Electromagnetic Induction and GeneratorsAQA GCSE Physics: Revision notes

Section 1

What is the generator effect?

The generator effect is the reverse of the motor effect: instead of a current producing motion, motion (or a changing field) produces a current.

A potential difference is induced across a conductor when:

the conductor moves relative to a magnetic field, or
the magnetic field around the conductor changes

If the conductor is part of a complete circuit, this induced potential difference drives an induced current through the circuit.

Key termsgenerator effectinduced potential differenceelectromagnetic induction

Exam tip

No relative movement or field change means no induced potential difference — a stationary conductor in a constant field produces nothing.

Section 2

What affects the size and direction of the induced potential difference?

The size of the induced potential difference (and current) is increased by:

moving the conductor faster
using a stronger magnetic field
increasing the number of turns on a coil
changing the angle between the conductor and the field (maximum effect at right angles)

The direction of the induced current depends on the direction of motion and the direction of the magnetic field — reversing either one reverses the induced current.

The induced current always flows in a direction that opposes the change that produced it (this is Lenz's law, in principle): the magnetic field created by the induced current pushes back against the motion or change causing it.

Key termsLenz's law

Think of it like this

Lenz's law is like friction resisting motion — nature 'fights back' against the change, which is why you have to do work to generate electricity (e.g. turning a dynamo feels harder than turning a disconnected wheel).

Section 3

How do alternators and dynamos generate electricity?

Both alternators and dynamos use the generator effect by rotating a coil within a magnetic field (or rotating a magnet within a coil):

Device	Output	How
Alternator	Alternating current (ac)	Uses slip rings so the current direction reverses every half turn
Dynamo	Direct current (dc)	Uses a split-ring commutator so the output current stays in one direction

A graph of potential difference generated against time for an alternator is a smooth sine wave (alternating between positive and negative), while a dynamo produces a series of positive humps.

Key termsalternatordynamo

Section 4

How does a moving-coil microphone work?

A moving-coil microphone converts sound into an electrical current — the reverse process to a loudspeaker.

Sound waves (pressure variations in air) hit a diaphragm attached to a coil
The diaphragm vibrates, moving the coil back and forth within a magnetic field
This relative movement between the coil and the field induces a current (generator effect)
The induced current varies in the same pattern as the original sound wave

Key termsmoving-coil microphone

Common mistake

Do not confuse a microphone (generator effect: sound → current) with a loudspeaker (motor effect: current → sound) — they are reverse processes.

Section 5

How does a transformer work?

A basic transformer consists of a primary coil and a secondary coil, both wound on a shared iron core.

An alternating current in the primary coil creates a constantly changing magnetic field in the iron core
This changing field passes through the secondary coil and, by the generator effect, induces an alternating current in the secondary coil
A transformer only works with alternating current — direct current produces no changing field, so no induction occurs

The relationship between the coils is given by:

$\frac{V_p}{V_s} = \frac{n_p}{n_s}$

where V = potential difference and n = number of turns on the primary (p) and secondary (s) coils.

For a transformer that is 100% efficient:

$V_s \times I_s = V_p \times I_p$

Key termstransformerprimary coilsecondary coil

Example

A step-up transformer has more turns on the secondary coil than the primary (ns > np), increasing the voltage but decreasing the current proportionally.

Section 6

Why does the National Grid use transformers?

The National Grid transmits electrical power from power stations to homes using step-up and step-down transformers.

Step-up transformers increase the potential difference (to very high voltages) for transmission along power lines
Since $P = VI$ , increasing the voltage means the current can be much lower for the same power
A lower current greatly reduces energy losses due to heating in the cables (heating losses depend on $I^2R$ )
Step-down transformers then reduce the voltage to safe levels before it reaches homes and businesses

This makes the National Grid a highly efficient way to transfer energy over long distances.

Key termsNational Gridstep-up transformerstep-down transformer

Exam tip

Remember the chain of reasoning: higher voltage → lower current → less energy wasted as heat in the cables (because power loss depends on current squared).

Must Know

The generator effect: a potential difference is induced when a conductor moves relative to a magnetic field, or the field changes
Induced current increases with faster movement, stronger field, more turns, and steeper angle to the field
Alternators produce ac (slip rings); dynamos produce dc (split-ring commutator)
A moving-coil microphone uses the generator effect to turn sound into a varying current
Transformers use a changing current in the primary coil to induce a current in the secondary coil, via Vp/Vs = np/ns
The National Grid uses step-up transformers (high voltage, low current, less energy loss) and step-down transformers for safe domestic use

That's the notes covered.

Carry on to the next subtopic.