First Law of Thermodynamics

The first law of thermodynamics is the principle of conservation of energy: Energy can be neither created or destroyed during a process; it can only change forms.

The first law cannot be proven mathematically, but no process in nature is known to have violated the first law, and this should be taken as sufficient proof.

See:

• First Law of Thermodynamics for Closed Systems
• First Law of Thermodynamics for Open Systems