AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy transfers in everyday appliances: energy transferred = power × time
Everyday appliances transfer energy from one store to another, and the amount transferred depends on two things: how quickly energy is transferred (the power) and how long the appliance is…
Study notes
From the checked teaching notes for this specification point.
The idea
Everyday appliances transfer energy from one store to another, and the amount transferred depends on two things: how quickly energy is transferred (the power) and how long the appliance is switched on (the time). Power is the rate of energy transfer, measured in watts (W), where 1 W means 1 joule per second. Time is measured in seconds (s). Multiplying power by time therefore gives energy transferred in joules (J). For example, a 2000 W kettle switched on for 120 s transfers 2000 × 120 = 240 000 J, which is 240 kJ. If time is given in minutes, convert to seconds first by multiplying by 60. This relationship lets you compare appliances: a low-power device left on for a long time can transfer more energy than a high-power device used briefly.
What earns the marks
- State that power is the rate of energy transfer, measured in watts, where 1 W = 1 J/s.
- Identify the correct values of power in watts and time in seconds from the question before substituting.
- Substitute into energy transferred = power × time and evaluate correctly, including powers of ten.
- Convert a time given in minutes or hours into seconds before multiplying.
- Give the final answer in joules, or convert to kilojoules by dividing by 1000 where appropriate.
- Interpret the result in context, for example linking a larger energy transfer to a longer operating time or a higher power rating.
Common mistakes and how to put them right
Using time in minutes instead of seconds.
Put it right by multiplying minutes by 60 before substituting.
Confusing power with energy.
Put it right by remembering that energy in joules is power multiplied by time.
Mixing units, such as kilowatts with seconds.
Put it right by converting kilowatts to watts first.
Exam tips
- Write the equation, then substitute values with units before calculating so unit errors are easy to spot.
- Check whether the time is in seconds; if not, convert it as your first step and show that conversion.
- Sanity-check the size of your answer: a 1 kW appliance for 1 minute should give about 60 000 J, not 60 J or 60 000 000 J.
Try the lesson
The start of the Energy transfers in everyday appliances: energy transferred = power × time lesson, made from checked teaching notes for this specification point.
Have a go first
A 2000 W kettle is switched on for 30 s. How much energy is transferred to the kettle?
Listen
Everyday appliances transfer energy from one store to another, and the amount transferred depends on two things: how quickly energy is transferred (the power) and how long the appliance is switched on (the time). Power is the rate of energy transfer, measured in watts (W), where 1 W means 1 joule per second. Time is measured in seconds (s). Multiplying power by time therefore gives energy transferred in joules (J). For example, a 2000 W kettle switched on for 120 s transfers 2000 × 120 = 240 000 J, which is 240 kJ. If time is given in minutes, convert to seconds first by multiplying by 60. This relationship lets you compare appliances: a low-power device left on for a long time can transfer more energy than a high-power device used briefly. A strong answer earns marks for these points. State that power is the rate of energy transfer, measured in watts, where 1 W = 1 J/s. Identify the correct values of power in watts and time in seconds from the question before substituting. Substitute into energy transferred = power × time and evaluate correctly, including powers of ten. Convert a time given in minutes or hours into seconds before multiplying. A common slip: Using time in minutes instead of seconds: the error gives an answer 60 times too small. Instead, multiplying minutes by 60 before substituting. Exam tip: Write the equation, then substitute values with units before calculating so unit errors are easy to spot.
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