AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy transfers in everyday appliances: the energy transferred over a given time.
Once the power of an appliance is known, the total energy transferred can be found by multiplying that power by the time the appliance is switched on.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Once the power of an appliance is known, the total energy transferred can be found by multiplying that power by the time the appliance is switched on. Power is the rate of energy transfer, so a 2000 W kettle running for 120 s transfers 2000 × 120 = 240 000 J, or 240 kJ. The same relationship applies to any appliance, from a phone charger to an electric shower, and links directly to the potential difference and current through the device.
What earns the marks
- States the relationship energy transferred = power × time, written as E = P × t.
- Uses consistent units, converting time to seconds when power is in watts so that energy is in joules.
- Rearranges the equation to calculate power when energy and time are given, or time when energy and power are given.
- Converts between watts and kilowatts, and between minutes or hours and seconds, where an everyday energy context requires it.
- Applies the equation to a named appliance, for example calculating the energy transferred by a 2000 W kettle in 120 s as 240 000 J.
- Links the calculation to the appliance's potential difference and current, recognising that P = V × I can be substituted into E = P × t.
Common mistakes and how to put them right
Using time in minutes or hours while power is in watts.
Put it right: Convert time to seconds first to ensure energy is calculated in joules.
Forgetting to rearrange the equation when the question asks for power or time rather than energy.
Put it right: Identify the unknown and divide or multiply accordingly before substituting.
Treating power and energy as the same quantity.
Put it right: Power is the rate of transfer in watts, while energy is the total transferred in joules.
Exam tips
- Write down the equation, substitute the values with units, then calculate; this makes unit errors easier to spot.
- If the time is given in minutes or hours, convert to seconds immediately and show the conversion in your working.
- Check that your final answer has the correct unit and a sensible magnitude for the appliance described.
Try the lesson
The start of the Energy transfers in everyday appliances: the energy transferred over a given time. lesson, made from checked teaching notes for this specification point.
Have a go first
A 60 W lamp is switched on for 20 s. How much energy is transferred by the lamp in this time?
Listen
Once the power of an appliance is known, the total energy transferred can be found by multiplying that power by the time the appliance is switched on. Power is the rate of energy transfer, so a 2000 W kettle running for 120 s transfers 2000 × 120 = 240 000 J, or 240 kJ. The same relationship applies to any appliance, from a phone charger to an electric shower, and links directly to the potential difference and current through the device. A strong answer earns marks for these points. States the relationship energy transferred = power × time, written as E = P × t. Uses consistent units, converting time to seconds when power is in watts so that energy is in joules. Rearranges the equation to calculate power when energy and time are given, or time when energy and power are given. Converts between watts and kilowatts, and between minutes or hours and seconds, where an everyday energy context requires it. A common slip: Using time in minutes or hours while power is in watts. Instead, convert time to seconds first to ensure energy is calculated in joules. Exam tip: Write down the equation, substitute the values with units, then calculate; this makes unit errors easier to spot.
Key words
Say what each word means before you turn the card.
Power
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Energy transferred
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Unit conversion
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Everyday appliance energy use
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See every definition
- Power
- The rate of energy transfer, measured in watts.
- Energy transferred
- Found by multiplying the power of an appliance by the time it is switched on.
- Unit conversion
- Changing between watts and kilowatts, and between minutes or hours and seconds, when calculating energy in joules.
- Everyday appliance energy use
- Applying the relationship between energy transferred, power and time to any appliance, from a phone charger to an electric shower.
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