Induction, generators and transformers

The generator effect (Higher)

• If a conductor moves relative to a magnetic field, or the magnetic field around a conductor changes, a potential difference is induced across the ends of the conductor.
• If the conductor is part of a complete circuit, a current is induced.
• The induced current makes its own magnetic field, which opposes the original change (the movement of the conductor or the change in the field).

Increasing the induced potential difference (Higher)

• Move the wire or magnet faster.
• Use a stronger magnetic field.
• Use a coil with more turns.
• Reversing the direction of motion, or the poles of the magnet, reverses the induced potential difference.

Uses of the generator effect (Higher)

• An alternator is a coil rotating in a magnetic field; it produces alternating current (ac) and uses slip rings.
• A dynamo produces direct current (dc) and uses a split-ring commutator.
• A microphone uses the generator effect: sound waves make a diaphragm and coil vibrate in a magnetic field, producing a changing current that matches the sound.

Transformers (Higher)

• A transformer has a primary coil and a secondary coil wound on an iron core. An alternating current in the primary coil makes a changing magnetic field in the core, which induces an alternating pd in the secondary coil.
• Vp ÷ Vs = np ÷ ns: the ratio of the potential differences equals the ratio of the numbers of turns.
• Step-up transformers have more turns on the secondary coil; step-down transformers have fewer.
• If a transformer is 100% efficient, the power in equals the power out: Vp × Ip = Vs × Is.

Key terms

Generator effect

Inducing a potential difference by moving a conductor in a magnetic field or changing the field around it.

Induced potential difference

A pd produced across a conductor by the generator effect.

Alternator

A generator that produces alternating current.

Dynamo

A generator that produces direct current.

Microphone

A device that uses the generator effect to turn sound into a changing current.

Transformer

A device with two coils on an iron core that changes the size of an alternating pd.

Primary coil

The input coil of a transformer.

Secondary coil

The output coil of a transformer.