Edexcel IGCSE Physics · Spec 4.4-4.5
Work Done & Power
Work done and power, and how they are related.
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Work Done & Power
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Explained
Work done, and the difference power makes
In physics, work is done whenever a force moves an object through a distance. Work done and energy transferred are the same quantity measured the same way, both in joules, and the specification treats them as interchangeable.
Work done equals force multiplied by distance moved in the direction of the force. Lifting a 20 N box 3 m does 60 J of work, and 60 J of energy has been transferred to its gravitational potential store.
In the direction of the force
That phrase is not padding. If you carry a suitcase horizontally across a room, you exert an upward force to hold it up, but the movement is sideways. The distance in the direction of that force is zero, so the work done against gravity is zero, however tired you feel.
Similarly, pushing hard against a wall that does not move does no work at all in the physics sense, because nothing has moved.
Power
Power is the rate of energy transfer, or work done per second. Power equals energy transferred divided by time taken, and the unit is the watt, where one watt is one joule per second.
Two people carrying identical loads up the same staircase do exactly the same work, because the force and the distance are the same. The one who runs up has the greater power, because the same energy was transferred in less time.
That is the distinction questions test. Work done depends only on force and distance. Power also depends on how long it took.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme works through this as a three part calculation: the gravitational potential energy gained, the mean of three timings, and then the power. The power mark is awarded for substituting into power equals energy divided by time, and it allows an error carried forward from both earlier parts.
Two details in the notes are worth copying. The mean is to be rounded to three significant figures, and that rounding is described as an independent mark, so it is credited even if the mean itself is wrong. And on the power part, a range of answers is accepted, because the examiners expect small differences from rounding along the way.
The evaluation part that follows credits four ideas about the quality of the data: that there are not enough data points to make a valid conclusion, that a greater range of masses should be tested, that values in between should be tested, and that more readings or students are needed. It then says to ignore whether the students were correct or incorrect.
That last instruction is the useful one. A question asking you to evaluate a conclusion is not asking whether you agree. It is asking what is wrong with the evidence.
Where the energy goes
Work done against gravity is stored as gravitational potential energy and can be recovered. Work done against friction is not: it transfers energy to the thermal store of the surfaces, which spreads to the surroundings and cannot be recovered usefully. That is why braking makes brake discs hot and why no machine is ever fully efficient.
Choosing between the two equations
If the question gives a force and a distance, use work done equals force times distance. If it gives an energy and a time, or asks how quickly something happened, use power equals energy over time.
Many questions give all three and expect both steps, so find the work done first and then divide by the time. Writing both equations down before you start makes the route obvious.
Spec 4.4-4.5
What you need to know
- Define work done as energy transferred
- Use work done equals force times distance
- Understand power as energy per second
Active recall
Quick check
Answer each question before opening the answer.
What is the equation for work done?
Work done = force × distance moved in the direction of the force (W = F × d).
What is power?
The rate of energy transfer (work done per second); P = E ÷ t.
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