What is energy?

Energy is in everything that happens. It is why a car goes, a lamp shines, a kettle boils and you can run. You cannot see it directly, but you can see what it does. It is measured in joules (J).

There are many forms of energy:

• Kinetic energy: energy of movement (a rolling ball).
• Potential energy: stored energy. Gravitational potential energy is stored in anything raised up, such as a book on a shelf. Elastic potential energy is stored in a stretched spring or rubber band.
• Thermal (heat) energy: from the movement of particles. See heat and temperature.
• Light energy and sound energy: they travel as waves.
• Electrical energy: from moving electric charges. See electricity.
• Chemical energy: stored in food, fuels and batteries.
• Nuclear energy: stored inside atoms.

The most important rule is that energy is never created or destroyed. It only transfers from one place to another or transforms into another form. This is the law of conservation of energy.

Follow the energy in a torch: chemical energy in the battery becomes electrical energy in the wires, which becomes light energy and a little heat from the bulb. When you ride a bike, the chemical energy in your food becomes kinetic energy as you move, and some becomes heat. Some energy always becomes heat that is hard to use again. We say that energy is "wasted" because it spreads out, not because it has disappeared.

Almost all the energy on Earth comes from the Sun. Plants capture it, animals eat plants, and fossil fuels such as coal and oil are the remains of ancient living things. Machines and fuels get their energy from the same source; see forces for how energy moves things. Some sources can be used again and again; read about renewable energy.