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.

Calculating power

The equation used to calculate the power is:

\[power = \frac {work \\ done}{time \\ taken}\]

\[P = \frac {E}{t}\]

This is when:

  • power (P) is measured in watts (W)
  • work done (E) 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 through a vertical height of 10 m.

Motor one does this in 5 seconds.

Motor two does this in 10 seconds.

An electric motor rests on a table lifting a weight. The motor is connected to a Joulemeter which in turn is connected to a low voltage supply.

For both motors, the energy transferred - the work done - is 20 J.

\[E = F \times d = 2 \times 10 = 20 \\ J\]

For motor one:

\[P = \frac {E}{t} = \frac {20}{5} = 4 \\ W\]

For motor two:

\[P = \frac {E}{t} = \frac {20}{10} = 2 \\ W\]

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.


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

\[P = \frac {E}{t}\]

\[P = \frac {48,000}{60}\]

\[P = 800 \\ W\]

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