IGCSE §4: Energy Resources and Energy Transfer · IGCSE
Revision notes on Conservation of Energy and Efficiency for the OCR A-level Physics specification (H556). Free to read, with 5 practice questions in the app.
The principle of conservation of energy — energy cannot be created or destroyed. It can only be transferred from one store to another.
This means the total energy before a change equals the total energy after it. Every joule is accounted for somewhere, including the joules that ended up spread through the surroundings.
Efficiency — the fraction of the energy supplied that ends up in the useful store:
efficiency = useful energy output / total energy input
Multiply by 100 for a percentage. Since power is energy per second, the same relationship works with powers:
efficiency = useful power output / total power input
Efficiency can never exceed 1, or 100%. An answer above that means the useful and total figures have been swapped, and it is worth checking every time.
Sankey diagrams show where the energy goes. The arrow enters from the left with a width proportional to the input energy, and splits into branches whose widths are proportional to each output. Because energy is conserved, the widths of all the branches add up to the width of the input arrow — so a Sankey diagram cannot be drawn wrongly without it being visible.
The useful output is conventionally drawn straight on; wasted energy branches downwards.
Example: a lamp supplied with 100 J produces 15 J of light and 85 J of heating. Its efficiency is 15 ÷ 100 = 0.15, or 15%. The Sankey arrow arrives 100 units wide, a 15-unit branch continues as light, and an 85-unit branch bends away as heating.
Why nothing is 100% efficient. Every real transfer moves some energy to the thermal store of the surroundings — through friction, air resistance, or the resistance of the wires. That energy is not lost from the universe, but it is lost to you.
Reducing waste. Lubricating moving parts cuts friction; streamlining cuts air resistance; insulation slows unwanted heating. Each one raises efficiency by reducing the energy dissipated, not by creating more.
Reading is not revising. This topic has 5 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
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