Two uses for a heat engine: a modern coal-burning power plant, and an old steam locomotive. Both produce steam which does work-on turbines to generate electricity, and on a piston that moves linkage to turn locomotive wheels The efficiency of any engine--no matter how carefully engineered-is limited by nature as described in the second law of thermodynamics. This great law is best stated in terms of a quantity called entropy, which is unlike any other. Entropy is not conserved, but instead is constrained always to increase in any real process. Entropy is a measure of disorder. The second 17 law of thermodynamics tells us that as time moves forward. the disorder in the universe increases. We also discuss many practical matters such as heat engines, heat pumps, and refrigeration CHAPTER 20