9702 Paper 4: How to Answer Structured Questions, and the Hardest Topics

Paper 4 is where Cambridge A Level Physics 9702 is won or lost. It is a two-hour, 100-mark structured paper, worth 38.5% of the A Level. It covers the A Level topics (syllabus topics 12–25: circular motion, gravitational fields, temperature, ideal gases, thermodynamics, oscillations, electric fields, capacitance, magnetic fields, alternating currents, quantum, nuclear and medical physics, and astronomy and cosmology), and it also requires AS knowledge. It rewards precise language as much as correct numbers. This guide sets out the technique we teach our own students, then applies it to the six topics that cause the most difficulty.

1. Know exactly what the paper is testing

Paper 4 questions usually follow a pattern: a definition or statement, then a derivation or explanation, then one or more calculations, sometimes a graph or sketch. Each part is worth a predictable number of marks. A 1-mark "define" needs one precise sentence; a 3-mark "explain" needs three distinct, linked points.

Tip: Count the marks and write that many creditable points. Padding does not score; precision does.

2. Treat definitions as exact wording

Many marks are lost on definitions that are almost right. Examples: - Gravitational field strength — force per unit mass (on a small test mass). Omitting "per unit mass" loses the mark. - Simple harmonic motion — acceleration proportional to displacement and directed towards a fixed point (in the opposite direction to displacement). Both conditions are required. - Magnetic flux density — force per unit length per unit current on a conductor placed at right angles to the field.

Build a definitions list from mark schemes and test yourself weekly. The 9702 syllabus is your checklist.

3. Respond to command words

"Show that" questions are a common trap: if the answer is given as 2.5 m s⁻¹, writing 2.5 proves nothing. Write 2.47 m s⁻¹ (or similar) and state the conclusion.

4. Lay out calculations for method marks

• Write the equation in symbols first.
• Substitute values with units converted (MeV to J, cm to m, °C to K).
• Give the answer with a unit and sensible significant figures (usually 2–3).
• If you get stuck in part (i), carry on: an error carried forward can still earn full marks in part (ii).

5. Derivations: learn the logic, not just the result

Common derivations include orbital period from gravitational force, the ideal-gas link between mean kinetic energy and temperature, and the capacitor discharge relationship. Examiners credit each logical step, so state your starting principle ("gravitational force provides the centripetal force") before writing equations.

6. Graphs and sketches

Label axes with quantity and unit, mark key values (intercepts, asymptotes, half-life points) and make curves smooth. For exponential decay, show the curve approaching but never meeting the axis.

7. Mistakes examiners see every series

Examiner reports for 9702 repeatedly highlight: 1. Confusing field strength with potential. 2. Missing directions for vectors (forces, fields, accelerations). 3. Using degrees instead of radians in SHM and circular motion. 4. Vague explanations ("it increases") without naming the quantity or cause. 5. Arithmetic slips from premature rounding — keep extra figures until the end.

Read the examiner report alongside every paper you sit; they are published on the official Cambridge 9702 page.

8. The six hardest Paper 4 topics, and how to crack them

"Hardest" here is our teaching judgement, based on where students most often lose marks. Check the examiner report for each session to see that session's specific weaknesses.

8.1 Gravitational and electric fields

Why it's hard: two similar-looking topics, each with field strength and potential, inverse-square and inverse laws, and signs that matter.

Crack it: build a side-by-side table (force, field strength, potential, potential energy) for gravitational and electric fields, and learn where they differ. Gravitational potential is always negative; electric potential takes the sign of the charge. Field strength is the negative gradient of potential. Original check: a satellite moves to a higher orbit. Does its gravitational potential energy increase or decrease? (It increases, becoming less negative.)

8.2 Capacitance

Why it's hard: combinations in series and parallel, energy stored, and exponential discharge all meet in one question.

Crack it: remember that the formulae for capacitors combine the opposite way from resistors. Know that energy stored is the area under a Q–V graph (½QV). For discharge, practise moving between x = x₀e^(−t/RC), the time constant and half-life–style reasoning. Original example: a 470 µF capacitor discharges through a 10 kΩ resistor. The time constant is RC = 470 × 10⁻⁶ × 10 × 10³ = 4.7 s, so after 4.7 s the charge has fallen to about 37% of its initial value.

