Physics 9702 · syllabus 20
Cambridge A Level Physics 9702 — Magnetic fields topical past papers
Magnetic forces on currents and moving charges, the Hall effect and velocity selection, and electromagnetic induction from Faraday's and Lenz's laws.
- Questions
- 180
- Years
- 2009–2026
- With mark scheme
- 100%
- Questions 2022–2026
- 33
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What Magnetic fields covers
A magnetic field is produced by moving charges or by a permanent magnet, and you need the field patterns of a long straight wire, a flat circular coil and a long solenoid, whose field a ferrous core strengthens. A current-carrying wire in a field feels a force F = BIL sin θ, in a direction given by Fleming's left-hand rule, and this defines the tesla. Two parallel currents therefore push or pull on each other: they attract when the currents flow the same way and repel when they flow in opposite directions.
A single charge moving through a field feels F = BQv sin θ, at right angles to its velocity, so a particle moving perpendicular to a uniform field follows a circle of radius r = mv/(BQ). Crossing an electric field with a magnetic field lets only particles of speed v = E/B through undeflected, which is velocity selection. The same sideways force on the carriers in a thin slice of conductor produces the Hall voltage, which a Hall probe uses to measure flux density.
Electromagnetic induction runs the other way. Magnetic flux is Φ = BA, flux linkage is NΦ, and Faraday's law says the induced e.m.f. equals the rate of change of flux linkage; Lenz's law says the e.m.f. acts to oppose the change that produces it, a consequence of energy conservation. This is A Level content, examined on Paper 4, and it draws on circular motion, electric fields and D.C. circuits.
In the syllabus
The headings this topic has in the Cambridge International 2025–2027 syllabus, as the board prints them.
- 20 Magnetic fields
- 20.1 Concept of a magnetic field
- 20.2 Force on a current-carrying conductor
- 20.3 Force on a moving charge
- 20.4 Magnetic fields due to currents
- 20.5 Electromagnetic induction
What you should be able to do
- Sketch the field patterns of a straight wire, a flat coil and a solenoid, and explain the forces between parallel currents
- Use F = BIL sin θ and F = BQv sin θ with Fleming's left-hand rule
- Explain the circular path of a charged particle in a uniform field and calculate its radius
- Derive and use the Hall voltage equation, and explain how a Hall probe measures flux density
- Explain how crossed electric and magnetic fields select particles of a single speed
- Use Faraday's and Lenz's laws to find the size and direction of an induced e.m.f.
Key ideas
- Force on a wire F = BIL sin θ; 1 T = 1 N A⁻¹ m⁻¹
- Force on a moving charge F = BQv sin θ; radius of the circular path r = mv/(BQ)
- Velocity selector: qE = Bqv, so v = E/B
- Hall voltage = BI/(ntq), where t is the thickness of the slice and n the number density of charge carriers
- Flux Φ = BA (Wb); induced e.m.f. = rate of change of flux linkage, E = −Δ(NΦ)/Δt
How Magnetic fields has been examined
From 2022 to 2026, Magnetic fields was on Paper 4 · A Level Structured Questions in 11 of 11 exam series, worth about 11% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 4 · A Level Structured Questions | 33 | 11% | 11 of 11 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 11.
Filed in Momentum under: Magnetic Fields, Electromagnetic Induction.
A worked example
Protons pass undeflected through a region where an electric field of 6.0 × 10⁴ V m⁻¹ is perpendicular to a magnetic field of 0.020 T. They then enter a region containing only a uniform magnetic field of 0.10 T, at right angles to their velocity. Find their speed and the radius of their path in the second region.
- Undeflected: electric force = magnetic force, eE = Bev, so v = E/B = 6.0 × 10⁴ / 0.020 = 3.0 × 10⁶ m s⁻¹
- In the second region the magnetic force provides the centripetal force: BQv = mv²/r
- r = mv/(BQ) = 1.67 × 10⁻²⁷ × 3.0 × 10⁶ / (0.10 × 1.60 × 10⁻¹⁹)
- r = 5.01 × 10⁻²¹ / 1.60 × 10⁻²⁰ = 0.31 m
Answer: Speed 3.0 × 10⁶ m s⁻¹; radius 0.31 m
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Applying Fleming's left-hand rule to an electron beam without reversing the current — conventional current flows opposite to the electrons' motion
- Saying the magnetic force on a moving charge speeds it up — the force is always perpendicular to the velocity, so it does no work; the speed stays constant and only the direction changes
- Forgetting N when calculating the e.m.f. induced in a coil — the e.m.f. depends on the rate of change of flux linkage NΦ, not of flux alone
- Stating Lenz's law as 'the induced current opposes the magnetic field' — it opposes the change in flux linkage that produced it
- Using the full coil area when the field passes through the coil at an angle — only the component of B perpendicular to the coil's plane contributes to the flux
Exam technique
- For direction questions, name the rule you used and what each finger represents in that case; a direction with no reasoning may not score
- When explaining an induction experiment, link three steps in order: the flux linkage changes, an e.m.f. is induced, and its size depends on the rate of change
- For a circular path, write the force balance BQv = mv²/r before rearranging, so the method is visible
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.