Energy transfers in everyday appliances: Students should be able to describe, with examples, the relationship between the power ratings for domestic electrical appliances and the changes in stored energy when they are in use.

The idea

Every mains domestic appliance transfers energy electrically from the mains supply to other stores. Its power rating, in watts or kilowatts, tells you the energy transferred each second, because power is the rate of energy transfer: P = E ÷ t, so E = P × t. A 2000 W (2 kW) kettle switched on for 180 s transfers E = 2000 × 180 = 360 000 J = 360 kJ, mostly to the thermal store of the water. A 100 W television left on for 3 hours (10 800 s) transfers 1 080 000 J = 1.08 MJ. Higher power ratings mean faster energy transfer, so the same change in stored energy happens in less time. Energy is conserved: the energy transferred electrically equals the increase in the useful stores plus any energy dissipated to the surroundings.

What earns the marks

• State that power is the rate of energy transfer, measured in watts (W) or kilowatts (kW), where 1 kW = 1000 W.
• Use the relationship E = P × t, converting time to seconds and power to watts before multiplying, to find energy transferred in joules.
• Explain that a higher power rating transfers the same amount of energy in a shorter time, or more energy in the same time, than a lower-rated appliance.
• Give a domestic example, such as a 2 kW kettle heating water or a 10 W LED lamp, and name the stores that gain or lose energy.
• Recognise that energy is conserved, so the electrical energy transferred equals the increase in useful stores plus energy dissipated to the surroundings.
• Compare two appliances quantitatively, for example showing that a 3 kW heater transfers three times the energy of a 1 kW heater in the same time.

Common mistakes and how to put them right

• Using minutes or hours directly in E = P × t without converting to seconds.

  Put it right by multiplying hours by 3600 or minutes by 60 first.

• Confusing power (the rate of transfer, in W) with energy (the total transferred, in J).

  Put it right by checking whether the question asks for a rate or a total.

• Assuming a higher power rating always means more energy transferred.

  Put it right by noting that a low-power appliance left on for a long time can transfer more energy than a high-power one used briefly.

Exam tips

• Write the equation, substitute values with units, then give the answer with the correct unit (J or kJ) and an appropriate number of significant figures.
• When comparing appliances, calculate the energy for each using the same time interval so the comparison is fair.
• Link each calculation back to named stores, such as the thermal store of water or the kinetic store of a motor, to show understanding of the change in stored energy.