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 V Topic 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.
Keep the induction toolkit tied to the rest of the electromagnetism arc via the H2 Physics notes hub; it bundles this chapter with electromagnetism, AC circuits, and Modern Physics extensions.
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.
2.4 What affects E?
Factor
Why it matters
Speed of motion
Faster change increases dtdϕ.
Number of turns
More turns raise Nϕ.
Coil area
Larger A intercepts more field lines.
Field strength
Bigger B gives bigger ϕ.
Timing hack: In WA calculations, write the chain rule in one line to earn a method mark even if algebra slips.
3 Power transformers
3.1 Principle of operation
Two coils share a laminated soft-iron core, enabling almost all magnetic flux from the primary to link the secondary via mutual induction.
3.2 Ideal transformer equation
Assuming perfect coupling and zero losses:
VpVs=NpNs.
3.3 Efficiency and real-world tweaks
Thin insulated laminations in the core slash eddy-current losses and cut heating.
High-grade silicon steel further reduces hysteresis loss.
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
ϕ=BAcosθ
Area component must be perpendicular to B to count toward flux.
Nϕ=NBAcosθ
Multiply single-coil flux by turn count for total linkage.
E=−dtd(Nϕ)
Minus sign encodes Lenz's law; magnitude follows the change rate.
E=BLv
For a length L moving perpendicular to B at speed v.
E(t)=NBAωsin(ωt)
Faraday's law applied to a rotating coil of angular speed ω.
VpVs=NpNs
VpIp=VsIs
η=PinPout×100%
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.
