# Energy and power

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

## Calculating power

The equation used to calculate 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.

### Example

Two electric motors are used to lift a 2 N weight 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 hair dryer 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.