Home › Learning Hub › IGCSE Physics › 4f Induction and transformers
Topic 4 · 4.5.1, 4.5.2, 4.5.6

Induction, generators and transformers

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Know that moving a conductor across a field, or changing the field linking it, induces an e.m.f.; describe an experiment and the factors affecting its size.
  • Know that the induced e.m.f. opposes the change causing it; use the relative directions of force, field and induced current EXTENDED.
  • Describe a simple a.c. generator with slip rings and brushes, and interpret its e.m.f.–time graph EXTENDED.
  • Describe transformers; use \( \dfrac{V_p}{V_s} = \dfrac{N_p}{N_s} \); describe high-voltage transmission and its advantages.
  • Explain how a transformer works; use \( I_pV_p = I_sV_s \) and \( P = I^2R \) EXTENDED.

📚The physics

Electromagnetic induction

An e.m.f. is induced in a conductor when it moves across a magnetic field, or when the magnetic field linking it changes. If the conductor is part of a complete circuit, a current flows.

A bar magnet and a coil connected to a sensitive meter. Moving the N pole into the coil makes the needle deflect; with the magnet stationary there is no reading; pulling the magnet out deflects the needle the other way.
No movement, no change, no e.m.f.

The size of the induced e.m.f. increases with a stronger magnet, faster movement (a faster change) and more turns on the coil. EXTENDED The direction of the induced e.m.f. opposes the change causing it: pushing an N pole into a coil induces a current that makes an N pole at that end, repelling the magnet. For a wire moving across a field, the force (motion), field and induced current are at right angles to each other (Fleming’s right-hand rule).

The a.c. generator EXTENDED

A coil turned between the N and S poles of a magnet, its ends connected to two slip rings touched by brushes that lead to the circuit. A graph of e.m.f. against time for one revolution is a sine curve: zero when the coil is vertical, a peak when it is horizontal, zero again when vertical and a trough when horizontal again.
EXTENDED Turn faster: higher peaks and more cycles per second.

EXTENDED A coil rotating in a magnetic field (or a magnet rotating inside a coil) has an e.m.f. induced in it that reverses every half turn: alternating current. Slip rings and brushes connect the turning coil to the external circuit without twisting the wires. The e.m.f. is greatest when the plane of the coil is parallel to the field (its sides cut the field lines fastest) and zero when the coil is at right angles to the field.

Transformers

A transformer: a primary coil connected to a 230 V a.c. supply and a secondary coil connected to a 12 V lamp, both wound on the same soft-iron core. The secondary has fewer turns, so it is a step-down transformer.
A transformer only works with alternating current.
\[ \frac{V_p}{V_s} = \frac{N_p}{N_s} \]

A step-up transformer has more turns on the secondary coil and increases the voltage; a step-down transformer has fewer. EXTENDED The alternating current in the primary coil produces a changing magnetic field in the soft-iron core; this changing field passes through the secondary coil and induces an alternating e.m.f. in it. For a 100% efficient transformer, power in = power out:

\[ I_pV_p = I_sV_s \]

High-voltage transmission

The national grid: a power station generates at 25 kV, a step-up transformer raises it to 400 kV for the transmission cables, and step-down transformers reduce it to 230 V for homes and factories.
Step up for transmission, step down for use.

Transmitting at a high voltage means a lower current for the same power, so less energy is wasted heating the cables, and thinner, lighter, cheaper cables and pylons can be used. EXTENDED The power wasted in the cables is \( P = I^2R \): halving the current quarters the loss.

✏️Worked example

A power station sends 1.0 MW along cables of total resistance 5.0 Ω. (a) EXTENDED Calculate the current and the power wasted in the cables if the power is sent at 10 kV. [3] (b) EXTENDED Repeat for 400 kV. [2] (c) A transformer steps 400 kV down to 11 kV. Its secondary coil has 550 turns. Calculate the number of turns on the primary. [2]

(a) I = P/V = 1.0 × 106 / 1.0 × 104 = 100 A; P = I2R = 1002 × 5.0 = 50 000 W (5% of the power).

(b) I = 1.0 × 106 / 4.0 × 105 = 2.5 A; P = 2.52 × 5.0 = 31 W.

(c) \( N_p = N_s \times \dfrac{V_p}{V_s} = 550 \times \dfrac{400}{11} = \) 20 000 turns.

Check it. The voltage went up 40 times, so the current fell 40 times and the loss fell 402 = 1600 times: 50 000 / 1600 = 31 W.
Using P = V2/R with the transmission voltage. The 400 kV is not across the cables; use the current in the cables and P = I2R.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) EXTENDED A 100% efficient transformer has 3000 turns on its primary and 600 on its secondary. The primary is supplied with 0.80 A at 230 V. What is the secondary current? A: 0.16 A. B: 0.80 A. C: 4.0 A. D: 46 A. (Modelled on 0625/22 June 2026 Q33.)
C. Vs = 230 × 600/3000 = 46 V; Is = IpVp/Vs = 0.80 × 230 / 46 = 4.0 A.
2. (Theory.) A magnet is held still inside a coil connected to a meter. State and explain the reading. [2]
Zero: the magnetic field linking the coil is not changing (no relative movement), so no e.m.f. is induced.
3. (Theory.) State three ways to increase the e.m.f. induced when a magnet is pushed into a coil. [3]
Push the magnet in faster; use a stronger magnet; use a coil with more turns.
4. (Theory.) A transformer changes 230 V to 6.0 V. The primary has 1150 turns. Calculate the number of secondary turns. [2]
Ns = Np × Vs/Vp = 1150 × 6.0/230 = 30 turns.
5. (Theory.) EXTENDED Explain how a transformer produces an output voltage. [3]
The alternating current in the primary coil produces a changing (alternating) magnetic field in the soft-iron core. The core carries this changing field through the secondary coil, where it induces an alternating e.m.f.
6. (Theory.) EXTENDED Explain why a transformer does not work with a steady d.c. supply. [2]
A steady current produces a constant magnetic field; the field linking the secondary does not change, so no e.m.f. is induced.
7. (Theory.) EXTENDED Sketch the e.m.f.–time graph for a simple a.c. generator, and describe the effect of turning the coil twice as fast. [3]
A sine-shaped curve alternating above and below zero. Turning twice as fast doubles the frequency (twice as many cycles in the same time) and increases the peak e.m.f. (roughly doubles it).
8. (Multiple choice.) A transformer gets warm when it is used. Which energy store increases? A: chemical. B: gravitational. C: internal (thermal) store of the surroundings. D: nuclear.
C.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Faraday’s Electromagnetic Lab” — induction, generators and transformers
  • BBC Bitesize — the national grid and transformers