GCSE Physics resources
AQA GCSE Energy
Efficiency compares the useful output with the total input; it does not measure how quickly a device works.
Start here
Efficiency compares the useful output with the total input. An efficiency improvement must name the unwanted transfer and the design change that reduces its rate, such as lubrication reducing friction or insulation reducing heating.
Before you revise
Choose an answer from memory. Your result tells you what to watch for in the guide.
Core revision guide
Efficiency and Unwanted Energy Transfers questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.
Efficiency compares the useful output with the total input. An efficiency improvement must name the unwanted transfer and the design change that reduces its rate, such as lubrication reducing friction or insulation reducing heating.
Useful energy transfer is not the same as all energy transfer. A motor can give a useful kinetic output while heating its surroundings through friction. Improving efficiency means reducing a named unwanted pathway, not claiming that the input energy has disappeared.
Efficiency: efficiency = useful output ÷ total input (decimal or percentage). Choose each relationship from the physical change described, convert quantities into compatible units and keep the unit beside the final answer.
Misconception clinic
Read each belief, say what is wrong with it, then compare your correction.
Tempting idea: Putting total input on the top of the fraction.
Use instead: Efficiency compares the useful output with the total input. An efficiency improvement must name the unwanted transfer and the design change that reduces its rate, such as lubrication reducing friction or insulation reducing heating.
Tempting idea: Mixing a decimal efficiency with a percentage.
Use instead: Useful energy transfer is not the same as all energy transfer. A motor can give a useful kinetic output while heating its surroundings through friction. Improving efficiency means reducing a named unwanted pathway, not claiming that the input energy has disappeared.
Tempting idea: Saying unwanted energy disappears.
Use instead: State the equation, identify useful and total values, give a percentage if requested, then explain the unwanted pathway in words.
Retrieval practice
Explore the relationship
Change the efficiency or total input. The bar length represents the total input, and its sections show useful and unwanted transfers.
What to notice: use the readout to describe how one variable changes when the other is controlled.
Worked examples
These examples expose the difference between a secure efficiency and dissipation method and the common error “Putting total input on the top of the fraction.”.
Efficiency = 0.70 = 70%.
Insulation reduces energy transfer by heating through the roof, so the house’s thermal energy is transferred to the surroundings more slowly.
Efficiency = 0.75, or 75%.
Exam precision
Do this: State the equation, identify useful and total values, give a percentage if requested, then explain the unwanted pathway in words.
Independent practice
Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.
Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.
Model answer: 24 ÷ 40 = 0.60, so 60%.
Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.
Model answer: It is transferred through unwanted pathways, usually to thermal stores of the machine and surroundings.
Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.
Model answer: It reduces friction, reducing unwanted transfer to thermal stores.
Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.
Model answer: 20 ÷ 80 = 0.25, or 25%.
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: Some energy is transferred to thermal stores by electrical resistance or friction.
Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.
Model answer: It has been transferred to less useful stores, not destroyed.
Questions pupils ask
Efficiency compares the useful output with the total input. An efficiency improvement must name the unwanted transfer and the design change that reduces its rate, such as lubrication reducing friction or insulation reducing heating.
Putting total input on the top of the fraction. Useful energy transfer is not the same as all energy transfer. A motor can give a useful kinetic output while heating its surroundings through friction. Improving efficiency means reducing a named unwanted pathway, not claiming that the input energy has disappeared.
Useful next steps