AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy transfers in everyday appliances: E = P t
This is the symbolic form of the equation linking energy transferred, power and time.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This is the symbolic form of the equation linking energy transferred, power and time. E stands for energy transferred in joules (J), P stands for power in watts (W) and t stands for time in seconds (s). Because 1 W = 1 J/s, multiplying watts by seconds gives joules. The equation can be rearranged: P = E ÷ t gives power when energy and time are known, and t = E ÷ P gives the time an appliance must run to transfer a given energy. For example, a 500 W heater running for 30 s transfers E = 500 × 30 = 15 000 J. Rearranged, a 1500 W iron transferring 90 000 J must run for t = 90 000 ÷ 1500 = 60 s. Always convert minutes to seconds and kilowatts to watts before substituting.
What earns the marks
- Identify E as energy in joules, P as power in watts and t as time in seconds.
- Substitute known values into E = P t and evaluate correctly.
- Rearrange the equation to P = E ÷ t when energy and time are given.
- Rearrange the equation to t = E ÷ P when energy and power are given.
- Convert kilowatts to watts and minutes to seconds before substituting.
- Give the answer with the correct unit for the quantity found: joules, watts or seconds.
Common mistakes and how to put them right
Rearranging incorrectly, for example writing P = E × t.
Put it right by dividing energy by time.
Forgetting to convert kilowatts to watts.
Put it right by multiplying kilowatts by 1000 before substituting.
Leaving time in minutes when finding energy.
Put it right by converting minutes to seconds.
Exam tips
- Write the equation in symbols first, then rearrange before substituting numbers to reduce algebra slips.
- Include units with every substituted value and with the final answer so the examiner can follow your reasoning.
- If the answer looks unrealistic, recheck the rearrangement and unit conversions rather than changing the numbers.
Try the lesson
The start of the Energy transfers in everyday appliances: E = P t lesson, made from checked teaching notes for this specification point.
Have a go first
A 50 W light bulb is switched on for 20 s. Calculate the energy transferred by the bulb.
Listen
This is the symbolic form of the equation linking energy transferred, power and time. E stands for energy transferred in joules (J), P stands for power in watts (W) and t stands for time in seconds (s). Because 1 W = 1 J/s, multiplying watts by seconds gives joules. The equation can be rearranged: P = E ÷ t gives power when energy and time are known, and t = E ÷ P gives the time an appliance must run to transfer a given energy. For example, a 500 W heater running for 30 s transfers E = 500 × 30 = 15 000 J. Rearranged, a 1500 W iron transferring 90 000 J must run for t = 90 000 ÷ 1500 = 60 s. Always convert minutes to seconds and kilowatts to watts before substituting. A strong answer earns marks for these points. Identify E as energy in joules, P as power in watts and t as time in seconds. Substitute known values into E = P t and evaluate correctly. Rearrange the equation to P = E ÷ t when energy and time are given. Rearrange the equation to t = E ÷ P when energy and power are given. A common slip: Rearranging incorrectly, for example writing P = E × t: the error gives a power far too large. Instead, dividing energy by time. Exam tip: Write the equation in symbols first, then rearrange before substituting numbers to reduce algebra slips.
Key words
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Energy transfer and conservation
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Power as the rate of energy transfer
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Rearranging equations
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Units and unit conversion
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See every definition
- Energy transfer and conservation
- Energy transferred is given by E in the equation E = P t, measured in joules (J).
- Power as the rate of energy transfer
- Power is given by P in the equation E = P t, measured in watts (W), where 1 W = 1 J/s.
- Rearranging equations
- The equation can be rearranged to P = E ÷ t to give power, or to t = E ÷ P to give time.
- Units and unit conversion
- Convert minutes to seconds and kilowatts to watts before substituting into the equation.
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