CCEA A Level Physics
CCEA A Level Physics 5.5 Magnetic Fields Exam Questions
Free CCEA A Level Physics revision for 5.5 Magnetic Fields: what you need to know, common mistakes and exam-style questions with answers. Covers field patterns, F = BIl, motors, flux linkage, faraday's and lenz's laws, generators, transformers and power transmission.
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About CCEA A Level Physics 5.5 Magnetic Fields
Magnetic fields is the largest section of Unit A2 2: Fields, Capacitors and Particle Physics. You need field patterns around straight wires and coils, the force on a current-carrying conductor (F = BIl) and its use in the electric motor. Electromagnetic induction follows: magnetic flux and flux linkage, Faraday's law and Lenz's law, the a.c. generator and the transformer. The section finishes with power losses in transmission cables and why electricity is transmitted at high voltage. Examiners set F = BIl calculations, induced e.m.f. from a changing flux linkage, transformer ratio problems, and explanations of Lenz's law as energy conservation.
What you need to know
- Field patterns Concentric circles around a straight wire (right-hand grip rule gives direction); a solenoid has a uniform field inside, like a bar magnet outside.
- Force on a conductor F = BIl when the wire is perpendicular to the field (BIl sin θ at angle θ). B is magnetic flux density in tesla. Fleming's left-hand rule gives the direction; this force turns the coil of an electric motor.
- Flux and flux linkage Magnetic flux Φ = BA (A perpendicular to B), unit weber. Flux linkage = NΦ for a coil of N turns.
- Faraday's and Lenz's laws Induced e.m.f. equals the rate of change of flux linkage, E = -Δ(NΦ)/Δt. Lenz's law: the induced current flows in a direction that opposes the change producing it, a consequence of conservation of energy.
- Generator and transformer A coil rotating in a field gives a sinusoidal e.m.f. A transformer uses an alternating current in the primary to create a changing flux in the core: Vs/Vp = Ns/Np, and for an ideal transformer VpIp = VsIs.
- Transmission Power lost in cables is I^2R. Stepping up to a high voltage for the same power lowers the current and greatly reduces losses; it is then stepped down for consumers.
Common mistakes in 5.5 Magnetic Fields
- Using flux density B where the question needs flux Φ or flux linkage NΦ.
- Stating Faraday's law as e.m.f. proportional to flux; it is proportional to the rate of change of flux linkage.
- Explaining Lenz's law without linking it to conservation of energy or opposing the change.
- Saying transformers work on d.c.; a steady current gives no changing flux and so no induced e.m.f.
- Calculating cable losses as V^2/R using the transmission voltage; the p.d. across the cable is much smaller, so use I^2R.
Try three 5.5 Magnetic Fields exam-style questions
1. A wire 0.15 m long carries 3.0 A at right angles to a field of 0.40 T. Calculate the force on it. (2 marks)
Show the mark scheme answer
F = BIl = 0.40 x 3.0 x 0.15; = 0.18 N.
2. A coil of 200 turns and area 4.0 x 10^-3 m^2 is in a field of 0.50 T perpendicular to its plane. The field falls to zero in 0.10 s. Calculate the average induced e.m.f. (3 marks)
Show the mark scheme answer
Initial flux linkage = 200 x 0.50 x 4.0 x 10^-3 = 0.40 Wb turns; change = 0.40 Wb turns; e.m.f. = 0.40/0.10 = 4.0 V.
3. Explain why electrical power is transmitted at high voltage. (3 marks)
Show the mark scheme answer
For a given power P = IV, a higher voltage means a smaller current; power lost in cables is I^2R; so losses are much lower (halving current quarters the loss).
5.5 Magnetic Fields FAQs
What does CCEA A Level Physics 5.5 Magnetic Fields cover?
Field patterns of wires and coils, F = BIl and the electric motor, flux and flux linkage, Faraday's and Lenz's laws, the a.c. generator, transformers and high-voltage power transmission.
Which unit is 5.5 examined in?
It is in Unit A2 2: Fields, Capacitors and Particle Physics.
How do I explain Lenz's law well?
Say the induced current produces a field opposing the change in flux, so work must be done to keep the change going; that work becomes electrical energy, which is why Lenz's law follows from conservation of energy.
