GCSE Physics resources
AQA GCSE Energy
Useful output is chosen by the purpose of the device. The “wasted” part is still present in the full energy account, usually spread through thermal stores.
Start here
Useful output is chosen by the purpose of the device. The “wasted” part is still present in the full energy account, usually spread through thermal stores.
Before you revise
Choose an answer from memory. Your result tells you what to watch for in the guide.
Core revision guide
Useful output is chosen by the purpose of the device. The “wasted” part is still present in the full energy account, usually spread through thermal stores.
Scope: 8463 and 8464 · Foundation and Higher
Efficiency is useful output energy / total input energy, or useful output power / total input power. Do not divide joules by watts. The result has no unit; multiply by 100 for a percentage. A 200 W motor delivering 150 W mechanically has efficiency 0.75 = 75% and an unwanted rate of 50 W.
Scope: 8463 and 8464 · Foundation and Higher
Lubrication reduces friction between moving parts, reducing mechanical work that warms the machine. Insulation lowers the rate of heating the surroundings. Thicker walls and smaller thermal conductivity reduce conduction. An improvement reduces a specified unwanted transfer; it cannot recover energy that has already dispersed spontaneously.
Scope: 8463 and 8464 · Foundation and Higher
Input = useful output + unwanted output. Arrow widths in a Sankey diagram represent energy or power in one consistent scale. A shorter unwanted arrow with the same input means a larger useful output and greater efficiency.
Misconception clinic
Read each belief, say what is wrong with it, then compare your correction.
Tempting idea: 400%.
Use instead: Input/output reverses the ratio. 20/80 × 100 = 25%.
Tempting idea: Oil creates extra input energy.
Use instead: Oil reduces loss, not creates energy. This identifies the pathway and mechanism.
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.
Model scope: 8463 and 8464 · Foundation and Higher
What to notice: use the readout to describe how one variable changes when the other is controlled.
Worked examples
Apply use the same kind of quantity in the ratio in the worked reasoning, then try the changed-context questions below.
Scope: 8463 and 8464 · Foundation and Higher
Efficiency = 0.70 = 70%.
Scope: 8463 and 8464 · Foundation and Higher
Insulation reduces energy transfer by heating through the roof, so the house’s thermal energy is transferred to the surroundings more slowly.
Scope: 8463 and 8464 · Foundation and Higher
Efficiency = 0.75, or 75%.
Exam precision
Do this: Use the same kind of quantity in numerator and denominator. Explain an improvement by naming the unwanted transfer it reduces.
Independent practice
Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.
Scope: 8463 and 8464 · Foundation and Higher
Original PhysicsUK mark scheme: Point 1 (1 mark): Efficiency = useful power/total power × 100. Point 2 (1 mark): Efficiency = 180/240 × 100. Point 3 (1 mark): Efficiency = 75% (no physical unit). Award each distinct point once. Accept equivalent scientifically correct wording. No half marks.
Model answer: Efficiency = useful power/total power × 100. Efficiency = 180/240 × 100. Efficiency = 75% (no physical unit).
Acceptable answers: Expected result: Efficiency = 75% (no physical unit). Accept the unrounded value or a correctly rounded equivalent to two or more significant figures; accept equivalent units if converted correctly. No additional measurement interval is implied.
Marking notes: Method marks remain available after an arithmetic error. Carry an earlier numerical error into a later valid step; award that method point if the unit conversion and physics are correct. A fully correct answer with unit earns all marks unless working is expressly requested.
Scope: 8463 and 8464 · Foundation and Higher
Original PhysicsUK mark scheme: Point 1 (1 mark): Lubrication reduces friction. Point 2 (1 mark): Less input energy is transferred to thermal stores through friction. Point 3 (1 mark): A greater fraction of the same total input becomes useful mechanical output. Award each distinct point once. Accept equivalent scientifically correct wording. No half marks.
Model answer: Lubrication reduces friction and therefore the heating of moving parts. A greater fraction of input energy becomes useful mechanical output.
Questions pupils ask
Yes. Heating water is the useful purpose of a kettle; heating the room is usually unwanted for that task.
Useful next steps