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.
Make a guess: where does the bulb's current come from?
Mission 1Where does the current come from?
Mission 2Why send electricity at deadly voltages?
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.
The generator effectHT
Inducing a potential difference
Move a magnet and a conductor relative to one another — so the conductor experiences a changing magnetic field — and a potential difference (a voltage) 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.
It works in the lab (pushing a magnet into a coil) and on a large scale (power-station generators). [13.1P]
The induced p.d. is bigger if you increase the speed of movement, the strength of the magnetic field, or the number of turns on the coil. [13.2]
The induced current always flows so that the magnetic field it produces opposes the change that caused it (Lenz's idea) — pushing the magnet in is resisted. [13.2]
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.
Quick check
When is a p.d. induced?
?A bar magnet is held completely still deep inside a coil connected to a galvanometer. What does the galvanometer read?
Alternators & dynamosHT
Generating a.c. and d.c.
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]
The alternator uses slip rings → a.c. (smooth sine that reverses). The dynamo uses a split-ring commutator → d.c. (bumps, but always one direction).
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.).
Match it
Generator features
Tap a feature on the left, then tap the generator it belongs to.
Mission 1 check
Where does the current come from?
?In a hand-crank lamp, turning a handle spins a magnet next to a coil. The coil is joined to a bulb. There is no battery. Why does the bulb light while the handle turns?
The microphone
Sound → induced current
A moving-coil microphone uses the generator effect to turn sound into a varying electrical signal. [13.4P]
Sound waves are pressure variations in the air. They make a thin diaphragm vibrate.
The diaphragm is attached to a coil that sits around a permanent magnet, so the coil moves through the magnetic field.
This relative motion induces a varying potential difference in the coil — an electrical copy of the sound.
A microphone is the reverse of a loudspeaker: a loudspeaker turns a current into sound; a microphone turns sound into a current.Quick check
How a microphone works
?In a moving-coil microphone, what is the role of the incoming sound wave?
Mission 2 of 2
Why send electricity at deadly voltages?
Make a guess: why send electricity at such a high voltage?
TransformersHT
Mutual induction
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]
The a.c. in the primary makes a changing magnetic flux in the iron core, which induces an alternating p.d. in the secondary.
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).
Equation 1 · turns ratioHT
The transformer equation
The voltages and the numbers of turns are linked by the turns-ratio equation: [13.7P]
A step-up transformer has more turns on the secondary (Ns > Np) → it increases the voltage.
A step-down transformer has fewer turns on the secondary (Ns < Np) → it decreases the voltage.
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.
Worked example
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).
Calculate
Your turn — turns ratio
1A transformer has a primary p.d. of 230 V and 460 turns on the primary. The secondary has 40 turns. Calculate the secondary p.d.
V
Hint: Vs = Vp × (Ns ÷ Np) = 230 × (40 ÷ 460).
Equation 1 · rearrangedHT
Finding a number of turns
The same equation can be rearranged to find a missing number of turns instead of a voltage:
Ns = Np × (Vs / Vp)rearranged from Vp / Vs = Np / Ns
2A step-up transformer takes 25 V on a 50-turn primary and must output 200 V. How many turns are needed on the secondary?
turns
Hint: Ns = Np × (Vs ÷ Vp) = 50 × (200 ÷ 25).
Equation 2 · powerHT
The power relationship
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]
Vp × Ip = Vs × Isprimary p.d. × primary current = secondary p.d. × secondary current
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.
Worked example
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.
Calculate
Your turn — Vp Ip = Vs Is
3A 100%-efficient transformer has a primary p.d. of 240 V drawing 0.5 A. The secondary p.d. is 12 V. Calculate the secondary current.
A
Hint: Is = (Vp × Ip) ÷ Vs = (240 × 0.5) ÷ 12.
The National Grid
Moving electricity around the country
The National Grid carries electrical energy from power stations to homes. Transformers change the voltage along the way. [13.8, 13.9]
Power station → step-up transformer → pylons (high V, low I) → step-down transformer → homes (about 230 V).
A step-up transformer raises the voltage to transmit across the country efficiently.
A step-down transformer lowers it to a safer ~230 V for local domestic use.
Why high voltage?HT
Reducing transmission losses
Transmission cables have resistance, so a current heats them and wastes energy. The power wasted as heat is given by P = I²R. [13.11P]
power lost in cables = I² × RP = I²R — current is squared, so it dominates the loss
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.
Quick check
Why transmit at high voltage?
?Electricity is sent across the country at very high voltage. What is the main reason?
Sort it
Step-up or step-down?
Tap a statement, then tap the box it belongs in.
⤴ Step-up
⤵ Step-down
Mission 2 check
Why send electricity at deadly voltages?
?A wind farm sends the same power to a city along the same cables. It could transmit at 33 000 V or at 132 000 V. Which should it choose, and why?
Recap
The key ideas & equations
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]