OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Newton's laws: recall and apply: power (W) = work done (J) / time (s)
Power measures how quickly work is done, so a machine that transfers 600 J in 3 s has a power of 600 J ÷ 3 s = 200 W. The equation power (W) = work done (J) / time (s) links three quantities: work…
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 4 of 20
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Power measures how quickly work is done, so a machine that transfers 600 J in 3 s has a power of 600 J ÷ 3 s = 200 W. The equation power (W) = work done (J) / time (s) links three quantities: work done in joules, time in seconds and power in watts, where 1 W = 1 J/s. To apply it, identify the work done and the time taken, divide, and give the answer in watts. To rearrange, multiply power by time to find work done, or divide work done by power to find time. Always convert milliseconds or minutes to seconds before dividing, and check that a larger power for the same work means a shorter time.
What earns the marks
- Selects the relationship power = work done ÷ time and substitutes the work done in joules and the time in seconds.
- Divides work done by time and states the result in watts, recognising that 1 W equals 1 J/s.
- Rearranges the equation to work done = power × time when the question supplies power and time.
- Rearranges the equation to time = work done ÷ power when the question supplies work done and power.
- Converts a time given in minutes or milliseconds into seconds before substituting it into the equation.
Common mistakes and how to put them right
Multiplies work done by time instead of dividing.
Put it right: Divide work done by time to obtain power.
Leaves a time in minutes without converting.
Put it right: Multiply minutes by 60 to obtain seconds first.
Reports power in joules.
Put it right: State power in watts because a watt is one joule per second.
Confuses power with energy transferred.
Put it right: Treat power as the rate of transferring energy or doing work.
Exam tips
- Write the equation, then substitute numbers with units before calculating so the unit of the answer is clear.
- Convert every time to seconds at the start, since the equation requires seconds.
- Sanity-check the answer: doubling the time for the same work should halve the power.
Try the lesson
The start of the Newton's laws: recall and apply: power (W) = work done (J) / time (s) lesson, made from checked teaching notes for this specification point.
Have a go first
A machine transfers 240 J of energy as work in 8 s. What is its power?
Listen
Power measures how quickly work is done, so a machine that transfers 600 J in 3 s has a power of 600 J ÷ 3 s = 200 W. The equation power (W) = work done (J) / time (s) links three quantities: work done in joules, time in seconds and power in watts, where 1 W = 1 J/s. To apply it, identify the work done and the time taken, divide, and give the answer in watts. To rearrange, multiply power by time to find work done, or divide work done by power to find time. Always convert milliseconds or minutes to seconds before dividing, and check that a larger power for the same work means a shorter time. A strong answer earns marks for these points. Selects the relationship power = work done ÷ time and substitutes the work done in joules and the time in seconds. Divides work done by time and states the result in watts, recognising that 1 W equals 1 J/s. Rearranges the equation to work done = power × time when the question supplies power and time. Rearranges the equation to time = work done ÷ power when the question supplies work done and power. Exam tip: Write the equation, then substitute numbers with units before calculating so the unit of the answer is clear.
See it
Key words
Say what each word means before you turn the card.
Power
Try answering aloud or in your head before turning the card.
Equation for power
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Unit relationship
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Rearranging the equation
Try answering aloud or in your head before turning the card.
See every definition
- Power
- Measures how quickly work is done, given by work done in joules divided by time in seconds, in watts.
- Equation for power
- Power (W) = work done (J) / time (s), linking work done in joules, time in seconds and power in watts.
- Unit relationship
- One watt equals one joule per second, so power is stated in watts rather than joules.
- Rearranging the equation
- Multiply power by time to find work done, or divide work done by power to find time.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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