This HL-only mini-lesson covers Theme D.4 — Electromagnetic induction: magnetic flux, Faraday's and Lenz's laws, motional emf, and generators and transformers.
Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Watch for the HL flag on higher-level extensions. Press Start when you're ready.
D.4 · flux
Magnetic flux
Magnetic flux Φ measures the field passing through a loop of area A; flux linkage for N turns is NΦ:
Φ = BA cosθθ is the angle between the field and the normal to the loop; unit weber (Wb)
Flux is maximum when the field is perpendicular to the plane of the coil (θ = 0, cosθ = 1) and zero when the field lies in the plane (θ = 90°).
Calculate
Calculate
#A field of 0.20 T passes perpendicularly through a coil of area 0.050 m². Find the magnetic flux.
Wb
Hint: Φ = BA cosθ, θ = 0 so cosθ = 1 → 0.20 × 0.050.
D.4 · Faraday & Lenz
Faraday's and Lenz's laws
A changing flux induces an emf. Faraday's law: the emf equals the rate of change of flux linkage. Lenz's law (the minus sign): the induced current opposes the change that causes it — this is energy conservation.
ε = −N (ΔΦ ÷ Δt)a faster change, or more turns, gives a larger induced emf
Worked example — a collapsing field
A 200-turn coil has its flux fall by ΔΦ = 0.010 Wb in Δt = 0.50 s.
ε = N ΔΦ ÷ Δt = 200 × 0.010 ÷ 0.50 = 4.0 V
Quick check
Quick check
?A magnet is pushed north-pole-first towards a coil. Which way does the induced current flow, and why?
Calculate
Calculate
#A 200-turn coil has its flux change by 0.010 Wb in 0.50 s. Find the magnitude of the induced emf.
V
Hint: ε = N ΔΦ ÷ Δt = 200 × 0.010 ÷ 0.50.
Calculate
Calculate
#A rod of length 0.20 m moves at 4.0 m s⁻¹ perpendicular to a 0.50 T field. Find the motional emf.
V
Hint: ε = BvL = 0.50 × 4.0 × 0.20.
D.4 · transformers
Generators & transformers
A rotating coil in a field is an AC generator — the flux linkage varies sinusoidally, inducing an alternating emf. A transformer changes AC voltage using two coils on an iron core:
V_s ÷ V_p = N_s ÷ N_pstep-up raises voltage (and lowers current); step-down does the reverse
A transformer only works with alternating current, because a steady current gives constant flux and hence no induced emf in the secondary.
Calculate
Calculate
#A transformer has 1000 primary turns and 100 secondary turns, with a 230 V input. Find the output voltage.
V
Hint: V_s = V_p × N_s ÷ N_p = 230 × 100 ÷ 1000.
Sort it
Which law or device does this describe?
Tap an item, then tap the group it belongs to.
⚡ Faraday's law
🛑 Lenz's law
🔌 Transformer
Match it
Match the expression to its name
Tap a statement on the left, then its match on the right.
Statement
Answer
Recap
The big ideas to know
Flux: Φ = BA cosθ; max when field ⟂ coil plane
Faraday: ε = −N ΔΦ/Δt; change is essential
Lenz: induced current opposes the change (energy conservation)
Motional emf: ε = BvL for a rod cutting field lines
Transformer: V_s/V_p = N_s/N_p; AC only
That completes Electromagnetic Induction for IB Diploma Physics HL. Press Finish to see your score.
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