OxfordAQA IGCSE Physics: Energy — Revision Notes

Condensed for the final weeks. For the full explanation, use the Energy study guide.

The core equations (3.2.1)

Quantity	Equation
Work done	W = Fd
Elastic potential energy	Stored in a stretched spring, within the limit of proportionality
Gravitational potential energy	Ep = mgh
Kinetic energy	Ek = ½mv²
Power	P = W/t = E/t

Doubling speed quadruples kinetic energy at constant mass (v² term) — a frequently tested consequence of the KE equation, not just the formula itself. Practise substituting and rearranging each equation until automatic, since exam questions regularly combine two equations in one multi-step problem — for example, finding kinetic energy from a measured loss of gravitational potential energy.

Energy transfers, conservation and dissipation (3.2.2)

Energy is never created or destroyed — only transferred usefully, stored, or dissipated (usually as heat via friction/air resistance). A simple pendulum illustrates continuous transfer between kinetic and gravitational potential energy.

efficiency = useful energy output / total energy input
           (as a decimal, or x 100 for a percentage)

Sankey diagrams represent this visually: arrow width is proportional to energy amount; the diagram must account for all input energy — useful output plus every wasted (dissipated) path. Practise both reading a given Sankey diagram to extract efficiency, and sketching one from a written description.

Worked example: efficiency and Sankey diagram

A motor is supplied 500 J; it usefully transfers 350 J as kinetic energy, the rest dissipated as heat.

Efficiency = 350 / 500 = 0.7, or 70%

Sankey diagram: input arrow (500 J) splits into a wider branch
(350 J useful output) and a narrower branch (150 J wasted as heat)

Being able to move between the numerical efficiency calculation and its Sankey-diagram representation is a common requirement — practise both directions.

Energy resources (3.2.3)

Fuels are chosen as energy stores based on ease of storage, energy content, and safety. When a fuel is used, energy transfers to the surroundings, and fuels differ in efficiency of that transfer.

Renewable vs. non-renewable: know named examples of both, and their implications for society — renewability, and environmental impact of extraction, use and disposal. Named renewables include wave, solar and geothermal power, each with specific advantages and drawbacks (e.g. solar depends on sunlight availability and location; geothermal needs suitable geology; wave power is site-dependent and can disrupt marine environments).

Don’t just list sources. Exam questions typically ask for a reasoned comparison for a specific context — for example, why solar suits one location better than wind — so practise weighing named advantages and drawbacks against a given scenario, not reciting a generic list.

Practising multi-step calculations

Because exam questions frequently chain two of the core equations together, practise problems that require this explicitly – for example, an object falling from a known height: use Ep = mgh to find the gravitational potential energy lost, then set this equal to Ek = 1/2 mv^2 (assuming no air resistance) to find the object’s speed just before impact. Working through several such chained problems, rather than only practising each equation in isolation, builds the specific skill multi-step exam questions on this topic actually test.

Common mistakes

• Confusing power (rate of energy transfer, watts) with energy itself (joules) — a higher power rating transfers energy faster, not necessarily more total energy, unless run for the same time.
• Describing an apparent energy “loss” without stating it was transferred or dissipated, never destroyed.
• Treating efficiency as a fixed property of a device, rather than depending on the specific transfer being described.
• Mixing up gravitational potential energy (depends on height) with kinetic energy (depends on speed) for an object moving both vertically and horizontally.

How this topic connects forward

Topic 1 (Forces and their Effects) established force, motion and momentum; Energy builds directly on that, since work, kinetic energy and gravitational potential energy are all defined using force and distance, or force and height. The conservation idea introduced here – energy redistributed but never created or destroyed – recurs through the rest of the specification, including Waves and the Particle Model of Matter, so treat the conservation principle as a tool you will keep reusing, not content specific to this topic alone. The specification itself frames energy as a useful accounting tool: tracking energy in and energy out lets you calculate how long a resource will last, or whether a proposed transfer or event is even physically possible in the first place.

Key terms

Work done — energy transferred when a force causes movement through a distance (W = Fd). Efficiency — the proportion of total energy input that is usefully transferred, as a decimal or percentage. Dissipation — energy spread into a less useful form, typically heat, via friction or air resistance. Sankey diagram — a diagram in which arrow width represents the proportion of energy following each path, accounting for all input energy. Renewable energy resource — a resource replenished naturally within a human timescale (e.g. solar, wind, wave, geothermal).

Quick self-test

• State all four core equations (work, GPE, KE, power) from memory.
• Calculate the kinetic energy of a 2 kg object moving at 5 m/s, and explain what happens to KE if speed doubles.
• Calculate efficiency for a device supplied with 200 J that usefully outputs 120 J, and sketch its Sankey diagram.
• Name two renewable energy resources and state one advantage and one drawback of each.
• Explain, in one sentence, why “energy loss” is imprecise language and what should be said instead.

Related resources

• Energy study guide
• Energy practice questions

Official syllabus

OxfordAQA International GCSE Physics (9203) specification — oxfordaqa.com.