Electromagnetic induction (Higher)

The generator effect

If a conductor moves through a magnetic field (or a magnetic field changes near a conductor), an emf (potential difference) is induced across the conductor. This is the generator effect — it is the reverse of the motor effect.

Three things must happen for an emf to be induced:

• A conductor and a magnetic field.
• Relative motion between them, OR a changing field.
• The conductor must cut field lines.

If the conductor forms a complete circuit, an induced current flows.

Factors that increase induced emf

• Faster relative motion (cut field lines more quickly).
• Stronger magnet (denser field lines).
• More turns on the coil.
• Larger coil area.

Direction of the induced current — Lenz's law (qualitative)

The induced current flows in the direction that opposes the change producing it. If you push a magnet north-pole-first into a coil, the coil acts like a magnet whose north pole faces the incoming magnet, repelling it. This is energy conservation in action — you have to do work against the repulsion, and that work becomes electrical energy.

Reverse the direction of motion (or flip the magnet) and the induced current reverses.

AC generator (alternator)

A coil rotates inside a magnetic field. As the coil rotates, the rate at which it cuts field lines changes:

• Coil parallel to field → cutting fastest → maximum induced emf.
• Coil perpendicular to field → not cutting → emf = 0.

The emf alternates direction every half rotation, producing alternating current (ac). Slip rings and brushes connect the rotating coil to the external circuit.

A graph of emf vs time is a sine wave; frequency = rotation frequency of the coil. Mains UK frequency is 50 Hz.

DC dynamo

Same as an alternator but uses a split-ring commutator (like a motor) instead of slip rings. The commutator reverses the connection every half turn, so the output is always positive — direct current with a pulsing magnitude.

Microphones and dynamos in everyday life

A moving-coil microphone uses the same effect: sound waves move a coil through a permanent magnet's field, inducing a tiny ac voltage that mirrors the sound wave. Bicycle dynamos drive lights from rotation of the wheel.

Edexcel exam tip

For "explain how the size of the induced current can be increased" the marks come in pairs: one method + the reason it works. "Use a stronger magnet so more field lines are cut per second" earns 2 marks; just "stronger magnet" earns 1.