7.7 - Conservation of Energy

The principle of conservation of energy

The principle of conservation of energy states:

Energy can be stored, transferred between stores, or dissipated, but it can never be created or destroyed.

The total amount of energy never changes: the total energy before a process always equals the total energy after it.

Dissipation and wasted energy

• Whenever a transfer happens, some energy is always dissipated: it spreads out into the surroundings and ends up in less useful stores.
• Dissipated energy is often called wasted energy, because it's no longer stored in a way you can usefully use.
• It almost always ends up in thermal energy stores, warming the device and the surrounding air.
• No transfer is perfectly efficient - Some energy is always wasted.
• Wasted energy isn't destroyed - It has simply spread out into the surroundings, making it very difficult to recover and use again.

Example: when a motor lifts a load, most of the input energy transfers usefully into the load's gravitational potential energy store, but some warms the motor and surrounding air as dissipated energy.