GCSE · Physics · AQA · Spec 8463

Efficiency

No machine is perfect — efficiency tells you exactly how much energy goes to waste.

What you need to know

  • Efficiency = useful output energy transfer ÷ total input energy transfer (efficiency = useful output energy transfer / total input energy transfer)
  • Efficiency = useful power output ÷ total power input (efficiency = useful power output / total power input)
  • Efficiency is a ratio with no unit — express it as a decimal (0 to 1) or as a percentage (0% to 100%)
  • Higher tier: efficiency can be increased by reducing unwanted energy transfers, e.g. lubrication to reduce friction or insulation to reduce heat loss

The big picture

Efficiency measures how much of the total input energy is converted into useful output energy. It is expressed as a decimal or a percentage and has no unit — it is a pure ratio. A perfectly efficient device would have an efficiency of 1 (or 100%), but in reality energy is always wasted to the surroundings. Higher-tier students also need to know how to increase efficiency by reducing unwanted energy transfers.

TONIGHT'S REVISION

Efficiency

How much of the energy you put in actually does something useful?

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%)?

Link 1 of 3

First link · your turn

Think about what happens to energy inside a device — does ALL of it reach the intended output?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
Higher

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

Wasted energy per joule inputUseful energy per joule inputMain way to increase efficiency
Low-efficiency devicee.g. old filament bulb
High-efficiency devicee.g. LED lamp

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.

CalculateBloom · Apply

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.

Exam line: On AQA efficiency questions: 'Calculate' wants the equation + substitution + answer; 'Describe' (Higher) wants a named method and the unwanted transfer it reduces; 'Explain' wants a causal chain.
Watch out: Do not treat 'state' and 'explain' as the same command — 'state' needs one line, 'explain' needs a because-chain.

Equations you need

Taken directly from the exam-board specification.

efficiency = useful output energy transfer / total input energy transfer

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 = useful power output / total power input

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

Key points

1efficiency = useful output energy transfer / total input energy transfer — use this when the question gives energy values in joules
2efficiency = useful power output / total power input — use this when the question gives power values in watts
3Efficiency has NO unit — it is a decimal or percentage; never write J or W after it
4An efficiency greater than 1 (or 100%) is impossible — it would mean creating energy from nothing
5Higher tier: reducing friction (lubrication) or heat loss (insulation) are the two key ways to increase efficiency

Worked example

Problem

A motor takes in 2000 J of electrical energy and transfers 1400 J as useful kinetic energy. Calculate the efficiency of the motor as a percentage.

🧠

Memory hook

Think of efficiency as a 'useful fraction' — cover the 'useful' part of the triangle and what's left tells you what you had to put in. Better still: GIGO backwards — Good In, Good Out. If half goes to waste, efficiency = 0.5.

★ Exam tip

On AQA Paper 1, always check whether the question gives energy (J) or power (W) — then pick the matching equation. If the answer comes out greater than 1, you've divided the wrong way round: flip numerator and denominator.

⚠ Watch out

Writing a unit after the efficiency value — efficiency is a dimensionless ratio. Never write J, W, or any other unit alongside it.

✓

Check yourself

A device has an efficiency of 0.35. Without calculating anything new, what percentage of its input energy is wasted?

Flashcards

(22)
State the efficiency equation using energy transfers.
efficiency = useful output energy transfer / total input energy transfer
State the efficiency equation using power.
efficiency = useful power output / total power input
What unit does efficiency have?
No unit — it is a dimensionless ratio.
What is the maximum possible efficiency of any real device?
1 (or 100%) — no device can transfer more useful energy than it receives.
A device receives 500 J and transfers 200 J usefully. What is its efficiency?
0.4 (or 40%)
How do you convert an efficiency decimal to a percentage?
Multiply by 100. For example, 0.6 → 60%.
A lamp has an efficiency of 0.15. What fraction of input energy is wasted?
0.85 (85%) is wasted as heat and other non-light forms.
Which equation do you use when the question gives values in watts?
efficiency = useful power output / total power input
Which equation do you use when the question gives values in joules?
efficiency = useful output energy transfer / total input energy transfer
A motor has a useful power output of 60 W and a total power input of 200 W. What is its efficiency?
0.3 (or 30%)
What does an efficiency of 1 mean?
All input energy is transferred usefully — 100% efficient. Impossible in practice.
What does an efficiency of 0 mean?
None of the input energy is transferred usefully — all energy is wasted.
Higher tier: Name two ways to increase the efficiency of a machine.
Lubrication (to reduce friction) and thermal insulation (to reduce unwanted heat loss).
Higher tier: Why does lubrication increase efficiency?
It reduces friction between moving parts, so less energy is wasted as heat.
Higher tier: Why does insulation increase efficiency in a heating system?
It reduces unwanted heat loss to the surroundings, so more energy reaches the intended place.
A device receives 800 J and wastes 320 J. What is its efficiency?
Useful output = 800 − 320 = 480 J. Efficiency = 480 ÷ 800 = 0.6 (60%).
Can efficiency ever be greater than 100%? Explain.
No — this would mean creating energy from nothing, which breaks conservation of energy.
What is meant by 'useful' energy transfer?
Energy transferred to the place and in the form intended by the device's purpose.
What happens to wasted energy?
It is dissipated to the surroundings — usually as heat, sound, or light where not intended.
A generator produces 9000 W of electrical power from 12 000 W of input power. What is its efficiency?
9000 ÷ 12 000 = 0.75 (75%)
State the two forms in which efficiency can be expressed.
As a decimal between 0 and 1, or as a percentage between 0% and 100%.
Higher tier: Give one example of reducing unwanted energy transfer to improve efficiency in a car engine.
Lubricating the pistons and crankshaft reduces friction, so less energy is wasted as heat.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learn Efficiency properly — interactive practice, marked questions and flashcards.

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