GCSE · Physics · AQA · Spec 8463
Efficiency
Where does input energy actually go?
Follow one joule of energy through a real device — see what becomes useful and what is wasted.
Stop and predict
Commit to an answer before you reveal — this is how memory sticks.
A motor transfers 800 J of useful kinetic energy from a total electrical input of 2000 J. A student writes the efficiency as 2.5. What has gone wrong?
Efficiency
Reason it through
Why must the efficiency of any real device always be less than 1 (less than 100%)?
First link · your turn
Think about what happens to energy inside a device — does ALL of it reach the intended output?
Relationship matrix
Tap any cell to reveal it. Tap a column header to read one property down every item.
Each cell hides a short answer and the reason behind it. Predict before you tap.
Cross-subject · Exam skill
Command word coach — Efficiency questions
AQA uses these command words on efficiency questions. Know what each one demands.
01Definition
Work out a numerical answer using the correct equation. Always show your substitution and give the answer with appropriate form (decimal or percentage).
02Mark-band signal
Full marks require: correct equation stated or used, correct substitution, correct numerical answer in correct form.
03Sentence stems
- Calculate the efficiency of the motor.
- Calculate the useful power output of the device.
- Calculate the total energy input given the efficiency and useful output.
04Common mistake
Giving efficiency with a unit (e.g. '0.6 J') — efficiency is dimensionless. Also: dividing total by useful instead of useful by total.
Equations you need
Taken directly from the exam-board specification.
efficiency = efficiency (dimensionless) · useful output energy transfer = useful output energy transfer (J) · total input energy transfer = total input energy transfer (J)
Learn it — you must recall this in the exam
efficiency = efficiency (dimensionless) · useful power output = useful power output (W) · total power input = total power input (W)
Learn it — you must recall this in the exam
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
How much of the energy you put in actually does something useful?
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