ABSTRACT
The first law of thermodynamics is the law of conservation of energy. It is stated in various forms, perhaps the most important of which are: although energy may be converted from one form to another, it cannot be created or destroyed; and whenever a quantity of one kind of energy is produced, an exactly equivalent amount of another kind (or other kinds) must be used up. The internal energy of the system under consideration must depend on its thermodynamic state. This is an evidence. In other words, the energy must be described by a single-valued function of the thermodynamic variables of the system. The first law requires immediately that the total energy of an isolated system is constant, although there may be changes from one form of energy to another in it. The isolated system may be considered, indeed, as being constituted by the system under study itself and by its surroundings.
