A-Level Physics: Magnetic Fields and Induction

Magnetic fields and electromagnetic induction close the A2 fields sequence, and they carry some of the paper's most reliable calculation marks — provided the angles, signs and laws are kept straight. This quiz drills exactly those pressure points.

The magnetic-force questions apply both force laws with real values: F = BIl sin θ for a wire at 30° to the field (0.40 N — a calculation the automated verifier itself got wrong during checking, double-applying the sine; the key was confirmed by manual computation, which says something about how easy the slip is), and the qualitative physics of F = BQv — why the magnetic force on a moving proton does no work, changes no kinetic energy, and produces circular motion by acting as a centripetal force.

The induction questions build from definitions to dynamics: flux linkage for a 50-turn coil in a stated field, Faraday's law applied to a changing flux through 200 turns (16 V, worked in full), the EMF across a rod moving through a field (Blv), which changes genuinely induce an EMF at all, and Lenz's law traced to its foundation — conservation of energy. A generator question identifies every factor that raises peak EMF: frequency, flux density, turns.

The transformer questions complete the set: the turns-ratio relation worked as a statement selection on a 1000:200 step-down transformer, and an efficiency calculation recovering secondary current from input power and output voltage at 90% efficiency.

All values are supplied in the questions, consistent with the exam's data booklet, and each explanation names the law being invoked before substituting — the habit that keeps six-mark induction explanations coherent.

• Apply F = BIl sin θ and F = BQv with given values and angles
• Explain circular motion of charges in magnetic fields via centripetal force
• Compute flux linkage and apply Faraday's law to changing flux
• Ground Lenz's law in conservation of energy and identify EMF-raising factors
• Work transformer turns-ratio and efficiency calculations

Topic scope follows section 3.7.5 (Magnetic fields) of the current A-level physics subject content: force on current-carrying conductors and moving charges, flux and flux linkage, electromagnetic induction, Faraday's and Lenz's laws, AC generators and transformers.