AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Energy › Conservation and dissipation of energy
Practise Efficiency. 9 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Efficiency tells you how much of the energy supplied to a device is transferred usefully. You need to calculate it from energy or power values, as a decimal or a percentage, and rearrange the equation. Most questions are 2 to 3 mark calculations.
Key facts
efficiency = useful output energy transfer ÷ total input energy transfer
efficiency = useful power output ÷ total power input
percentage efficiency = efficiency × 100
total input = useful output + wasted
Efficiency has no unit and is never more than 1 (100%)
useful output = efficiency × total input
Notes
What efficiency means
diagram
Energy supplied to a device is either transferred usefully (to the store you want) or wasted (dissipated, usually to the thermal store of the surroundings).
total input energy = useful output energy + wasted energy
Sankey diagram: widths show the energy. Efficiency = 75 ÷ 100 = 0.75 (75%).
Efficiency = useful output energy transfer ÷ total input energy transfer
You can also use power: efficiency = useful power output ÷ total power input.
Efficiency has no unit. Give it as a decimal (e.g. 0.35) or as a percentage (× 100, e.g. 35%).
No real device is 100% efficient, because some energy is always dissipated. Efficiency can never be more than 1 (100%).
Using the equation
Both energies (or both powers) must be in the same unit, e.g. both in J or both in kJ.
If you are given a percentage, divide it by 100 before you use it in the equation.
To find the useful output: useful output = efficiency × total input.
How to answer each type of question
Calculate the efficiency
2 marksGrade 4
Pick out the useful output and the total input.
Divide useful by total.
Give a decimal, or multiply by 100 for a percentage (with the % sign).
Example. An electric motor is supplied with 2400 J of energy. It transfers 1800 J of this energy usefully. Calculate the efficiency of the motor.
Putting the wasted energy on top instead of the useful energy.
Using one value in J and the other in kJ.
Giving efficiency a unit (such as J), or forgetting the % sign.
Putting a percentage (e.g. 80) into the equation instead of the decimal (0.80).
More tips
Memory tricks
'Useful over total': the smaller number goes on top.
Sense check: if your efficiency is more than 1 (or 100%), you have divided the wrong way round.
Decimal to percentage: multiply by 100, so 0.35 becomes 35%.
An electric heater is almost 100% efficient, because heating the room is its useful output.
Exam technique
Check whether the question wants a decimal or a percentage.
Show the subtraction when you have to find the useful energy first; it can earn a mark on its own.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Identify useful and wasted energy transfersE.g. for a lamp, light is the useful output and heating the surroundings is wasted.
Grade 4
Calculate efficiency from energy valuesDivide the useful output energy by the total input energy.
Grade 5
Give efficiency as a decimal or percentageMultiply the decimal by 100 to get a percentage; efficiency is never more than 1 (100%).
Grade 5
Calculate efficiency from power valuesDivide the useful power output by the total power input.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Write down the equation that links efficiency, useful output energy transfer and total input energy transfer.
efficiency = useful output energy transfer ÷ total input energy transfer
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
An electric motor is used in a toy car.
(a) The motor is supplied with 500 J of energy. 350 J is transferred usefully to the kinetic energy store of the car. Calculate the efficiency of the motor. Use the equation: efficiency = useful output energy transfer ÷ total input energy transfer Give your answer as a decimal.[2]
(b) Calculate the energy wasted by the motor.[1]
(c) What happens to the energy wasted by the motor? Tick (✓) one box.[1]
It is destroyed.
It is dissipated to the surroundings.
It is stored in the battery.
It is transferred to the kinetic energy store of the car.
Show the answer and mark scheme
(a)Answer: 0.70
efficiency = 350 ÷ 500
efficiency = 0.70
(b)Answer: 150 J
150 (J)
(c)Answer: It is dissipated to the surroundings.
Question 2Medium6 marks
An electric motor in a fan has an efficiency of 0.65.
(a) The useful power output of the motor is 390 W. Calculate the total power input to the motor.[3]
(b) Calculate the power wasted by the motor.[1]
(c) The fan runs for 5.0 minutes. Calculate the energy wasted by the motor in this time.[2]