IB DP Physics · Exam special · SL & HL · calculations, "explain" answers and multi-concept questions · ~8 min read
IB Physics Paper 2: How to Write Full-Mark Answers
How to write IB Physics Paper 2 answers that earn every mark: setting out calculations, "show that" questions, model answers for the "explain" questions examiners see go wrong, and how to tackle the long multi-concept questions.
Command termsCalculation layoutShow thatFree-body diagramsExplain answersModel answers by topicMulti-concept questionsPresentation
RSby Riyaz Sir
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mentorriyaz.in
No tag = SL and HLHL ONLY HL students onlyDATA BOOKLET given in the examM1A1 how marks are awardedhighlight = comes up again and againHANDY worth memorisingTest yourself at the end
In 30 seconds
- Paper 2 is 44% of your grade: 90 marks at HL and 50 at SL, all structured questions.
- The new syllabus ends Paper 2 with long multi-concept questions that mix several topics. In May 2025 these had the most blank answers.
- Examiners mark what is written. A wrong final answer with no working scores zero; a wrong answer with clear working can still earn most of the marks.
By the end you can
- Match the length of each answer to the command term and the marks.
- Set out calculations so every method mark is visible.
- Write "explain" answers with precise physics, not a rewording of the question.
- Tackle multi-concept questions part by part.
1Command terms decide the answer
| Command term | What examiners want | Typical marks |
|---|---|---|
| State | A name, value or short fact; no explanation | 1 |
| Outline | A brief account: one or two linked points | 2 |
| Describe | A detailed account of what happens (no reasons needed) | 2–3 |
| Explain | Reasons or causes: "because…", "so…" | 2–4 |
| Calculate / Determine | A numerical answer with working | 2–4 |
| Estimate | An approximate value, with the working shown | 2–3 |
| Show that | Every step to a given answer, finishing with more s.f. than given | 2–3 |
| Deduce / Suggest | A conclusion from given information; a plausible idea | 2–3 |
| Sketch | The shape and key features, labelled axes, no exact scale | 2–3 |
Exam tipCount the marks. A 3-mark "explain" needs three separate, correct physics points.
2Calculation layout
- Line 1: the equation, in symbols (from the data booklet or derived).
- Line 2: the equation with the numbers substituted, in SI units. This line earns the method mark; copying the equation alone doesn't.
- Line 3: the answer, with units and sensible significant figures (usually 2–3 s.f.).
Worked example
A 0.20 kg ball moves in a vertical circle of radius 0.50 m. At the top its speed is 4.0 m s⁻¹. Find the normal force from the track at the top.
- At the top both forces point down towards the centre: N + mg = mv²/r
- N + 0.20 × 9.81 = 0.20 × 4.0² ÷ 0.50 = 6.4
- N = 6.4 − 1.96 = 4.4 N
Common mistakeWriting N − mg = mv²/r at the top, or leaving out mg. Draw a free-body diagram first: the reports say this single step prevents most circular-motion errors.
- Decimals, not fractions or surds: write 0.67, not 2/3.
- Kelvin in every gas and thermal-radiation calculation: 27 °C = 300 K.
- Joules or electronvolts? Don't mix them. With photons, hc = 1240 eV nm saves time.
Worked example
Light of wavelength 250 nm falls on a metal with work function 2.3 eV. Find the maximum KE of the photoelectrons.
- E = hc/λ = 1240 ÷ 250 = 4.96 eV
- KEmax = E − Φ = 4.96 − 2.3 = 2.7 eV (≈ 4.3 × 10⁻¹⁹ J)
- Horizontal and vertical separately in projectile questions. Never put both into one suvat equation.
- Constant acceleration only. If the force or acceleration changes (a spring, a varying field), suvat is invalid: use energy, work or the area under a graph.
- Collisions: momentum is conserved in every collision; kinetic energy only in elastic ones. If in doubt, use momentum.
3"Show that" questions
- Work forwards from the data in the question; don't start from the given answer.
