Power, work and time

Appliances, power and energy

All electrical appliances transfer energy from one store to another, for example chemical energy in the fuel in power stations. This is transferred into kinetic energy in a fan or heat energy in a cooker.

The amount of energy transferred depends on the power (the energy transferred each second) and the amount of time the appliance is switched on for. The energy transferred by an appliance can be calculated using the equation:

energy = power × time

\text{E} = \text P \times \text t

This is when:

  • energy ( \text{E}) is measured in joules (J)
  • power ( \text{P}) is measured in watts (W)
  • time ( \text{t}) is measured in seconds (s)
curriculum-key-fact
One watt is the power when one joule of energy is transferred in one second.

The equation above is often written with power as the subject.

\text{power} = \frac{\text{energy}}{\text{time}}

\text{P} = \frac{\text{E}}{\text{t}}

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Time should be converted from minutes into seconds - this is done by multiplying the number of minutes by 60.

Example

How much energy is transferred by a 1,500 W hairdryer in 15 minutes?

\text{E} = \text{P} \times \text{t}

= 1,500 × (15 × 60)

= 1,500 × (900)

\text{E} = 1,350,000 J or 1.35 mega joules (MJ)

Power can also be calculated using the equation:

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

\text{P} = \frac{\text{W}}{\text{t}}

This is when:

  • work done ( \text{W}) is measured in joules (J)
  • power ( \text{P}) is measured in watts (W)
  • time ( \text{t}) is measured in seconds (s)