Conservation of Energy

Principle that the total energy of an isolated system stays constant; it changes form but is never created or destroyed.

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The principle was first stated in its modern form by the German surgeon Julius Robert von Mayer in 1842.
In 1843 James Prescott Joule independently discovered the mechanical equivalent in experiments using falling weights turning a paddle in water.
Noether's theorem rigorously proves energy conservation as a consequence of continuous time-translation symmetry.