IGCSE Physics: energy stores, work, power and efficiency

Key ideas, board by board

Cambridge 0625 Core (1.7.1) lists the stores: kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal), and asks you to describe transfers between them by forces (mechanical work), electrical currents, heating and waves. Core students apply conservation of energy to simple flow diagrams; Supplement students recall and use Ek = 1/2 mv^2 and change in Ep = mg x change in h, and interpret Sankey diagrams for multi-stage processes. Work W = Fd = change in E and power P = W/t = change in E / t are Core. Efficiency is qualitative in Core; the percentage equations are Supplement (1.7.3.7).

Edexcel 4PH1 lists the stores slightly differently (chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear) and the transfers as mechanical, electrical, by heating and by radiation (light and sound). Every student uses conservation of energy, efficiency with Sankey diagrams, work done, GPE and KE, and power. Appendix 7 of the specification says the efficiency, work, KE and GPE relationships will not be given in the exam, so they must be memorised.

Energy resources: Cambridge Core 1.7.3 covers fossil fuels, biofuels, water (waves, tides, hydroelectric), geothermal, nuclear, solar cells and solar panels, and wind, with advantages and disadvantages; Supplement adds that the Sun is the main source for most resources and that the Sun's energy comes from nuclear fusion. Edexcel puts electricity generation and its advantages and disadvantages in 4.18P and 4.19P, so they appear only on Paper 2.

The equations and their units

"Recall" means the board expects you to know the equation without being given it.

Worked example 1: falling ball (conservation of energy)

Question: a 0.50 kg ball is dropped from a height of 3.2 m. Ignoring air resistance, calculate its speed just before it hits the ground. Use g = 9.8 N/kg.

GPE lost = m x g x h = 0.50 x 9.8 x 3.2 = 15.68 J. All of it becomes kinetic energy, so 1/2 x 0.50 x v^2 = 15.68. Then v^2 = 15.68 ÷ 0.25 = 62.72 and v = sqrt(62.72) = 7.9 m/s.

Notice the mass cancels: v = sqrt(2gh) for any mass. In a real drop some energy is transferred to the thermal store of the air and ball by air resistance, so the real speed is a little lower.

Worked example 2: power and efficiency

Question: a motor lifts a 500 kg load through 12 m in 20 s. The motor takes in 4000 W of electrical power. Calculate the useful power output and the efficiency.

Useful work = GPE gained = 500 x 9.8 x 12 = 58 800 J. Useful power = 58 800 ÷ 20 = 2940 W (about 2.9 kW).

Efficiency = useful power output / total power input x 100% = 2940 ÷ 4000 x 100% = 73.5%, about 74%. The other 26% is transferred to the thermal store of the motor and surroundings, mostly by friction and electrical heating in the wires. An answer above 100% means the fraction is upside down.

Common mistakes

• Writing that energy is "used up", "lost" or "created". Say it is transferred, often to the thermal store of the surroundings, where it is dissipated.
• Squaring the whole of 1/2 mv instead of only v.
• Using the distance moved sideways in a GPE calculation instead of the vertical height.
• Forgetting to convert grams to kilograms or kilojoules to joules.
• Giving efficiency a unit, or a value greater than 1 (or 100%).
• Saying solar energy is reliable. It is renewable but depends on time of day and cloud cover.
• Confusing power (rate, in watts) with energy (amount, in joules).

Exam technique and mark-scheme language

In "describe the energy transfers" questions, name the stores and the pathway: "chemical energy store of the fuel decreases, energy is transferred by heating to the thermal store of the water". Both syllabuses are written in this store and pathway language, so use it; "chemical energy turns into heat energy" is vaguer than the wording examiners look for.

For energy resources, compare on the criteria Cambridge lists: renewability, availability, reliability, scale and environmental impact. A strong answer pairs each point with a reason: "wind is renewable but unreliable because wind speed varies". In Sankey diagrams, the width of each arrow is proportional to the energy, and the input width equals the sum of the outputs.

How one-to-one lessons help with this topic

Energy is the topic that links the whole course, so students who write vague transfer descriptions lose marks everywhere. A tutor rehearses the store and pathway vocabulary until it is automatic, then works through multi-step calculations (GPE to KE, power from work, efficiency) on the shared whiteboard, checking each line. The tutor also sets the six-mark resource comparisons and marks them against real mark schemes.

Self-check

1. List the energy stores your board names.
2. Describe the energy transfers when a car brakes to a stop.
3. Use W = Fd, KE = 1/2 mv^2 and GPE = mgh, rearranging for any quantity.
4. Solve a dropped-object problem using conservation of energy.
5. Calculate power from work done and time.
6. Calculate efficiency and explain where the wasted energy goes.
7. Compare two energy resources using at least three criteria.