This mini-lesson walks you through the whole of Edexcel Topic 13 — Electromagnetic induction: the generator effect, alternators & dynamos, the microphone, transformers and their equations, and the National Grid.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. The purple HT tags mark Higher-Tier-only ideas. Press Start when you're ready.
Move a magnet and a conductor relative to one another — so the conductor experiences a changing magnetic field — and a potential difference is induced across it. If the conductor is part of a complete circuit, that p.d. drives an induced current. This is the generator effect.
Watch out: there must be relative motion / a changing field. Hold the magnet still inside the coil and the field is steady — no p.d. is induced, even though the magnet is right there.
Spin a coil in a magnetic field and the generator effect produces a current. The difference between an alternator and a dynamo is only how the coil connects to the circuit. [13.3P]
Key contrast: slip rings keep each brush on the same end of the coil, so the output swaps direction (a.c.). A split-ring swaps the connections every half-turn, so the output never reverses (d.c.).
Tap a feature on the left, then tap the generator it belongs to.
A moving-coil microphone uses the generator effect to turn sound into a varying electrical signal. [13.4P]
A transformer has two coils — a primary and a secondary — wound on the same iron core. It can change the size of an alternating voltage. [13.5, 13.6]
How it works: the a.c. in the primary coil makes a continually changing magnetic field. The iron core channels this changing flux through the secondary coil, where it induces an alternating p.d. — this is mutual induction.
Misconception alert: a transformer needs a changing field, so it works on a.c. only. Steady d.c. gives a constant flux → nothing is induced in the secondary (except a brief blip when you switch on or off).
The voltages and the numbers of turns are linked by the turns-ratio equation: [13.7P]
Analogy: a transformer is like gears for voltage. Just as gears trade speed for turning force, a transformer trades voltage for current — turn the voltage up and the current goes down (and vice-versa), so the power stays (nearly) the same.
A transformer has Np = 200 turns, Ns = 1000 turns and Vp = 12 V. Find Vs.
Vs = Vp × (Ns / Np) = 12 × (1000 / 200) = 12 × 5 = 60 V (a step-up).
The same equation can be rearranged to find a missing number of turns instead of a voltage:
Np = 100, Vp = 20 V, and you need Vs = 5 V.
Ns = 100 × (5 / 20) = 100 × 0.25 = 25 turns (a step-down).
For an ideal (100%-efficient) transformer, no power is lost, so the power going in equals the power coming out (remember P = I × V): [13.10]
This is why a step-up transformer that raises the voltage must lower the current by the same factor — and a step-down does the reverse.
A transformer has Vp = 230 V, Ip = 2 A, and Vs = 23 V. Find Is.
Is = (Vp × Ip) / Vs = (230 × 2) / 23 = 460 / 23 = 20 A.
The National Grid carries electrical energy from power stations to homes. Transformers change the voltage along the way. [13.8, 13.9]
Transmission cables have resistance, so a current heats them and wastes energy. The power wasted as heat is given by P = I²R. [13.11P]
Because the loss depends on the current squared, even a small drop in current makes a big drop in wasted power. From P = I × V, sending the power at a high voltage means a low current — which is exactly what a step-up transformer does before transmission.
Misconception alert: the high voltage is not there to "push harder" — its job is to make the current low, because it is the I²R heating in the wires (not the voltage) that wastes energy. Halving the current cuts the cable loss to a quarter.
Tap a statement, then tap the box it belongs in.
Generator effect: a changing field / relative motion induces a p.d. (and current if in a circuit). [HT]
Bigger induced p.d.: more speed, stronger field, more turns. [HT]
Alternator = slip rings → a.c.; dynamo = split-ring → d.c. [HT]
Microphone: sound vibrates a coil near a magnet → varying p.d. (reverse of a loudspeaker).
Transformer (a.c. only): changing flux in an iron core → mutual induction. [HT]
Turns ratio: Vp / Vs = Np / Ns. [HT]
Power (ideal): Vp × Ip = Vs × Is. [HT]
National Grid: step-up for transmission, step-down for homes.
High V → low I → small I²R loss in the cables. [HT]
You've covered all of Edexcel Topic 13 — Electromagnetic induction. Press Finish to see your score.
You've worked through Electromagnetic Induction for Edexcel GCSE Physics. 🎉
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Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.