Energy Stores and Transfers

Energy is not a substance and it does not "get used up". It sits in stores, and it moves between them along pathways. Describing a change means naming the store it left and the store it arrived in.

The energy stores:

Chemical — held in the bonds of fuels, food and batteries.

Kinetic — held by anything that is moving.

Gravitational potential — held by a mass raised above the ground.

Elastic (strain) — held by a stretched or compressed spring, rubber band or spring-loaded toy.

Thermal (internal) — the total kinetic and potential energy of the particles in an object. A hot object has more of it than a cold one of the same material and mass.

Nuclear — held in the nuclei of atoms, released in fission and fusion.

Electrostatic — held by separated charges.

Magnetic — held by magnets in a field.

The transfer pathways — the four ways energy moves between stores:

• Mechanically — a force moves through a distance. Pushing, pulling, stretching.
• Electrically — a current flows through a component.
• By heating — energy moves because of a temperature difference.
• By radiation — energy travels as waves, such as light, infrared or sound.

Naming a change properly. "The energy turns into movement" earns nothing. "Energy is transferred from the chemical store of the fuel to the kinetic store of the car, with some to the thermal store of the surroundings" earns the marks, because it names both ends of each transfer.

Example: a torch. Chemical store of the battery → transferred electrically → thermal and light output from the bulb. A falling ball: gravitational potential store → kinetic store, transferred mechanically by the force of gravity.

Dissipation — energy spreading out into the thermal store of the surroundings, where it is too spread out to be useful. This is what people mean by "wasted" energy. It has not been destroyed; it has become useless, which is a different and more interesting thing.