Name the main forms of energy, describe how energy is transferred and changed from one form to another, and apply the idea that energy is never destroyed

What this dot point is asking

This dot point wants you to name the main forms that energy can take, describe how energy is transferred and changed from one form into another, and use the important rule that energy is never created or destroyed. The big idea is that energy is what makes things happen, it comes in several forms, and whenever something happens, energy simply changes from one form into another. The total amount of energy always stays the same.

The answer

What energy is

Energy is what is needed to make things happen. Anything that moves, heats up, lights up or makes a sound is using energy. Energy is measured in a unit called the joule (J).

The main forms of energy

Energy comes in several forms. The ones you need to know are:

• Kinetic energy: the energy of anything that is moving, such as a running person or a moving car.
• Potential energy: stored energy. Gravitational potential energy is stored when something is lifted up high; elastic potential energy is stored when something is stretched or squashed, like a stretched rubber band.
• Chemical energy: energy stored in fuels, food and batteries, ready to be released.
• Heat (thermal) energy: the energy of a hot object.
• Light energy: the energy given out by the Sun, a lamp or a flame.
• Sound energy: the energy carried by sounds.
• Electrical energy: the energy carried by an electric current.

Energy transfers and changes

Energy does not stay still. It is transferred (moved) from place to place, and it is changed from one form into another. This happens all the time.

For example, when you switch on a lamp, electrical energy is changed into light energy (and some heat). When you eat food and then run, chemical energy in the food is changed into kinetic energy in your moving body (and heat, which is why you get warm). Almost every event is really an energy change.

A useful way to show an energy change is an energy chain, written with arrows. For a torch it is: chemical energy (battery) goes to electrical energy goes to light energy (and heat).

Useful and wasted energy

When energy changes form, you usually get a useful form plus some wasted energy. The wasted energy is almost always heat, and sometimes sound. For example, a light bulb gives out useful light but also wastes some energy as heat, which is why a bulb feels warm. The wasted energy is not destroyed; it just spreads out and is no longer useful.

Energy is never destroyed

The most important rule about energy is the conservation of energy: energy can never be created or destroyed, only changed from one form into another. So the total amount of energy before a change is always equal to the total amount after it. When energy seems to disappear, it has really turned into heat or sound that has spread out.

Examples in context

Example 1. A mobile phone charging and ringing. When you charge a phone, electrical energy from the wall is changed into chemical energy stored in the battery. When the phone later rings, that chemical energy is changed into sound and light energy (the ringtone and the screen), with some wasted as heat, which is why a busy phone feels warm.

Example 2. A roller coaster. At the top of the first big hill, the cars have a lot of stored gravitational potential energy. As they rush down, this is changed into kinetic energy, so they go fastest at the bottom. Going back up the next hill changes kinetic energy back into potential energy. A little is wasted as heat and sound, so each hill is lower than the one before.

Try this

• Cue. Name the energy change that happens in a loudspeaker. A loudspeaker changes electrical energy into sound energy (with a little heat wasted).

• Cue. A stretched catapult fires a stone. Name the stored energy in the stretched catapult and the energy the stone has as it flies. The stretched catapult stores elastic potential energy, and the flying stone has kinetic energy.

• Cue. Explain what happens to the energy when a moving car brakes and stops. The car's kinetic energy is changed into heat (and a little sound) at the brakes; the energy is not destroyed, it spreads out into the surroundings.