Page:Elementary Text-book of Physics (Anthony, 1897).djvu/258

244 an engine. In the ideal form it consists of a body called the source, from which heat may be drawn, another body called the refrigerator, into which heat may be sent, and a third body called the working body, which expands or contracts on the reception or emission of heat. The working body will itself always possess energy in the form of heat and possibly also in other forms. If heat be supplied to it from the source, it will expand and do work, but no relation can be stated between the work done and the heat supplied to it, because the change in its own energy experienced during the expansion is, in general, unknown. In order to obtain a relation between the heat supplied to the working body and the work done by it, the operations performed with it must be so conducted as to bring the working body back to its original state. It will then possess the same energy as at the outset, and the first law of thermodynamics enables us to assert that the difference between the heat which leaves the source and the" heat which enters the refrigerator is equal to the work done by the working body. Such a series of operations is called a cycle. The ratio of the work done to the heat which leaves the source is called the efficiency of the engine.

228. The Carnot's Cycle.—In order to study the efficiency of an engine we restrict the conditions under which the transformation of heat into work goes on. We suppose that the source is so large and furnishes so unlimited a supply of heat that its temperature $$S$$ remains constant, notwithstanding the loss or gain of heat which it may receive from the working body. Similarly, we suppose the refrigerator to have a constant temperature $$R,$$ notwithstanding the gain or loss of heat it may receive from the working body. The changes by which the working body does work are supposed to occur only when the working body is either at the temperature of the source or of the refrigerator, or when it is so conditioned that it neither receives nor emits heat. While it is kept at a constant temperature, its change is isothermal; when it neither receives nor emits heat, its change is adiabatic (§ 213).

In order to exhibit the operation of this simple engine most clearly, we will assume that the working body is one which