the conservation of energy is a physical Principe according to which the total energy of an isolated system is Invariant E during time.

In the simple systems of the Newtonian Mechanical , the sum of the kinetic energies and potential energies is a constant.

In Physique of the particles, the law of conservation of energy imposes that at the time of a nuclear Reaction or a Désintégration, the sum of energies of the starting particles is the same one as the sum of energies of the emitted particles.

The application of this principle allowed the discovery of the Photon in 1909.

Bond with thermodynamics

The First principle of thermodynamics reflects the principle of conservation of energy: $\backslash \; Delta\; U\; =\; Q\; +\; W\; \backslash ,$

The internal variation of energy of a system is equal to heat and work which it exchanges with the external medium. For an isolated system, one a: $\backslash \; Delta\; U\; =\; Q\; +\; W=\; 0\; \backslash ,$ thus $Q\; =-W\; \backslash ,$