Energy can be described as being in different 'stores'. It cannot be created or destroyed but it can be transferred. This is doing work. The rate of energy transfer or work done is the power.

Part of

When a force causes a body to move, work is being done on the object by the force. Work is the measure of energy transfer when a force '*F*' moves an object through a distance '*d*'.

So when work is done, energy has been transferred from one energy store to another, and so:

energy transferred = work done

Energy transferred and work done are both measured in joules (J).

The amount of work done when a force acts on a body depends on two things:

- the
**size of the force**acting on the object - the distance through which the force causes the body to move in the direction of the force

The equation used to calculate the work done is:

work done = force × distance

This is when:

- work done (
*W*) is measured in joules (J) - force (
*F*) is measured in newtons (N) - distance (
*x*) is in the same direction as the force and is measured in metres (m)

In this example, a force of 10 N causes the box to move a horizontal distance of 2 m, so:

One joule of work is done (or one joule of energy is transferred) when a force of one newton causes a body to move through a distance of one metre.

- Question
A horizontal force of 50 N causes a trolley to move a horizontal distance of 30 m. How much work is done on the trolley by the force?

- Question
12,000 J of energy is supplied to move a small truck a distance of 80 m. What is the size of the force applied?

A current will flow in a circuit when there is a potential difference applied to the circuit. The power supply (or cell or battery) gives an amount of energy to each coulomb of charge flowing.

A 6 volt cell, for example, gives 6 joules of energy to each coulomb. We can also use the word 'work' instead of the word 'energy' because:

work done = energy transferred

so

which can be rearranged to

energy = voltage × charge

can be called energy transferred or work done by the power supply.

- Question
How much energy is transferred (or work done) when 3 C of charge moves through a potential difference of 6 V across a resistor?

- Question
If the 3 C of charge flows in 6 seconds, how big is the current in the resistor?

- Question
What is the total energy dissipated in the resistor?

Note that the first answer and the last answer agree.