15.7

The Second Law of Thermodynamics

Ice cream melts when left out on a warm day. A cold can of soda warms up on a hot day at a picnic. Ice cream and soda never become colder when left in a hot environment, for heat always flows spontaneously from hot to cold, and never from cold to hot. The spontaneous flow of heat is the focus of one of the most profound laws in all of science, the second law of thermodynamics.

| The Second Law of Thermodynamics: The Heat Flow Statement
| Heat flows spontaneously from a substance at a higher temperature to a substance at a lower temperature and does not flow spontaneously in the reverse direction.

CONCEPTS AT A GLANCE It is important to realize that the second law of thermodynamics deals with a different aspect of nature than does the first law of thermodynamics. The second law is a statement about the natural tendency of heat to flow from hot to cold, whereas the first law deals with energy conservation and focuses on both heat and work. A number of important devices depend on heat and work in their operation, and to understand such devices both laws are needed. For instance, an automobile engine is a type of heat engine because it uses heat to produce work. In discussing heat engines, Sections 15.8 and 15.9 will bring together the first and second laws to analyze engine efficiency, as the Concepts-at-a-Glance chart in Figure 15-11 indicates. Then, in Section 15.10 we will see that refrigerators, air conditioners, and heat pumps also utilize heat and work and are closely related to heat engines. The way in which these three appliances operate also depends on both the first and second laws of thermodynamics.

Figure 15-11 CONCEPTS AT A GLANCE The first and second laws of thermodynamics are used to evaluate the performance of heat engines, as well as refrigerators, air conditioners, and heat pumps. The high-performance race car in the photograph uses a heat engine for propulsion. (AutoImagery)

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