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What is conservation of energy?

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred between stores.

Conservation of energy explained

Energy questions become bookkeeping: list the stores before and after, and account for every joule, including the energy dissipated to the surroundings.
Efficiency compares useful output energy with total input energy, and no real device reaches 100% because some energy always dissipates as heat.

Key formula

efficiency=useful output energytotal input energy×100%\text{efficiency} = \dfrac{\text{useful output energy}}{\text{total input energy}} \times 100\%

Worked example

A ball dropped from 2 m2\,\text{m} converts mghmgh into kinetic energy, reaching about 6.3 m s−16.3\,\text{m s}^{-1} if air resistance is ignored.

Examiner tip

Say 'dissipated to the surroundings by heating', not 'lost'. Examiners reject 'energy is lost' because it contradicts the principle.

Conservation of energy: common questions

What is a closed system?
A system with no net transfer of energy or matter across its boundary, so the total energy inside stays constant.
Why is no machine 100% efficient?
Friction and electrical resistance always transfer some energy to thermal stores in the surroundings.

Related terms

Kinetic energy

Kinetic energy is the energy an object has because of its motion, equal to half its mass times its velocity squared.

Work done

Work done is the energy transferred when a force moves an object through a distance in the direction of the force.

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