Energy, Law of Conservation of

one of the most fundamental laws, according to which a very important physical quantity—energy—is conserved in an isolated system. All known processes in nature, without exception, conform to the law. In an isolated system, energy can be converted from one form to another, but its amount remains constant. If the system is not isolated, its energy may change either upon a simultaneous change in energy of the bodies surrounding the system or upon a change in the energy of interaction of the system with surrounding bodies. Upon a transition of the system from one state to another, the change in energy of the system does not depend on the means (the kinds of interactions) by which the transition occurred. This is the case because energy is a single-valued function of the state of the system. The energy in a system changes when work is done and when heat is transmitted to the system.

The conservation of energy is associated with uniformity of time—that is, with the fact that all moments of time are equivalent and that physical laws do not change with time (seeSYMMETRY in physics). The law of conservation of mechanical energy was established by G. W. von Leibniz (1686), and the law of conservation of energy of nonmechanical phenomena was established by J. R. von Mayer (1845), J. P. Joule (1843–50), and H. L. von Helmholtz (1847). In thermodynamics, the law of conservation of energy is called the first law of thermodynamics (seeTHERMODYNAMICS, FIRST LAW OF).

Before Einstein created the special theory of relativity (1905), the laws of conservation of mass and energy were two independent laws. In relativity theory they were unified into the law of conservation of energy.