Q: What does A-Level Physics: 18) Electromagnetic Induction Guide cover? A: Magnetic flux, Faraday's law, Lenz's minus sign and the transformer equation - this post unpacks Section I Topic 1.18 of the 2026 H2 Physics syllabus.
TL;DR Induction is the bridge between electricity and magnetism that powers bike dynamos, phone chargers and the entire national grid. For Paper 2 and Paper 4, memorise ϕ=BA, drill the chain rule for E=−dtd(Nϕ)
, and practise sketching transformer energy flow diagrams under timed conditions.
1 Magnetic flux (ϕ) and flux linkage
1.1 Definition
Magnetic flux is the product of magnetic flux density (B) and the area (A)perpendicular to that field:
ϕ=BA
1.2 Flux linkage
For a coil with N identical turns, the magnetic flux linkage is
Nϕ=NBA.
IP Exam Cue: The unit for flux is the weber (Wb=T⋅m2). Always show the superscript “2” - examiners dock marks for m2 written as m.
2 Faraday's and Lenz's laws
2.1 Faraday's law
A changing magnetic flux linkage induces an e.m.f.:
E=−dtd(Nϕ).
The magnitude is proportional to the rate of change of Nϕ.
2.2 Lenz's law
The negative sign shows that the induced e.m.f. acts to oppose the change producing it - a direct consequence of energy conservation.
2.3 Required practical
Push a bar magnet into a coil connected to a galvanometer: the needle deflects; pull it out and the needle swings the opposite way. The faster you move, or the stronger the magnet, the larger the deflection.
Flux linkage changes drive induced e.m.f. Use ϕ=BA to compute flux, then multiply by turns N. Faraday's law links the rate of change to induced voltage via E=−dtd(Nϕ); Lenz's law fixes the opposition direction. In moving conductors, the shortcut E=BLv applies. Transformers rely on alternating flux sharing a core between windings.
Key relations
Relation
Comment
\( \phi = B A \cos\theta \)
Area component must be perpendicular to \( \mathbf{B} \) to count toward flux.
\( N\phi = N B A \cos\theta \)
Multiply single-coil flux by turn count for total linkage.
Efficiency expressed as a percentage of input power transmitted.
Derivations & reasoning to master
Bar cutting flux: derive E=BLv from changing linkage or Lorentz-force arguments.
Rotating coil waveform: compute ϕ=BAcosθ, differentiate to recover the sinusoidal e.m.f.
Lenz's minus sign: articulate energy conservation argument to justify opposition.
Transformer losses: relate eddy currents, hysteresis, copper losses to design features (laminations, soft iron, thick windings).
Worked example 1 - induced e.m.f. in rotating coil
A 200-turn coil of area 0.015m2 rotates at 25Hz in a 0.35T uniform field. Find the maximum induced e.m.f. and write its time dependence.
Solution: E0=NBAω with ω=2πf. Therefore E(t)=E0sin(2πft).
Worked example 2 - transformer efficiency
A transformer steps 240V down to 12V for a 2.0A load. The primary current is 0.12A. Determine efficiency and estimate copper loss if the efficiency is 92%.
Method: Calculate Pout=VsIs and Pin=VpIp; the difference approximates copper loss through I2R.
Practical & data tasks
Use datalogger to record induced e.m.f. vs time when dropping a magnet through a coil; integrate area to estimate flux change.
Build a simple generator with hand crank and LED; relate speed to brightness (frequency).
Perform transformer experiment measuring VpVs vs NpNs; evaluate deviations and discuss causes.
Common misconceptions & exam traps
Forgetting the cosine factor in ϕ=BAcosθ when the area is tilted.
Dropping the negative sign in E=−dtd(Nϕ) (lose credit for direction).
Confusing flux ϕ (webers) with flux density B (tesla).
Assuming transformers work with DC (they require changing flux).
Quick self-check quiz
Define magnetic flux. - Product of magnetic flux density and perpendicular area ϕ=BA.
What does Lenz's law state? - Induced e.m.f. acts to oppose the change causing it.
How can you increase induced e.m.f. in a generator? - Increase N, A, B or rotation speed so ∣E∣ grows.
Why are transformer cores laminated? - To reduce eddy current losses.
If a conductor moves parallel to magnetic field lines, what is induced e.m.f.? - Zero: E=0 (no flux change).
Revision workflow
Re-derive E=BLv and the rotating-coil waveform E(t)=NBAωsin(ωt) weekly to keep intuition sharp.
Practise interpreting flux linkage graphs and calculating gradients quickly.
Summarise transformer loss mechanisms (I2R, eddy currents, hysteresis) and their mitigation strategies on a cheat sheet.
Attempt past practical questions involving induced e.m.f. measurements and graph analysis.
Practice Quiz
Test yourself on the key concepts from this guide.
