Power: power = energy transferred / time

The idea

This equation lets you calculate power from the energy transferred and the time taken. Power is measured in watts, energy transferred in joules and time in seconds, so 1 W = 1 J/s. To use it, identify the energy transferred and the time, convert time to seconds if needed, then divide energy by time. For example, a heater transferring 36 000 J in 60 s has power 36 000 J ÷ 60 s = 600 W. Rearranged, energy transferred = power × time and time = energy transferred ÷ power. The same relationship applies when work done is used instead of energy transferred, because work done and energy transferred are equivalent in this context. Always check that time is in seconds and energy is in joules before dividing.

What earns the marks

• Selects the equation power = energy transferred ÷ time and substitutes the correct values.
• Converts time to seconds and energy to joules before dividing, for example 3 minutes becomes 180 s.
• Calculates power correctly, such as 36 000 J ÷ 60 s = 600 W.
• Rearranges the equation to find energy transferred or time when those are the unknown quantities.
• States the unit of the answer as W and checks that the value is reasonable for the device described.

Common mistakes and how to put them right

• Dividing time by energy instead of energy by time.

  Put it right by writing the equation with energy on top and time underneath.

• Using minutes or hours directly in the calculation.

  Put it right by converting all times to seconds first.

• Forgetting to include the unit W with the final answer.

  Put it right by writing the unit after every calculated power.

Exam tips

• Write down the equation, then substitute values with units before doing the arithmetic.
• Convert minutes to seconds by multiplying by 60, and check the conversion before calculating.
• If asked for energy or time, rearrange the equation first and then substitute to reduce errors.