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P6F
The dynamo effect
Electricity can be generated by moving a
wire near a magnet, or by moving a
magnet near a wire. This is called the
dynamo effect.
A voltage is induced (made to happen)
across a wire when the wire moves
relative to a magnetic field. If the wire is
moved in the opposite direction, the
voltage is reversed.
A voltage is also induced across a coil
when the magnetic field in it changes. If
the magnet is moved in the opposite
direction, the voltage is reversed.
Just Imagine the arrow moving to positive
everytime it goes it and out.
If the wire or coil is part of a complete
circuit, a current will flow.
Changing the size of the induced voltage
The size of the induced voltage depends
on the rate that the magnetic field
changes. The faster it changes, the greater
the induced voltage.
The AC generator
The AC (alternating current) generator
makes use of the dynamo effect to
produce electricity.
One simple example is the bicycle
dynamo. It has a wheel that touches the
back tyre. As the bicycle moves, the
wheel turns a magnet inside a coil. This
induces enough electricity to run the
bicycle’s lights.
The faster the bicycle moves, the greater
the induced current and the brighter the
lights.
A generator can be described as an
electric motor working in reverse –
instead of using electrical energy to
produce kinetic energy, kinetic energy is
used to produce electrical energy.
You should be able to label a diagram of
an AC generator. These are the main
features you should identify:

coil

magnets

slip rings

brushes
(alternating current) electricity
produced
Mains electricity is supplied at 50 Hz (50
complete waves per second) in the UK.
How an AC generator works
The slip rings are connected to the coil.
The brushes are connected to the
external circuit. The brushes touch the
spinning slip rings, which maintain
electrical contact between the coil and
the external circuit.
At the power station
Electricity is a useful form of energy. It
allows energy to be transmitted over long
distances easily through cables, and it
allows energy to be stored for use later
on.
The large generators used in power
stations use electromagnets instead of
permanent magnets. Electricity is
generated when the electromagnet
rotates inside coils of wire. The more
turns there are on the coils of the
electromagnet, the greater the size of the
voltage generated.
The speed at which the electromagnet is
rotated is important. The faster the
electromagnet is rotated:

the greater the size of the voltage
generated

the higher the frequency of the AC
As the electromagnet turns, the direction
of the induced voltage and current
changes every half-turn.
This is why AC electricity is produced.