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Unformatted text preview: Chapter 7 The second law of thermodynamics Read BS, Chapter 7 Conservation of mass and energy are fine concepts that allow us to quantify and predict well many phenomena which are observed in nature. And if a phenomena can be repeated, it becomes subject to prediction, and can be thought of as a science. However, conservation of mass and energy, by themselves, admit as possibilities phenom- ena which are not observed in nature! For instance consider an isolated system composed of two equal masses of liquid water. See Fig. 7.1. The first is at T A = 310 K , the second is at A B T = 310 K T = 290 K A B T = 320 K T = 280 K A B T = 300 K T = 300 K t not observed in nature observed in nature Figure 7.1: Sketch of two scenarios, both of which satisfy mass and energy conservation. T B = 290 K . A long time elapses. Because the combined system is isolated, there are no 185 186 CHAPTER 7. THE SECOND LAW OF THERMODYNAMICS external heat or work exchanges with the environment. But we will allow heat exchanges between mass A and mass B . Consider two possibilities, both admitted by mass and energy conservation, as t : T A 320 K , T B 280 K . The thermal energy that is gained by A is lost by B , such that the net energy is conserved and the first law is satisfied. This is never observed in nature. T A 300 K , T B 300 K . The thermal energy that is lost by A is gained by B , so once again the first law is satisfied. This is always observed in nature. So mass conservation and the first law of thermodynamics, both of which speak to this gedankenexperiment , are insufficient to guarantee that we will predict what is observed in nature. We need another axiom! In a similar way, there are a variety of phenomena which may satisfy mass and energy conservation, but are not observed in nature. Some include water running uphill without an external assist, CO 2 and H 2 O reacting spontaneously to form CH 4 and O 2 , and air separating into its constituents spontaneously. 7.1 Statements of the second law The second law of thermodynamics is an attempt to provide a single all-encompassing state- ment which expands our thermodynamic theory so as to predict the just-described behavior. Though our statement of the second law will be simple enough, it will be obtuse and some- times difficult to reconcile with nature. It is also a profound concept which has wide ranging ramifications. Its origins are firmly rooted in the engineering sciences, as it was motivated by optimization of steam engines. However, it has found applications in many realms of physics, chemistry, ecology, economics, computer science, and other fields. In his influential essay contrasting scientific and humanistic cultures, C. P. Snow 1 sees understanding of the second law as an indicator of scientific literacy and goes on to chide his humanist colleagues: A good many times I have been present at gatherings of people who, by the stan- dards of the traditional culture, are thought highly educated and who have with...
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This note was uploaded on 03/02/2012 for the course THERMO 20231 taught by Professor Powers during the Spring '10 term at Notre Dame.
- Spring '10
- Things Fall Apart