserving the 

 term vapor for a gaseous substance that can be liqui- 

 fied more easily, by slight changes of pressure or 

 temperature. Other writers define a vapor as a gas 

 which is in equilibrium with its liquid. For a great 

 many purposes, the properties of a vapor are essen- 

 tially the same as the properties of real gases; hence 

 the distinction is not always important. 



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