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Power is the rate of transfer of energy between energy stores.

Energy transferred

The energy transferred can be calculated using the equation:

energy transferred = power × time

This is when:

• energy is measured in joules (J)
• power is measured in watts (W)
• time is measured in seconds (s)

Example

A 50 W lamp is switched on for 60 s. Calculate the energy transferred.

energy transferred = power × time

= 50 × 60

= 3,000 J (or 3 kJ)

The energy transferred can be calculated using the same equation but with different units:

• energy is measured in kilowatt-hours (kWh)
• power is measured in kilowatts (kW)
• time is measured in hours (h)

Example

A 500 W television set is switched on for 4 hours. Calculate the energy transferred.

500 W = 500/1000 = 0.5 kW

energy transferred = power × time

= 0.5 × 4

= 2 kWh

Power, potential difference and current

Electrical power can be calculated using the equation:

power = potential difference × current

This is when:

• power is measured in watts (W)
• potential difference is measured in volts (V)
• current is measured in amps (A)

Example

A current flows through a 2.0 A electric drill at a potential difference of 230 V. Calculate the power dissipated.

power = potential difference × current

= 230 × 2.0

= 460 W

Power, current and resistance

Electrical power can also be calculated using the equation:

power = (current)² × resistance

This is when:

Example

power = (current)² × resistance

= 3² × 10

= 9 × 10

= 90 W