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.

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When work is done on an object, energy is transferred. The rate at which this energy is transferred is called power. So the more powerful a device is, the more energy it will transfer each second.

The equation used to calculate the power is:

This is when:

- power (
*P*) is measured in watts (W) - work done (
*W*) is measured in joules (J) - time (
*t*) is measured in seconds (s)

One watt is equal to one joule per second (J/s). This means that for every extra joule that is transferred per second, the power increases by one watt.

Two electric motors are used to lift a 2 N weight at a constant speed through a vertical height of 10 m.

Motor one does this in 5 seconds.

Motor two does this in 10 seconds.

For both motors:

For motor one:

For motor two:

Since twice as much energy is transferred by motor one each second, it is possible to say that motor one is twice as powerful as motor two.

- Question
A hairdryer transfers 48,000 J of energy in one minute. What is the power rating of the hairdryer?

Power is the amount of energy that is transferred per second.