Power: P = E / t

The idea

Power measures how quickly energy is transferred. The equation P = E / t links power P in watts (W) to energy transferred E in joules (J) and time t in seconds (s). One watt equals one joule per second, so a 60 W lamp transfers 60 J each second. To use the equation, identify the energy transferred and the time taken, convert time to seconds and energy to joules, then divide. For example, a heater transferring 7200 J in 60 s has P = 7200 J ÷ 60 s = 120 W. Rearranged, E = P × t finds total energy and t = E / P finds time. This relationship applies to electrical devices, mechanical work and any energy transfer, making it central to efficiency and cost calculations.

What earns the marks

• State that power is the rate of energy transfer, measured in watts (W), where 1 W = 1 J/s.
• Select P = E / t and substitute the energy transferred in joules and the time in seconds correctly.
• Convert units before calculating, for example minutes to seconds or kilojoules to joules.
• Rearrange the equation to E = P × t or t = E / P when the question asks for energy or time.
• Give the answer with the correct unit, usually W, and a sensible number of significant figures.

Common mistakes and how to put them right

• Using time in minutes instead of seconds.

  Put it right: Multiply minutes by 60 before dividing.

• Confusing power with energy.

  Put it right: Power is the rate in watts, while energy is the total in joules.

• Dividing time by energy instead of energy by time.

  Put it right: Check that P = E / t gives a larger power for a shorter time.

Exam tips

• Write the equation, then substitute values with units before calculating to reduce errors.
• Check the unit asked for; if the answer is in kilowatts, divide watts by 1000.
• Use the rearranged forms E = P × t and t = E / P when the question gives power and asks for energy or time.