Energy is the ability to do work. When we do work, we use energy. Three ideas are closely related here: force, energy and work.

Force is a push or a pull. A force acting on a body may cause the body to move faster, move slower or stop moving.

We need to use energy to apply a force, and to do work. Energy and work are measured in a unit called the joule (J).

Here are three very important statements about energy.

• There are many different forms of energy.
• Energy can be converted from one form to another.
• Energy cannot be created or destroyed.

Some of the different forms of energy are;

• Mechanical energy (including kinetic energy and potential energy)
• Chemical energy
• Electrical energy
• Light
• Heat (which is sometimes called thermal energy)
• Sound.

Machines that convert energy from one form to another are called transducers. Motor car engines, steam engines, generators and electric motors are all examples of transducers.

• Motor car engines convert chemical energy (fuel) into heat energy (by burning the fuel), and then into mechanical energy.

• ‘Steam engines' also convert chemical energy into heat energy, but they work in a different way. Steam engines burn substances (usually coal or wood) and use the heat to boil water. The steam of the boiling water is used to push pistons and make wheels turn. This is then mechanical energy.

• A generator is a machine that converts mechanical energy into electrical energy.

• An electric motor is a machine that converts electrical energy into mechanical energy. It could be said that an electric motor is a generator 'working backwards'.

A very important point about converting energy from one form to another is that we cannot make any conversions without wasting some energy. In other words, we cannot convert energy from one form into any other with 100% efficiency. The 'wasted' or 'lost' energy usually escapes as heat. Think, for example, of a motor car using fuel (chemical energy) to produce mechanical energy. The engine gets hot while it is doing this conversion. The heat escapes into the environment, and is ‘lost'. We cannot use it.