Glossary — Energy

Closed system — a system that no energy can enter or leave. In a closed system the total energy stored does not change.

Conservation of energy — the principle that energy is never created and never destroyed; it is only transferred between stores.

Dissipation — the transfer of energy to the surroundings in a way that is not useful, usually to the thermal store of the surroundings.

Efficiency — the useful output energy transfer divided by the total input energy transfer. It has no unit and can be written as a decimal or a percentage.

Elastic potential store — the energy stored in an object that is stretched or squashed, such as a spring.

Energy store — a way in which energy is held in a system, such as the kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, magnetic or electrostatic store.

Energy transfer — the movement of energy from one store to another, by heating, by work done by a force, or by work done when a current flows.

Gravitational potential store — the energy an object has because of its height above the ground.

Kinetic store — the energy an object has because it is moving.

Lubrication — putting oil or another slippery substance between moving surfaces to reduce friction and the energy transferred to the thermal store.

Non-renewable resource — an energy resource that is used up faster than it is replaced, such as coal, oil, gas or nuclear fuel.

Power — the rate at which energy is transferred, measured in watts. One watt is one joule per second.

Renewable resource — an energy resource that is replaced as it is used, so it will not run out, such as wind, hydroelectricity or the Sun.

Specific heat capacity — the energy needed to raise the temperature of one kilogram of a substance by one degree Celsius.

System — an object, or a group of objects, chosen for study.

Thermal conductivity — a measure of how quickly a material transfers energy by conduction. A material with a higher thermal conductivity transfers energy faster.

Thermal store — the energy held by the particles of a warm object because of their motion.