Power, work and time

Appliances, power and energy

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

The amount of energy transferred depends on the (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)

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

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

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

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)

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Mains electricity - AQA SynergyPower, work and time