Electromagnetic Induction

Faraday's Law and Lenz's Law

Electromagnetic induction: when a conductor experiences a changing magnetic flux, an electromotive force (emf) is induced across it.

Faraday's Law: the magnitude of the induced emf is proportional to the rate of change of magnetic flux.

Increasing the rate of change:

• Move the magnet faster.
• Use a stronger magnet.
• Increase the number of turns on the coil.

Lenz's Law: the induced current flows in a direction that opposes the change causing it (conservation of energy — it takes work to push the magnet in/out of the coil).

Practical implication: to induce an emf, there must be relative motion between the conductor and the field (or a changing current in an adjacent coil).

The AC Generator (Dynamo)

A coil of wire rotates in a magnetic field (or magnets rotate around a fixed coil). As the coil rotates, the flux through it changes → emf is induced.

• Maximum emf when coil is parallel to the field (rate of flux change is maximum).
• Zero emf when coil is perpendicular to field (flux is at maximum but not changing at that instant).

Output is alternating current AC — the emf reverses direction every half rotation, producing a sinusoidal wave.

Slip rings and brushes maintain continuous contact with the rotating coil without reversing the current (unlike the commutator in a DC motor).

UK mains supply: 230 V, 50 Hz. Frequency = number of complete rotations per second.

Transformers

A transformer consists of two coils (primary and secondary) wound on a shared soft iron core.

Operation:

1. Alternating current in the primary coil creates a changing magnetic field.
2. Changing magnetic field is concentrated in the iron core.
3. Changing flux through the secondary coil induces an alternating emf (Faraday's Law).

Transformer equation: V_p/V_s = N_p/N_s (voltage ratio = turns ratio)

For an ideal transformer (100% efficient): V_p I_p = V_s I_s (power in = power out).

Why AC and not DC? DC produces a constant (non-changing) field → no changing flux → no induction in secondary coil. Transformers only work with AC.

Microphone (Generator in Reverse for Sound)

A microphone converts sound waves to electrical signals. In a dynamic microphone:

• A diaphragm moves with sound pressure waves.
• A coil attached to the diaphragm moves in a magnetic field.
• Moving coil → changing flux → induced alternating emf in the coil → electrical signal.

Core Practical 8 — Investigating electromagnetic induction (motor/generator)

Equipment: coil/solenoid, bar magnet, galvanometer (sensitive ammeter), leads.

Observations:

• Push north pole into coil → galvanometer deflects one way (induced current in one direction).
• Pull north pole out → deflects opposite way (induced current reverses — Lenz's Law).
• Hold magnet still → no deflection (no changing flux → no induction).
• Faster movement → larger deflection (greater emf — Faraday's Law).
• More turns on coil → larger deflection.
• South pole in → deflection opposite to north pole in.

Lenz's Law confirmation: the coil end nearest the approaching north pole becomes a north pole (repels incoming magnet — opposing the change).