CIE A Level Physics
CIE A Level Physics Magnetic Fields Exam Questions
Free CIE A Level Physics exam questions for Magnetic Fields, with a mark scheme for every pack. Covers magnetic field patterns, force on currents and moving charges, Hall voltage, velocity selection, magnetic flux and electromagnetic induction.
Magnetic Fields exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics Magnetic Fields
Magnetic fields is Topic 20 of the Cambridge International AS & A Level Physics (9702) syllabus. It covers fields produced by magnets and currents, the force on a current-carrying conductor and on a moving charge, the Hall effect, and crossed electric and magnetic fields used for velocity selection. It then introduces magnetic flux, flux linkage, and Faraday's and Lenz's laws of electromagnetic induction. The topic links to Motion in a circle and leads into Alternating currents. Examiners often ask for the definition of the tesla, the radius of a charged particle's path and induced e.m.f. calculations.
What you need to know
- Magnetic fields A magnetic field is a region where a magnet, a current-carrying conductor or a moving charge experiences a force. Know the field patterns for a long straight wire, a flat circular coil and a solenoid, and how an iron core strengthens the field.
- Force on a conductor F = BIL sin θ, with direction given by Fleming's left-hand rule. One tesla is the flux density that causes a force of 1 N on a 1 m wire carrying 1 A at right angles to the field.
- Force on moving charges F = BQv sin θ. A charged particle moving perpendicular to a uniform field moves in a circle, with r = mv/BQ.
- Hall voltage and velocity selection Hall voltage VH = BI/(ntq), so it is larger for materials with low n such as semiconductors. In crossed fields, particles with v = E/B pass undeflected.
- Currents and forces Parallel wires carrying currents in the same direction attract; opposite directions repel.
- Electromagnetic induction Magnetic flux Φ = BA (with A normal to B) in webers; flux linkage = NΦ. Faraday's law: induced e.m.f. equals rate of change of flux linkage. Lenz's law: the induced e.m.f. opposes the change that produces it.
Common mistakes in Magnetic Fields
- Using the right-hand rule for motors. Use Fleming's left-hand rule for force, remembering current is in the direction of positive charge flow.
- Saying a magnetic force does work on a charged particle moving in a circle. The force is perpendicular to velocity, so speed and kinetic energy are constant.
- Confusing flux density B with flux Φ. Flux is BA, and flux linkage multiplies by the number of turns N.
- Explaining Lenz's law without linking it to conservation of energy.
- Forgetting the sin θ term when the wire or velocity is not perpendicular to the field.
Try three Magnetic Fields exam-style questions
1. A wire of length 0.25 m carries a current of 3.0 A at right angles to a uniform field of flux density 0.080 T. Calculate the force on the wire. (1 mark)
Show the mark scheme answer
F = BIL = 0.080 × 3.0 × 0.25 = 0.060 N.
2. An electron moving at 2.0 × 10^7 m s^-1 enters a uniform magnetic field of 5.0 mT at right angles. Calculate the radius of its circular path. (3 marks)
Show the mark scheme answer
Bev = mv^2/r; r = mv/(Be) = 9.11 × 10^-31 × 2.0 × 10^7/(5.0 × 10^-3 × 1.60 × 10^-19); r = 0.023 m.
3. A coil of 200 turns and area 4.0 × 10^-3 m^2 is in a field of 0.50 T perpendicular to its plane. The field falls uniformly to zero in 0.10 s. Calculate the average induced e.m.f. (2 marks)
Show the mark scheme answer
Change in flux linkage = NBA = 200 × 0.50 × 4.0 × 10^-3 = 0.40 Wb; e.m.f. = 0.40/0.10 = 4.0 V.
Magnetic Fields FAQs
What does Cambridge AS & A Level Physics Magnetic fields cover?
It covers magnetic field patterns, forces on current-carrying conductors and moving charges, the Hall effect, velocity selection, forces between currents, magnetic flux, and Faraday's and Lenz's laws.
Which paper is Magnetic fields examined in?
It is an A Level topic assessed in Paper 4 (A Level Structured Questions).
What equations do I need for Magnetic fields?
Learn F = BIL sin θ, F = BQv sin θ, VH = BI/(ntq), v = E/B, Φ = BA and induced e.m.f. = rate of change of NΦ.
