The First Law of Thermodynamics

Abstract

The first law of thermodynamics is a special consequence of the law of conservation of energy in physics. We state the law of conservation of energy in the following simple form: A given amount of a particular type of energy may be converted into another type, and the process may be reversed to obtain the initial type of energy without destroying or augmenting any portion of the initial amount. The historical and outmoded enunciation of the first law as “the perpetual motion of the first kind is impossible, i.e., it is impossible to construct an engine that does work without using any form of energy” is not incorrect, but it is unsatisfactory to the thermodynamicist. We enunciate the first law of thermodynamics in the following simple mathematical form:* There is a function U, called the energy of a system, which is a function of its variables of state, and the change in the value of U, ΔU, from a given initial state to a final state is always equal to the same amount of energy irrespective of the path followed by the system.