What is energy?

Jonny Nelson introduces an animated explanation of electromagnetism

Energy can be stored or transferred but it cannot be used up. For example, energy can be stored in the chemical bonds of diesel oil. It can be transferred when the diesel oil burns in a car engine, allowing the car to move and accelerate, or reach a certain distance. Where there are energy transfers in a system, the total energy associated with the system stays the same. Energy is measured in joules (J).

Energy can be transferred usefully, stored or dissipated, but it cannot be created or destroyed.

Energy stores

There are eight energy stores which are used to keep track of energy in a system.

Energy storeDescriptionExamples
MagneticThe energy stored when like poles are pushed closer together or when unlike poles are pulled further apart.Fridge magnets, compasses and maglev trains which use magnetic levitation.
ThermalIn most cases this is the vibrations of the particles in the object. In hotter objects, the particles vibrate faster.The human bodies, hot coffees, stoves or hobs. Also: ice particles vibrate slower, but still have energy.
ChemicalThe energy store associated with chemical bonds, such as those between molecules.Food, muscles, electrical cells.
Kinetic The energy associated with a moving object.Runners, busses, comets.
ElectrostaticThe energy stored when like charges are moved closer together or when unlike charges are pulled further apart.Thunderclouds, Van De Graaff generators.
Elastic The energy stored when an object is stretched, squashed or twisted.Drawn catapults, compressed springs, inflated balloons.
Gravitational The energy associated with an object at height.Aeroplanes, kites, mugs on a table.
NuclearThe energy associated with nuclear interactions.Nuclear fusion of hydrogen and helium in the Sun, Uranium-245 undergoing fission.

A store is not a physical place, and energy is not a physical substance. The idea of an energy store is to allow calculations to be made and to state how many joules are available for an action to happen. Each of the stores has an equation associated with it.