The Second Law of Thermodynamics

Abstract

Not all processes allowed by the first law of thermodynamics actually occur; there are limitations that are expressed in a number of generalisations of experience that are known as the second law of thermodynamics. The simplest statement of the second law is that when two closed systems interact thermally, the hotter system always becomes cooler and the cooler system always becomes hotter. In terms of the heat flow that occurs, this statement may be put in the form: when two closed systems are placed in thermal contact, the direction of the energy transfer as heat is always from the system at the higher temperature to that at the lower temperature. This is the Clausius statement of the second law.