What is the name for the principle that states the total energy of an isolated system remains constant?

The principle stating that the total energy of an isolated system remains constant is the conservation of energy. Energy may move between parts of the system or change form, but the total amount does not change.

For example, a falling object converts gravitational potential energy into kinetic energy. Friction can transform some mechanical energy into thermal energy, while a battery converts chemical energy into electrical energy. These changes do not destroy energy; they redistribute it among different forms.

The wording “isolated system” matters. An isolated system exchanges neither matter nor energy with its surroundings. A closed system may exchange energy as heat or work, even though it does not exchange matter, so its internal energy can change. Conservation of mass is a separate classical principle, while modern physics treats mass and energy as related through Einstein’s E = mc².

A deeper mathematical explanation comes from Emmy Noether’s theorem: energy conservation follows from the fact that the laws of physics are unchanged by shifts in time. The principle also rules out a perpetual-motion machine that creates usable energy without an external supply.