The conservation of energy states that energy cannot be created or destroyed. It can only change form. A falling rock converts potential energy into kinetic energy. A battery converts chemical energy into electrical energy. A light bulb converts electrical energy into light and heat. The total amount stays the same.

This principle is one of the pillars of physics. It was formalized in the 19th century by scientists like Joule, Mayer, and Helmholtz. It applies everywhere, from subatomic particles to galaxies. It rules out perpetual motion machines. No device can produce more energy than it takes in.

In practice, the phrase can confuse. People say "energy conservation" when they mean using less energy, as in turning off lights. That is a different idea, better called energy efficiency or energy saving. The physics principle says energy is always conserved. The practical goal is to waste less of it on unwanted heat and friction.

Key points

• Energy changes form, not amount
• Applies to all physical systems
• Rules out perpetual motion
• Distinct from energy saving
• Related to the first law of thermodynamics