Electromagnetic induction and transformers – GCSE Combined Science Physics

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Every power station, wind turbine and bicycle dynamo makes electricity in the same way. Something moves a magnet near a coil of wire, or moves a wire through a magnetic field, and a potential difference appears. No battery is pushing the charge. The movement itself creates the potential difference.

This is called electromagnetic induction, or the generator effect. It is the reverse of the motor effect. The motor effect uses a current to make movement. The generator effect uses movement to make a current.

The same idea explains the transformer, which changes the size of an alternating potential difference. Without transformers the National Grid could not send electricity across the country without wasting a huge amount of energy as heat. By the end you can explain induction, describe a generator and explain how a transformer works.

Inducing a potential difference

If a wire cuts through magnetic field lines, a potential difference is induced across the ends of the wire. If the wire is part of a complete circuit, an induced current flows. You get the same result if the wire stays still and the magnet moves, because only the movement of one relative to the other matters.

A simple test uses a coil of wire joined to a sensitive voltmeter. Push a bar magnet into the coil and the meter flicks one way. Hold the magnet still inside the coil and the reading falls to zero. Pull the magnet out and the meter flicks the opposite way.

So a potential difference is induced only while the magnetic field through the coil is changing. A steady field that is not moving relative to the wire induces nothing, however strong the magnet is.

What changes the size and the direction

The induced potential difference gets larger when the magnet or wire moves faster, when the magnet is stronger, and when the coil has more turns. A coil with a larger area also cuts more field lines. All of these increase how quickly the magnetic field through the coil changes.

The direction of the induced potential difference, and of the current it drives, reverses if the movement is reversed. Pulling the magnet out instead of pushing it in reverses it. Turning the magnet round so the other pole goes in first also reverses it. If you reverse both at once, the direction stays the same.

In an exam, a question that asks for two ways to increase the induced potential difference wants two of these: move faster, use a stronger magnet, or add more turns to the coil. Using a thicker wire is not one of them.

Generators and dynamos

A generator spins a coil in a magnetic field, or spins magnets inside fixed coils. As the wires of the coil cut the field lines, a potential difference is induced. In a power station, steam, water or wind turns a turbine, and the turbine turns the generator.

An alternator keeps the coil connected to the outside circuit with slip rings and carbon brushes. The output is an alternating potential difference that keeps reversing. A dynamo uses a split-ring commutator instead, which swaps the connections every half turn, so the output stays in one direction. A dynamo gives direct current, although the size of the potential difference still rises and falls.

A bicycle dynamo spins a magnet next to a fixed coil as the wheel turns. The faster you cycle, the faster the magnet spins, so the induced potential difference is larger and the lamp is brighter.

Reading an alternating graph

When a coil spins at a steady rate, the potential difference it produces rises to a maximum in one direction, falls to zero, reaches a maximum in the opposite direction and falls to zero again. A graph of potential difference against time is a repeating wave. One full repeat is one cycle.

The potential difference is greatest when the sides of the coil are moving straight across the field lines, and zero when the sides move along the field lines. Spin the coil twice as fast and two things happen: the maximum potential difference doubles and the cycles come twice as often, so the frequency doubles.

Mains electricity in the UK is alternating at 50 hertz, which is 50 complete cycles every second, at 230 volts. A graph question may ask you to read the maximum value or work out the frequency from the time for one cycle, using frequency = 1 ÷ time period.

How a transformer works

A transformer has two coils of insulated wire wound on the same laminated soft iron core. The coil connected to the supply is the primary coil. The other is the secondary coil. The two coils are not connected by wire.

An alternating potential difference across the primary coil drives an alternating current, which magnetises the iron core with a field that keeps changing. The core carries this changing field through the secondary coil, and a potential difference is induced across it. The soft iron core is used because it is easily magnetised and demagnetised.

A transformer only works with alternating current. Direct current gives a steady magnetic field, so the field through the secondary coil is not changing and nothing is induced. A step-up transformer has more turns on the secondary coil than on the primary and raises the potential difference. A step-down transformer has fewer turns on the secondary and lowers it.

Transformer calculations (Higher tier)

Higher tier. The potential differences across the two coils are in the same ratio as the numbers of turns: Vₚ ÷ Vₛ = Nₚ ÷ Nₛ. Here Vₚ and Vₛ are the primary and secondary potential differences and Nₚ and Nₛ are the numbers of turns. A step-down transformer with 1000 turns on the primary and 50 on the secondary, connected to 230 V, gives Vₛ = 230 × 50 ÷ 1000 = 11.5 V.

A transformer cannot make energy, so for an ideal transformer that is 100% efficient the power in equals the power out: Vₚ × Iₚ = Vₛ × Iₛ. If a transformer steps the potential difference up by a factor of ten, the current drops by a factor of ten. A transformer with 12 V and 5 A going in gives 60 W. If the output is 120 V the output current is 60 ÷ 120 = 0.5 A.

This is why the National Grid works. Power lost as heat in a cable depends on the current squared, so a high potential difference, often called high voltage, with a low current wastes far less energy. Step-up transformers raise the potential difference to hundreds of thousands of volts for transmission, and step-down transformers lower it to 230 V near homes.

Recap

Induction creates a potential difference when the magnetic field through a coil changes. It is larger with faster movement, a stronger magnet or more turns, and it reverses when the movement reverses. A generator uses it, and a transformer uses it to change the size of an alternating potential difference.

Check the common slips. A magnet that is not moving induces nothing. A transformer needs alternating current. More turns on the secondary means a step-up transformer. Power in equals power out for an ideal transformer, so a bigger potential difference means a smaller current.