Electromagnetic Induction » Generators

From a pushed magnet to a spinning one

In the last lesson, Electromagnetic Induction, you saw that a voltage is induced when a magnet and a coil move relative to each other. Pushing a magnet in and out by hand only works for a moment, though. To make electricity for a long time, we need the movement to keep going. The easiest way is to make something spin. A machine that does this is called a generator.

Key terms:

• Generator: a device that uses a spinning magnet or coil to induce a voltage.
• Slip rings: two metal rings fixed to the ends of a rotating coil. They turn with the coil.
• Brushes: two carbon blocks that press against the slip rings and pass the voltage to the outside circuit.

The bicycle dynamo

A bicycle dynamo is the simplest generator. It makes the electricity for the lights.

• A small magnet sits inside a fixed coil of wire.
• A little wheel on the dynamo presses against the bicycle tyre. When the bike moves, the tyre turns the wheel, and the wheel rotates the magnet.
• As the magnet spins, the magnetic field through the coil keeps changing. A voltage is induced in the coil.
• The coil is joined to the lamp, so the circuit is complete and a current flows.

Stop pedalling and the magnet stops turning. The field stops changing, and the light goes out. This matches what you saw before: no movement, no induced voltage. If you pedal faster, the light gets brighter, because the voltage is bigger.

The a.c. generator

Big generators work the other way round. The magnet stays still and the coil rotates between its poles. To draw and describe one in the exam:

1. Draw a north pole on the left and a south pole on the right, facing each other. The magnetic field runs between them from left to right.
2. Draw a rectangular coil of wire in the gap, free to turn around a middle axle.
3. Join each end of the coil to its own slip ring on the axle.
4. Draw a brush touching each slip ring, and join the brushes to the outside circuit, such as a lamp.

Without slip rings, the wires from the coil would twist up as it turned. The rings and brushes let the coil spin freely and still stay connected.

A d.c. motor (see Electric Motors and Loudspeakers) has a split-ring commutator that reverses the current every half turn. A generator is different. It uses two full slip rings, so nothing is reversed on the way out.

Why the output is alternating

As the coil turns, its two long sides move through the magnetic field. For half of each turn, one side moves up through the field while the other moves down. For the other half of the turn, they swap: the side that was moving up is now moving down.

Moving the other way through the field reverses the induced voltage (you saw this with the magnet pushed in and pulled out). So the voltage changes direction every half turn, and the current in the circuit changes direction too. This is alternating current, the a.c. you met in Alternating and Direct Current.

One full turn of the coil gives one complete cycle: voltage one way, then the other way. If the coil makes 50 turns every second, the frequency of the output is 50 Hz.

The voltage-time trace

A trace is a graph of voltage (up the side) against time (along the bottom). For a generator, describe it like this:

1. Draw a horizontal zero line.
2. Draw a smooth wave that rises above the line to a peak, falls back through zero, goes below the line to an equal trough, then returns to zero. This is one cycle.
3. The wave repeats, with the same peaks above and below the line.

Position of the coil	What the trace shows
Coil upright, sides moving along the field lines (not cutting across them)	Voltage is zero, on the zero line
Coil flat, sides moving straight across the field lines	Voltage is at its largest, a peak or a trough
Coil upright again, after half a turn	Zero again, then the voltage reverses direction

The voltage is biggest when the coil's sides cut across the field lines fastest. It is zero when they briefly move along the field lines, at the moment the voltage reverses.

What changes the size of the voltage?

The same three changes you saw in the last lesson work in a generator too.

Change	Effect on the induced voltage
Rotate the coil (or magnet) faster	Bigger voltage, and higher frequency
Use a stronger magnet	Bigger voltage
Use a coil with more turns	Bigger voltage

Worked example

Common mistakes

Exam-style question

A simple a.c. generator has a coil that rotates between the poles of a magnet. The coil is joined to a lamp by slip rings and brushes.

(a) State the purpose of the slip rings and brushes. [2 marks]

(b) Explain why the generator produces an alternating voltage. [2 marks]

(c) The coil is now rotated faster. Describe two changes to the voltage-time trace. [2 marks]