Electromagnetic Induction, Generators & Transformers (Years 10–11) — Science

How moving magnets induce voltage, how a.c. generators work, and how transformers change voltage using Vp/Vs = Np/Ns.

Almost all the electricity that reaches your home was made by spinning a magnet near a coil of wire. Move a magnet past a wire and, remarkably, a voltage appears across that wire — no battery required. This effect, electromagnetic induction, is how generators produce power and how transformers move it efficiently across the country. It is arguably the most important discovery in the history of technology.

Why this matters Every power station, wind turbine, dynamo and phone charger relies on induction. Understanding transformers explains why the National Grid transmits electricity at hundreds of thousands of volts — the answer is all about reducing wasted energy.

Inducing a voltage

A voltage is induced whenever a conductor cuts through magnetic field lines (or the field through a coil changes):

The faster the movement, the greater the induced voltage.
More turns on the coil, or a stronger magnet, also increase the voltage.
If the wire is stationary in a steady field, no voltage is induced — there must be a change.
Reversing the direction of movement reverses the direction of the induced voltage.

Moving the magnet into the coil induces a voltage shown on the meter.

The a.c. generator

A generator turns motion into electricity. A coil is spun between the poles of a magnet; as its sides sweep through the field they cut field lines and a voltage is induced. Because each side reverses direction every half-turn, the output is alternating current (a.c.). Slip rings and brushes carry the current to the external circuit. Spinning the coil faster gives a larger and more frequent alternating voltage.

The transformer

A transformer changes the size of an alternating voltage. Two coils are wound on an iron core: the alternating current in the primary coil makes a changing magnetic field, which induces a voltage in the secondary coil. The voltages depend on the number of turns:

V_p ÷ V_s = N_p ÷ N_s
Step-up (more secondary turns) increases voltage; step-down (fewer secondary turns) decreases it.

Symbol	Meaning
V_p, V_s	primary and secondary voltage
N_p, N_s	primary and secondary turns

More turns on the secondary than the primary makes this a step-up transformer.

Why the grid uses high voltage

Power stations step voltage up to about 400 000 V for transmission. Since power P = VI, a high voltage means a small current for the same power. Because energy wasted as heat in the cables is I²R, a small current wastes far less energy. Local substations then step the voltage back down to a safe 230 V for homes.

Worked example (3 marks). A transformer has 1000 primary turns and 50 secondary turns. The primary voltage is 230 V. Find the secondary voltage. V_p÷V_s = N_p÷N_s so V_s = V_p × N_s÷N_p [1] = 230 × 50 ÷ 1000 [1] = 11.5 V [1]. Fewer secondary turns, so it is a step-down transformer.

Common exam mistake: putting the turns ratio upside down. Keep primary quantities together on top: V_p/V_s = N_p/N_s. Also remember transformers only work with a.c., never steady d.c.

Memory hook: "more turns, more volts." The side with more turns has the higher voltage. Grid logic: high voltage → low current → low heat loss (I²R).