The amount of energy an appliance transfers depends on how long the…

The idea

The energy transferred is the power multiplied by the time, E = P t, so the two factors matter equally: a high power appliance used briefly can transfer more energy than a low power one left on for hours. A 2000 W kettle boiling for 180 s transfers 2000 × 180 = 360 000 J. A 10 W phone charger left on for five hours, 18 000 s, transfers only 10 × 18 000 = 180 000 J, half as much. To use the equation the power must be in watts and the time in seconds, so convert kilowatts and minutes first. Energy bills use the kilowatt-hour, which is power in kW multiplied by time in hours, but every exam calculation works in joules unless you are told otherwise.

What earns the marks

• Selecting E = P t.
• Converting the time to seconds and the power to watts before substituting.
• Correct substitution of both values.
• The energy transferred with the unit joules, J.

Common mistakes and how to put them right

• Leaving the time in minutes or hours, so the answer is 60 or 3600 times too small.

• Leaving the power in kilowatts while reporting an answer in joules.

• Assuming the appliance with the larger power rating always transfers more energy whatever the running time.

• Adding power and time instead of multiplying them.

Exam tips

• Underline the time in the question and convert it to seconds before writing anything else.
• When comparing two appliances, calculate both energies in full rather than arguing from the power ratings.
• Write large answers in standard form, or with a space every three digits, so the magnitude is readable.