Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall that one watt is equal to one joule per second, J/s
Full specification wording
Topic 8 – Energy – forces doing work: 8.14 Recall that one watt is equal to one joule per second, J/s
Power measures how quickly energy is transferred, so the unit of power is defined by combining the unit of energy with the unit of time.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 14 of 15
Study notes
From the checked teaching notes for this specification point.
The idea
Power measures how quickly energy is transferred, so the unit of power is defined by combining the unit of energy with the unit of time. One watt is the power of a device that transfers one joule of energy each second, which is why 1 W = 1 J/s. To convert, divide a quantity in joules by the time in seconds to obtain watts, or multiply a power in watts by a time in seconds to recover energy in joules.
For example: A lamp transferring 60 J in 3 s has a power of 60 J ÷ 3 s = 20 W. A 500 W heater running for 4 s transfers 500 W × 4 s = 2000 J. Recognise that J/s and W are interchangeable, and that a larger power means the same energy is transferred in less time.
What earns the marks
- States that one watt equals one joule per second, written 1 W = 1 J/s.
- Divides energy in joules by time in seconds to obtain power in watts.
- Multiplies power in watts by time in seconds to obtain energy in joules.
- Explains that a higher power transfers the same energy in a shorter time.
Common mistakes and how to put them right
Multiplies joules by seconds when power is required.
Put it right: Divide energy by time instead.
Treats watts and joules as the same unit.
Put it right: Keep watts for power and joules for energy.
Uses minutes without converting to seconds.
Put it right: Convert time to seconds before dividing.
Exam tips
- Write the unit relationship as 1 W = 1 J/s before substituting numbers.
- Check that time is in seconds, converting minutes by multiplying by 60.
- Sanity-check the answer: a small energy over a long time gives a small power.
Try the lesson
The start of the Topic 8 – Energy – forces doing work: 8.14 Recall that one watt is equal to one joule per second, J/s lesson, made from checked teaching notes for this specification point.
Have a go first
A fan transfers 150 J of energy each second. What is its power?
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