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• Energy can be described as being in different ‘stores’.

• Energy cannot be created or destroyed, but it can be transferred from one energy store to other energy stores.

• Some energy transfers are useful, but others are not.

Energy is a very important idea in physics – it allows us to work out what might happen in a system.

Energy is a helpful way to describe what has happened in a system, but it isn’t a very good way to explain why things happen. To explain why something happens, it is usually better to think about forces.

Energy can be a difficult idea to understand because we can’t see energy itself. One way we can understand energy is to use money as a model.

Money can be stored in different ways:

• in a bank account

• as bank notes in your wallet

• as coins in a piggy bank.

Money can be transferred from one store to another. Money doesn’t make things happen, but it does help to explain what might happen. For example, if you have a lot of money in your bank account, you could buy lots of expensive things.

Energy can also be stored in different stores, like the thermal store of a hot object, or the kinetic store of a moving object. The unit of energy is the joule (J).

There are many different stores of energy.

Have a look at this slideshow to explore more about different stores of energy.

It is helpful to describe how energy moves from one store to another during a process. Do this by choosing a start point (where you can identify a store of energy) and an end point.

Energy can be transferred from one store to another in four ways:

1. Mechanical work – a force is applied to move an object, for example when a person lifts a book onto a high shelf.

2. Electrical work – charges flow in the form of electricity, for example in a battery powered toy train.

3. Heat transfer – energy moves from the thermal store of a hotter object to the thermal store of a cooler object, for example when a handwarmer is used to warm up your hands.

4. Radiation - when energy is transferred as a wave, for example a grill cooking food using infra-red radiation.

When a force causes an object to move, work is being done. Work is a measure of the energy transferred when a force acts over a distance. This is often when a force moves an object, but work is also done when a force compresses or extends a spring or other flexible object.

This means that:

energy transferred = work

Work and energy are both measured in joules (J).

Work is not a store of energy - it is one of the ways in which energy can be transferred.