Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall and use the equation: power (watt, W) = work done (joule, J) ÷ time…
Full specification wording
Topic 8 – Energy – forces doing work: 8.13 Recall and use the equation: power (watt, W) = work done (joule, J) ÷ time taken (second, s)
The equation P = W ÷ t links power in watts (W), work done in joules (J) and time in seconds (s).
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 13 of 15
Study notes
From the checked teaching notes for this specification point.
The idea
The equation P = W ÷ t links power in watts (W), work done in joules (J) and time in seconds (s). Work done equals the energy transferred when a force moves an object, so a motor that does 600 J of work in 4 s has a power of 600 J ÷ 4 s = 150 W. Rearranged, W = P × t and t = W ÷ P, which lets you find work done from a known power and time, or the time needed for a given power to do a set amount of work. Always convert time to seconds first: 2 minutes is 120 s, so 2400 J ÷ 120 s = 20 W. Check units at the end, because joules divided by seconds must give watts.
What earns the marks
- Recalls the equation power equals work done divided by time taken.
- Substitutes work done in joules and time in seconds into the equation to calculate power in watts.
- Rearranges the equation to find work done as power multiplied by time.
- Rearranges the equation to find time taken as work done divided by power.
- Converts a time given in minutes into seconds before substituting into the equation.
Common mistakes and how to put them right
Divides time by work done instead of work done by time.
Put it right: Divide the joules by the seconds to obtain watts.
Substitutes time in minutes without converting.
Put it right: Multiply minutes by 60 to obtain seconds first.
Mixes units and reports the answer in joules.
Put it right: Report power in watts after dividing joules by seconds.
Multiplies power by time when asked for power.
Put it right: Use division for power and multiplication only when finding work done.
Exam tips
- Write the equation, then the rearranged form you need, before substituting numbers.
- Convert every time to seconds and check that the final unit is the watt.
- Estimate the answer first, so a result far from your estimate signals a rearrangement error.
- Carry the unit through the calculation to confirm joules divided by seconds gives watts.
Try the lesson
The start of the Topic 8 – Energy – forces doing work: 8.13 Recall and use the equation: power (watt, W) = work done (joule, J) ÷ time taken (second, s) lesson, made from checked teaching notes for this specification point.
Have a go first
A machine does 200 J of work in 4 s. What is its power?
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