Principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant, though it may take different forms (e.g., kinetic energy , potential energy , thermal energy , energy in an electric current , or energy stored in an electric field , in a magnetic field , or in chemical bonds [see bonding ]).
With the advent of relativity physics in 1905, mass was recognized as equivalent to energy. When accounting for a system of high-speed particles whose mass increases as a consequence of their speed, the laws of conservation of energy and conservation of mass become one conservation law . See also Hermann von Helmholtz .