Published by Australian Academy of Science
Wind power gathers speed Box 3 | Energy basics

There are two different sorts of energy:

• stored energy (which we call potential energy); and
• energy of motion (which is also called kinetic energy, from the Greek word kineo, to move).

Potential energy

Potential energy can have many forms. The wound-up spring of a toy has potential energy that can be converted into energy of motion when the toy is set running. Water in a high dam similarly has potential energy that can be converted into energy of motion when we open the sluice gates of the dam and the water streams down.

One of the important things about energy is that it can be used to do things. In formal terms we call this 'useful work', but it might not be useful and it might be play rather than work! Simply making something move might be useful work, but so could be stirring liquids, lifting loads or heating foods.

Energy comes in so many forms that it is worthwhile, first of all, to take stock of these. Energy can be stored in many ways. One way is energy of position. We have already mentioned the energy of water stored in a high dam, and there is a similar energy of position stored in any heavy object that has been lifted up. The heavier the object and the higher it is lifted, the more energy of position it has, and the more useful work it could do if it was allowed to fall.

Another very important way in which energy can be stored is as chemical energy. Petrol clearly has stored energy because it can be used to run a car, and electric batteries similarly have stored energy because they can be used to run many things.

Energy of motion

Heat is a special sort of energy which is actually a form of energy of motion, but we can't see any movement because it is the random motion of atoms. When atoms receive energy they move more rapidly – moving around in different directions in a gas or a liquid, or simply vibrating in one position in a solid. Although we can't see anything happening, we have nerve cell endings that detect this heat energy and tell us whether something is hot or cold.

Light energy is rather different from heat energy, though it is given out by things that are very hot. Boiling water is 100°C, a red-hot electric radiator is about 1000°C, a light bulb filament is about 3500°C and the surface of the sun is about 6000°C. (Some things however, such as the phosphors in fluorescent lights or in the screens of television sets, give out light by electrical processes at room temperature.)

Conversion of energy

One form of energy can be readily converted to another. The elastic energy of a wound spring can be converted to energy of motion of a toy. The chemical energy of petrol can be converted to the energy of motion of a car, and also to heat energy in the engine. The chemical energy in an electric battery can be converted into electrical energy and then into light (in a torch) or into motion (in a toy). It can even be converted into sound energy (in a radio or CD player).

Conservation of energy

One of the most important things recognised by scientists about 100 years ago was that energy is never actually created or destroyed. It simply changes from one form to another or moves from one place to another. But all along this chain of conversion there is 'waste' energy that appears as heat.

It is important to realise that heat energy is different from temperature. In fact the amount of heat energy in something depends on its mass as well as its temperature. There is more total heat energy in the lukewarm water of a large bath than in the small amount of boiling water inside a kettle.

Other boxes

Box 1. What causes the wind?

Box 2. The environmental credentials of wind power

Box 4. The power of the winds