What is power, and how do we measure how efficiently a device transfers energy?
Power as the rate of energy transfer, P = E / t, and efficiency as useful output over total input.
A CCEA GCSE Physics answer on power as the rate of doing work or transferring energy, the equation P = E / t, and how efficiency is calculated as useful output divided by total input.
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What this dot point is asking
CCEA wants you to define power as the rate of transferring energy or doing work, use the equation P = E / t, define efficiency, and calculate it as useful output over total input. Efficiency questions link back to conservation of energy and dissipation.
The answer
Power
A more powerful device transfers the same energy in less time, or more energy in the same time. A kettle transfers energy twice as fast as a one.
Efficiency
No device transfers all its input energy usefully; some is always dissipated, usually as heat.
The energy that is not transferred usefully is dissipated to the surroundings, mostly as heat, in line with conservation of energy.
Worked example: an efficient and a wasteful bulb
Examples in context
- Example 1. Two cyclists
- Two cyclists climb the same hill, gaining the same gravitational potential energy. The one who reaches the top in less time has the greater power output, even though both did the same work.
- Example 2. A power station
- A typical fossil-fuel power station is about 35 to 40 percent efficient: most of the chemical energy in the fuel is dissipated as heat in the cooling towers and exhaust, so only part becomes useful electrical energy.
- Example 3. A smartphone charger
- A charger takes in mains electrical energy and usefully transfers most of it to the phone's battery, but a small amount is dissipated as heat, which is why the charger feels warm. A more efficient charger wastes less heat for the same charging job.
Efficiency and power often appear together. If a device is very powerful but inefficient, much of the energy it transfers each second is wasted as heat rather than doing the useful job. Designers aim to raise efficiency so that more of the input power becomes useful output, cutting both running costs and wasted heat. This is why energy-efficiency labels are now placed on appliances such as fridges, washing machines and light bulbs.
Try this
Q1. State the equation for power and the unit of power. [2 marks]
- Cue. ; power is measured in watts (W).
Q2. A device transfers in . Find its power. [2 marks]
- Cue. .
Q3. A machine takes in and usefully transfers . Find its efficiency as a percentage. [2 marks]
- Cue. Efficiency .
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style3 marksA motor transfers 9000 J of energy in 30 s. Calculate its power output.Show worked answer →
Power is the rate of energy transfer:
So the power output is 300 W.
Markers reward , the substitution, and the value 300 W.
CCEA style4 marksAn electric drill takes in 600 W of power and delivers 420 W of useful power. Calculate its efficiency as a percentage and state what happens to the rest of the energy.Show worked answer →
Efficiency is useful power output divided by total power input:
The remaining is dissipated, mostly as heat in the motor and in friction, plus some sound.
Markers reward the efficiency ratio, the value 70 percent, and naming heat (and sound) as the wasted energy.
Related dot points
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A CCEA GCSE Physics answer on calculating kinetic energy with half m v squared and gravitational potential energy with mgh, and using conservation of energy to link the two stores in falling objects.
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A CCEA GCSE Physics answer on the main energy stores and the ways energy is transferred between them, the principle of conservation of energy, and how energy is dissipated as heat to the surroundings.
- Renewable and non-renewable energy resources, how electricity is generated from them, and the advantages and disadvantages of each.
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)