- Finish with one more significant figure than the given value, then the statement.
Worked example
Show that a ball dropped from 7.5 m hits the ground at about 12 m s⁻¹ (ignore air resistance).
- mgh = ½mv², so v = √(2gh)
- v = √(2 × 9.81 × 7.5) = √147.2
- v = 12.1 m s⁻¹ ≈ 12 m s⁻¹
Common mistakeInventing a number to make it work (a different g, an emissivity of 0.8, an extra factor of 10). Examiners spot it and award nothing for that step. If you can't get the given value, use it in the next part anyway.
4"Explain" answers
- Add physics the question didn't give you. Restating the stem ("the gases re-radiate energy back to the surface") scores nothing.
- Use the precise term: "constant phase difference", not "in sync"; "rate of change of flux linkage", not "the magnet moves".
- Link cause and effect with "so", "therefore", "which means".
Model answers
| Question | Points that score |
|---|---|
| Explain the greenhouse effect | The surface emits infrared; greenhouse gas molecules absorb IR (their natural frequency matches); they re-emit it in all directions, so some returns to the surface, which warms further |
| Explain why the wave model fails for the photoelectric effect | There is a threshold frequency, whatever the intensity; emission is instantaneous; max KE depends on frequency, not intensity. Each photon gives its energy hf to one electron |
| Explain why a constant current in a coil induces no emf in a nearby coil | A constant current gives a constant B, so the flux linkage doesn't change; emf needs a rate of change of flux (Faraday) |
| Explain how a star stays stable | The inward gravitational force is balanced by the outward pressure of the hot gas and radiation from fusion in the core |
| Outline the conditions for fusion in a star's core | Very high temperature (so nuclei move fast enough to overcome the electrostatic repulsion) and high density/pressure (so collisions are frequent) |
| Explain how we know a star contains helium | Its spectrum shows absorption lines at wavelengths that match helium's emission lines |
| Explain thermal conduction in a metal | Lattice vibrations pass energy between neighbouring ions, and free electrons carry energy quickly through the metal |
| Explain the role of the iron core in a transformer | It concentrates the magnetic flux and links it from the primary to the secondary coil; it doesn't carry the current |
| State the conditions for observable two-source interference | Coherent sources: same frequency and constant phase difference; similar amplitudes |
Common mistakeInterference "destroys" energy at the dark fringes. Energy is redistributed: bright fringes get more.
5Multi-concept questions
- They link two or more topics (for example a satellite's orbit and its solar-panel power, or a nuclear reaction and its thermal output).
- Each part can usually be started on its own. Given values ("show that…") let you continue; error carried forward (ECF) means a wrong earlier answer used correctly still scores.
- Make a quick list of the quantities you know and the one you need, then find the link in the data booklet.
- Budget the time. Leave about 25–30 minutes at HL (15–20 at SL) for these questions, so they're not left blank.
6Presentation
- Write inside the answer boxes. Writing outside them might not be scanned for marking.
- Clear digits: examiners flag 4 vs 9, 1 vs 7, and powers of ten (×10⁻³ written as ×10³).
- Cross out with one line. If you give two answers, only the first may be marked.
Linking questionRevise the topics behind these answers: A.2 Forces and momentum, B.2 Greenhouse effect, D.4 Induction, E.2 Quantum physics and E.5 Fusion and stars. See also Paper 1B data skills.
Test yourself — cover the answers first
Quick check
- What is the equation at the top of a vertical loop?
- Is kinetic energy conserved in an inelastic collision?
- How many s.f. should you give in "show that v ≈ 12"?
- Photon energy for λ = 620 nm in eV?
- Why does a constant current not induce an emf?
- What makes two sources coherent?
Show answers
- N + mg = mv²/r
- no; only momentum is conserved
- 3, e.g. 12.1
- 2.0 eV (1240 ÷ 620)
- the flux doesn't change
- a constant phase difference
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