Thermodynamics

Chapter 8 | 467Children are born with different levels of exergies (talents) in differentareas. Giving aptitude tests to children at an early age is simply an attempt touncover the extent of their “hidden” exergies, or talents. The children arethen directed to areas in which they have the greatest exergy. As adults, theyare more likely to perform at high levels without stretching the limits if theyare naturally fit to be in that area.We can view the level of alertness of a person as his or her exergy forintellectual affairs. When a person is well-rested, the degree of alertness,and thus intellectual exergy, is at a maximum and this exergy decreaseswith time as the person gets tired, as illustrated in Fig. 8–49. Differenttasks in daily life require different levels of intellectual exergy, and thedifference between available and required alertness can be viewed as thewasted alertness or exergy destruction. To minimize exergy destruction,there should be a close match between available alertness and requiredalertness.Consider a well-rested student who is planning to spend her next 4 hstudying and watching a 2-h-long movie. From the first-law point of view, itmakes no difference in what order these tasks are performed. But from thesecond-law point of view, it makes a lot of difference. Of these two tasks,studying requires more intellectual alertness than watching a movie does,and thus it makes thermodynamic sense to study first when the alertness ishigh and to watch the movie later when the alertness is lower, as shown inthe figure. A student who does it backwards wastes a lot of alertness whilewatching the movie, as illustrated in Fig. 8–49, and she has to keep goingback and forth while studying because of insufficient alertness, thus gettingless done in the same time period.MentalalertnessVariation of mentalalertness with timeWasted alertness(irreversibility)MentalalertnessVariation of mentalalertness with timeWasted alertness(irreversibility)Alertnessrequired forstudyingAlertnessrequired forwatching TVAlertnessrequired forwatching TVAlertnessrequired forstudying0 2 4Time (h)0 2 4Time (h)(a) Studying first(b) Watching a movie firstFIGURE 8–49The irreversibility associated with a student studying and watching a movie ontelevision, each for two hours.