OxfordAQA International A Level Physics
OxfordAQA International A Level Physics 3.10 Magnetic Fields Exam Questions
Free OxfordAQA International A Level Physics revision for 3.10 Magnetic Fields: what you need to know, common mistakes and exam-style questions with answers. Covers magnetic flux density, force on conductors and moving charges, flux linkage, faraday's and lenz's laws, alternating current, oscilloscopes and transformers.
Practise 3.10 Magnetic Fields with past papers
Once you have learnt this topic, find the 3.10 Magnetic Fields questions in real OxfordAQA International A Level Physics past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
StemUp mark scheme flashcards
A-Level Physics Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £10.99View flashcards
About OxfordAQA International A Level Physics 3.10 Magnetic Fields
Magnetic fields is the final section of Unit 3 (PH03), Fields and their consequences, in the OxfordAQA International A2 year. You calculate the force on a current-carrying wire and on moving charges, describe circular paths of charged particles and the cyclotron, then study magnetic flux, flux linkage and electromagnetic induction using Faraday's and Lenz's laws. The section ends with alternating current, oscilloscope readings and transformers, including power transmission. Examiners regularly ask for the radius of a charged particle's path, the emf induced in a moving rod or rotating coil, explanations using Lenz's law, rms values, and transformer and transmission loss calculations.
What you need to know
- Force on a conductor F = BIL when the wire is perpendicular to the field. The direction is given by Fleming's left-hand rule. One tesla is the flux density that gives a force of 1 N on 1 m of wire carrying 1 A at right angles to the field.
- Force on moving charges F = BQv. The force is perpendicular to the velocity, so a charged particle moves in a circle of radius r = mv / BQ. This is the principle of the cyclotron.
- Flux and flux linkage Magnetic flux Phi = BA (in webers) when the field is normal to the area; flux linkage = N Phi. For a coil at angle theta, N Phi = BAN cos theta.
- Faraday's and Lenz's laws The induced emf equals the rate of change of flux linkage, epsilon = N delta Phi / delta t. Lenz's law: the induced current opposes the change that produces it. A coil rotating at angular speed omega gives epsilon = BAN omega sin omega t.
- Alternating current Irms = I0 / square root 2 and Vrms = V0 / square root 2. Read peak and peak-to-peak values and time period from an oscilloscope using its Y-gain and time-base settings.
- Transformers Ns / Np = Vs / Vp for an ideal transformer, efficiency = IsVs / IpVp. Losses come from resistance of the coils, eddy currents (reduced by a laminated core) and magnetic flux leakage. Transmitting power at high voltage reduces the current and so the I^2 R losses.
Common mistakes in 3.10 Magnetic Fields
- Using the right-hand rule for the force on a wire. Fleming's left-hand rule gives the motor effect force.
- Saying the magnetic force on a moving charge does work. It is always perpendicular to the velocity, so the speed is constant.
- Confusing flux with flux linkage. Flux linkage is N times the flux and is what Faraday's law uses.
- Saying a transformer works with direct current. A changing current is needed to produce a changing flux and induce an emf.
- Quoting a peak value when the question asks for the rms value of mains voltage, or vice versa.
Try three 3.10 Magnetic Fields exam-style questions
1. A proton travels at 2.0 x 10^6 m s-1 perpendicular to a uniform magnetic field of 0.50 T. Calculate the radius of its path. Mass of proton 1.67 x 10^-27 kg, charge 1.60 x 10^-19 C. (2 marks)
Show the mark scheme answer
r = mv / BQ = 1.67 x 10^-27 x 2.0 x 10^6 / (0.50 x 1.60 x 10^-19); r = 4.2 x 10^-2 m.
2. An ideal transformer steps 230 V down to 12 V. The primary coil has 2300 turns. Calculate the number of turns on the secondary. (2 marks)
Show the mark scheme answer
Ns = Np x Vs / Vp = 2300 x 12 / 230; Ns = 120 turns.
3. A magnet is pushed into a coil connected to a meter. Use Lenz's law to explain the direction of the induced current and why work must be done on the magnet. (3 marks)
Show the mark scheme answer
The changing flux induces an emf and current in the coil; the current makes the end of the coil facing the magnet the same pole as the approaching pole, opposing the change; so the magnet is repelled and work must be done to push it in, which is converted to electrical energy.
3.10 Magnetic Fields FAQs
What does OxfordAQA International A Level Physics 3.10 Magnetic Fields cover?
It covers magnetic flux density, the force on wires and moving charges, circular motion of charged particles, flux linkage, electromagnetic induction, alternating current and oscilloscopes, and transformers and power transmission.
Which unit is 3.10 Magnetic Fields examined in?
It is examined in Unit 3 (PH03), Fields and their consequences.
How should I revise electromagnetic induction?
Learn Faraday's and Lenz's laws word for word, practise flux linkage calculations including the cos theta factor, and work through examples of a rod moving in a field and a rotating coil generator.
