AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy transfers in everyday appliances: energy transferred, E, in joules, J
Energy transferred, E, measured in joules, J, is the quantity of energy moved when a device operates.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Energy transferred, E, measured in joules, J, is the quantity of energy moved when a device operates. In everyday appliances, electrical energy from the mains is transferred by the appliance into useful stores or pathways, such as thermal energy in a heater or kinetic energy in a motor. To calculate E, use the equation E = P × t, where P is power in watts and t is time in seconds. For example, a 2000 W kettle switched on for 120 s transfers E = 2000 × 120 = 240 000 J. Because 1 W = 1 J/s, multiplying watts by seconds gives joules directly. Always convert minutes to seconds before substituting, and remember that energy is conserved: the total energy transferred equals the useful energy plus any energy dissipated to the surroundings.
What earns the marks
- State that energy transferred is measured in joules, J, and that 1 J is the energy transferred by a power of 1 W acting for 1 s.
- Select and apply the equation E = P × t, substituting power in watts and time in seconds to obtain energy in joules.
- Convert time units correctly, for example 5 minutes = 300 s, before calculating energy transferred.
- Interpret everyday appliance contexts, such as kettles, lamps or motors, and identify the useful energy transfer and any wasted transfer.
- Use the conservation of energy to explain that the total energy transferred by an appliance equals the sum of useful and wasted energy transfers.
Common mistakes and how to put them right
Using time in minutes instead of seconds.
Put it right by multiplying minutes by 60 before using E = P × t.
Confusing power and energy.
Put it right by stating that power is the rate of energy transfer in watts, while energy transferred is in joules.
Forgetting to include units or writing J without showing the calculation.
Put it right by writing the equation, substitution and answer with the unit J.
Exam tips
- Write the equation E = P × t before substituting values so the examiner can see your method.
- Check that time is in seconds and power is in watts; convert first, then calculate.
- Give the unit J with your final answer and, where asked, comment on useful and wasted energy transfers.
Try the lesson
The start of the Energy transfers in everyday appliances: energy transferred, E, in joules, J lesson, made from checked teaching notes for this specification point.
Have a go first
A 60 W lamp is switched on for 10 s. Calculate the energy transferred, E, in joules.
Listen
Energy transferred, E, measured in joules, J, is the quantity of energy moved when a device operates. In everyday appliances, electrical energy from the mains is transferred by the appliance into useful stores or pathways, such as thermal energy in a heater or kinetic energy in a motor. To calculate E, use the equation E = P × t, where P is power in watts and t is time in seconds. For example, a 2000 W kettle switched on for 120 s transfers E = 2000 × 120 = 240 000 J. Because 1 W = 1 J/s, multiplying watts by seconds gives joules directly. Always convert minutes to seconds before substituting, and remember that energy is conserved: the total energy transferred equals the useful energy plus any energy dissipated to the surroundings. A strong answer earns marks for these points. State that energy transferred is measured in joules, J, and that 1 J is the energy transferred by a power of 1 W acting for 1 s. Select and apply the equation E = P × t, substituting power in watts and time in seconds to obtain energy in joules. Convert time units correctly, for example 5 minutes = 300 s, before calculating energy transferred. Interpret everyday appliance contexts, such as kettles, lamps or motors, and identify the useful energy transfer and any wasted transfer. A common slip: Using time in minutes instead of seconds. Instead, multiplying minutes by 60 before using E = P × t. Exam tip: Write the equation E = P × t before substituting values so the examiner can see your method.
Key words
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Energy transferred
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Power
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Conservation of energy
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Everyday appliances
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See every definition
- Energy transferred
- The quantity of energy moved when a device operates, measured in joules, J.
- Power
- The rate of energy transfer, measured in watts, where 1 W = 1 J/s.
- Conservation of energy
- The total energy transferred equals the useful energy plus any energy dissipated to the surroundings.
- Everyday appliances
- Devices in which electrical energy from the mains is transferred into useful stores or pathways, such as thermal energy in a heater or kinetic energy in a motor.
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