Thermodynamics

468 | ThermodynamicsI have only just a minute,Only 60 seconds in it,Forced upon me—can’t refuse itDidn’t seek it, didn’t choose it.But it is up to me to use it.I must suffer if I lose it.Give account if I abuse it,Just a tiny little minute—But eternity is in it.(anonymous)FIGURE 8–50A poetic expression of exergy andexergy destruction.In thermodynamics, the first-law efficiency (or thermal efficiency) of aheat engine is defined as the ratio of net work output to total heat input.That is, it is the fraction of the heat supplied that is converted to net work.In general, the first-law efficiency can be viewed as the ratio of the desiredoutput to the required input. The first-law efficiency makes no reference tothe best possible performance, and thus the first-law efficiency alone is nota realistic measure of performance. To overcome this deficiency, we definedthe second-law efficiency, which is a measure of actual performance relativeto the best possible performance under the same conditions. For heatengines, the second-law efficiency is defined as the ratio of the actual thermalefficiency to the maximum possible (reversible) thermal efficiencyunder the same conditions.In daily life, the first-law efficiency or performance of a person can beviewed as the accomplishment of that person relative to the effort he or sheputs in. The second-law efficiency of a person, on the other hand, can beviewed as the performance of that person relative to the best possible performanceunder the circumstances.Happiness is closely related to the second-law efficiency. Small childrenare probably the happiest human beings because there is so little they can do,but they do it so well, considering their limited capabilities. That is, childrenhave very high second-law efficiencies in their daily lives. The term “fulllife” also refers to second-law efficiency. A person is considered to have afull life, and thus a very high second-law efficiency, if he or she has utilizedall of his or her abilities to the limit during a lifetime.Even a person with some disabilities has to put in considerably more effortto accomplish what a physically fit person accomplishes. Yet, despite accomplishingless with more effort, the person with disabilities who gives animpressive performance often gets more praise. Thus we can say that thisperson with disabilities had a low first-law efficiency (accomplishing littlewith a lot of effort) but a very high second-law efficiency (accomplishing asmuch as possible under the circumstances).In daily life, exergy can also be viewed as the opportunities that we haveand the exergy destruction as the opportunities wasted. Time is the biggestasset that we have, and the time wasted is the wasted opportunity to dosomething useful (Fig. 8–50).The examples above show that several parallels can be drawn between thesupposedly abstract concepts of thermodynamics related to the second lawand daily life, and that the second-law concepts can be used in daily life asfrequently and authoritatively as the first-law concepts. Relating the abstractconcepts of thermodynamics to ordinary events of life benefits both engineersand social scientists: it helps engineers to have a clearer picture ofthose concepts and to understand them better, and it enables social scientiststo use these concepts to describe and formulate some social or psychologicalphenomena better and with more precision. This is like mathematics and sciencesbeing used in support of each other: abstract mathematical conceptsare best understood using examples from sciences, and scientific phenomenaare best described and formulated with the help of mathematics.