Transformers

Two circuits linked by magnetism

A transformer transfers electrical energy between two separate circuits using electromagnetic induction. Its main parts are an input coil called the primary coil, an output coil called the secondary coil, and an iron core around which both coils are wound. The coils are not joined by a conducting wire, so charge does not travel directly from one circuit to the other.

From alternating current to a changing field

When an alternating potential difference is applied across the primary coil, it drives an alternating current. Because a current produces a magnetic field, this current magnetises the iron core. As the current changes in size and direction, the magnetic field also changes in size and direction.

The core provides a path that links this changing magnetic field to the secondary coil. It therefore makes the magnetic link between the two coils much stronger than it would be through air alone.

Induction in the secondary circuit

The magnetic field passing through the secondary coil is continually changing. This changing field induces an alternating potential difference across the ends of the secondary coil. If the secondary circuit is complete—for example, if it is connected to a lamp—the induced potential difference drives an alternating current through the load.

The sequence is therefore:

1. an alternating current flows in the primary coil;
2. it produces a changing magnetic field in the iron core;
3. the changing field links with the secondary coil;
4. an alternating potential difference is induced across the secondary coil;
5. an alternating current flows if the secondary circuit is closed.

The induced potential difference exists even when the secondary circuit is open, but an induced current requires a complete circuit.

Why a steady current does not operate a transformer

Electromagnetic induction requires a change in the magnetic field. A steady direct current produces a steady magnetic field once it has settled, so it cannot produce a continuous induced potential difference in the secondary coil.

There can be a brief induced effect when a direct-current circuit is switched on or off because the primary current—and therefore its magnetic field—is changing at that moment. Once the current becomes steady, the induced effect stops. An a.c. supply keeps the field changing, so the transformer can operate continuously.

Transformer action

• The primary and secondary coils are separate circuits wound around an iron core.
• An a.c. in the primary produces a changing magnetic field.
• The core links this changing field to the secondary coil.
• The changing field induces an alternating p.d. across the secondary.
• A current flows in the secondary only when its circuit is complete.

Essential distinction

A transformer transfers energy magnetically; current does not pass directly from one coil to the other.

A steady d.c. produces no continuous secondary output because its magnetic field is steady. Only changes at switch-on or switch-off cause brief induction.