Chapter 8-INTRODUCTION TO THERMODYNAMICS 



statement indicates that water will not freeze 

 when heat is applied. Note that the Clausius 

 statement includes and goes somewhat beyond 

 the common observation that heat flows only 

 from a hotter to a colder substance. 



The statement that no process is possible 

 where the sole result is the removal of heat 

 from a single reservoir and the performance 

 of an equivalent amount of work is known as the 

 Kelvin-Planck statement of the second law. 

 Among other things, this statement says that 

 we cannot expect the heat of friction to reverse 

 itself and perform mechanical work. More 

 broadly, this statement indicates a certain one- 

 sidedness that is inherent in thermodynamic 

 processes. Energy in the form of work can be 

 converted entirely to energy in the form of heat; 

 but energy in the form of heat can never be 

 entirely converted to energy in the form of 

 work. 



A very important inference to be drawn 

 from the second law is that no engine, actual 

 or ideal, can convert all the heat supplied to it 

 into work, since some heat must always be re- 

 jected to a receiver which is at a lower tem- 

 perature than the source. In other words, there 

 can be no heat flow without a temperature dif- 

 ference and there can be no conversion to work 

 without a flow of heat. A further inference from 

 this inference is sometimes given as a statement 

 of the second law: No thermodynamic cycle can 

 have a thermal efficiency of 100 percent. 



We must say, then, that the first law of 

 thermodynamics deals with the conservation of 

 energy and with the mutual convertibility of 

 heat and work, while the second law limits the 

 direction of thermodynamic processes and the 

 extent of heat-to-work energy conversions. 



THE CONCEPT OF ENTROPY 



The concept of reversibility and the second 

 law of thermodynamics are closely related to 

 the concept of entropy . In fact, the second law 

 may be stated as: No process can occur in which 

 the total entropy of an isolated system de- 

 creases; the total entropy of an isolated system 

 can theoretically remain constant in some re- 

 versible (ideal) processes, but in all irreversible 

 (real) processes the total entropy of an isolated 

 system must increase. 



From other statements of the second law, we 

 know that the transformation of heat to work is 

 always dependent upon a flow of heat from a high 



temperature region to a low temperature region. 

 The concept of the unavailability of a certain 

 portion of the energy supplied as heat to any 

 thermodynamic system is clearly implied in the 

 second law, since it is apparent that some heat 

 must always be rejected to a receiver which is 

 at a lower temperature than the source, if there 

 is to be any conversion of heat to work. The 

 heat which must be so rejected is therefore 

 unavailable for conversion into mechanical work. 



Entropy is an index of the unavailability of 

 energy. Since heat can never be completely con- 

 verted into work, we may think of entropy as a 

 measure or an indication of how much heat 

 must be rejected to a low temperature receiver 

 if we are to utilize the rest of the heat for the 

 production of useful work. We may also think of 

 entropy as an index or measure of the reversi- 

 bility of a process. All real processes are 

 irreversible to some degree, and all real proc- 

 esses involve a "growth" or increase of en- 

 tropy. Irreversibility and entropy are closely 

 related; any process in which entropy has in- 

 creased is an irreversible process. 



The entropy of an isolated system is at its 

 maximum value when the system is in a state 

 of equilibrium. The concept of an absolute 

 minimum— that is, an absolute zero— value of 

 entropy is sometimes referred to as the third 

 law of thermodynamics (or Nernst's law). This 

 principle states that the absolute zero of en- 

 tropy would occur at the absolute zero of tem- 

 perature for any pure material in the crystalline 

 state. By extension, therefore, it should be 

 possible to assign absolute values to the entropy 

 of pure materials, if such absolute values were 

 needed. For most purposes, however, we are 

 interested in knowing the values of the changes 

 in entropy rather than the absolute values of 

 entropy. Hence an arbitrary zero point for 

 entropy has been established at 32° F. 



Entropy changes depend upon the amount of 

 heat transferred to or from the working fluid, 

 upon the absolute temperature of the heat source, 

 and upon the absolute temperature of the heat 

 receiver. Although actual entropy calculations 

 are complex beyond the scope of this text, one 

 equation is given here to indicate the units in 

 which entropy is measured and to give the 

 relationship between entropy and heat and tem- 

 perature. Note that this equation applies only to 

 a reversible isothermal process in which Tj = 

 T2. 



S S Q 



^2 ~ ''l = T 



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