8.3 Electromagnetic induction

Why it's hard: flux linkage, Faraday's and Lenz's laws demand both calculation and precise explanation.

Crack it: for explanations, use a three-step chain: (1) there is a change in magnetic flux linkage, (2) so an e.m.f. is induced (Faraday), (3) its direction opposes the change that causes it (Lenz). Name the quantity that changes.

8.4 Oscillations (simple harmonic motion)

Why it's hard: the defining equation, energy graphs and damping all depend on reading graphs correctly, and on radians.

Crack it: learn the definition exactly (see section 2), and use a = −ω²x with ω = 2πf. Remember that maximum speed is ωx₀ at the equilibrium position. Sketch kinetic and potential energy against displacement until it is automatic. Original example: an oscillator has amplitude 0.040 m and frequency 2.5 Hz. ω = 2π × 2.5 = 15.7 rad s⁻¹, so the maximum speed is 15.7 × 0.040 = 0.63 m s⁻¹.

8.5 Thermal physics and ideal gases

Why it's hard: kinetic theory derivations, the first law of thermodynamics sign convention, and unit conversions (°C to K, cm³ to m³).

Crack it: state the assumptions of kinetic theory precisely. Fix the first-law convention used in the syllabus (ΔU = q + W, where q is energy transferred to the system by heating and W is the work done on the system) and stick to it.

8.6 Quantum physics

Why it's hard: it asks for explanations of phenomena that contradict everyday intuition, such as the photoelectric effect, energy levels and wave–particle duality.

Crack it: for the photoelectric effect, explain why the wave model fails: emission is instantaneous, there is a threshold frequency, and maximum kinetic energy depends on frequency, not intensity. Practise hf = φ + ½mv²max and energy-level transitions with ΔE = hf. Original example: light of photon energy 3.2 eV falls on a metal with work function 2.3 eV. Maximum kinetic energy of the photoelectrons = 3.2 − 2.3 = 0.9 eV (1.4 × 10⁻¹⁹ J).

Practise each of these with 9702 topical questions, filtered to Paper 4, before you attempt full papers.

9. A six-week Paper 4 practice plan

• Weeks 1–3: start with the six hardest topics above, then cover the rest; one topic every two days using 9702 topical questions, Paper 4 filter, mark schemes interleaved.
• Weeks 4–5: two full timed papers per week from the 9702 past papers library.
• Week 6: redo every question you got wrong; drill definitions daily.

10. Time management

Roughly a mark a minute leaves about 20 minutes to check. Do not spend ten minutes on a 2-mark part — mark it, move on and return.

Practise by topic

Download topical PDFs of past-paper questions, with mark schemes: Gravitational fields · Electric fields · Capacitance · Magnetic fields · Alternating currents · Oscillations · Ideal gases · Thermodynamics · Quantum physics · Nuclear physics. Or browse all 9702 past papers.

Frequently asked questions

How long is 9702 Paper 4? Two hours, 100 marks, worth 38.5% of the A Level. The data and formulae are printed on pages 2 and 3 of the paper.

What is the hardest topic in 9702 Paper 4? It depends on the student, but fields, capacitance, electromagnetic induction, oscillations, thermal physics and quantum physics cause the most lost marks in our experience.

Is Paper 4 harder than Paper 2? It covers more abstract A Level content and longer chains of reasoning, but the technique is the same: precise definitions and clear working.

How many past papers should I do? Enough to cover every topic at least twice; quality of correction matters more than quantity.

Want structured help with Paper 4?

Engr. Muhammad Waseem works with A Level students on exactly these skills — definitions, derivations and calculation discipline. He has taught Cambridge A Level Physics since 2022.

Generate a Paper 4 topical PDF

Sources: Cambridge International AS & A Level Physics 9702 syllabus for examination in 2025–2027: assessment overview, topics 12–25, 16.2 (first law), 17 and 20.5, and the definitions quoted in section 2. https://www.cambridgeinternational.org/Images/664565-2025-2027-syllabus.pdf. Students sitting in 2028 or later should use the 2028–2030 syllabus.

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