Energy and power

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 power is:

power = \frac{work~done}{time}

power = \frac{W}{t}

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 through a vertical height of 10 m.

Motor one does this in 5 seconds.

Motor two does this in 10 seconds.

For both motors:

W = F \times d = 2 \times 10 = 20~J

For motor one:

P = \frac{W}{t} = \frac{20}{5} = 4~W

For motor two:

P = \frac{W}{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 hair dryer transfers 48,000 J of energy in one minute. What is the power rating of the hairdryer?

P = \frac{W}{t}

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

P = 800~W

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