Page:Elementary Text-book of Physics (Anthony, 1897).djvu/257

 CHAPTER IV.

THERMODYNAMICS.

226. First Law of Thermodynamics.—The law of the conservation of energy, in the special case of heat and mechanical work, is called the first law of thermodynamics. It may be thus stated: When heat is transformed into work, or work into heat, the quantity of work is equivalent to the quantity of heat. The experiments of Joule, Rowland, and Hirn establishing this law, and determining the mechanical equivalent, have already been described (§ 171).

227. The Thermodynamic Engine.—When a body does work against non-conservative forces, so that heat is evolved, the operations may be so regulated that all, or practically all, of the work done is transformed into heat. On the other hand, if a certain quantity of heat be present in a body, from which it may be drawn in any manner, so that it can be used for the doing of work, it is never possible, under conditions attainable on the earth's surface, even if they were ideally perfect, to transform the whole of this heat into work. The operations necessary for the transformation of some of it involve the transfer of the rest to other bodies of lower temperature.

The operation of transforming heat into work is in general very complicated; it is, however, possible to conceive of a simple operation by means of which heat may be transformed into work, and in which a relation may be found between the quantities of heat and the temperatures concerned. The relations thus developed may then be extended to far more complicated cases.

An arrangement designed to transform heat into work is called