Download Electromagnetism Y 10

Electromagnetism
• A magnetic field is produced when an
electric current flows through a coil of wire.
This is the basis of the electromagnet.
• Electricity & magnetism are different aspects of
the same basic phenomenon: ‘Electromagnetism’
• We can make an electromagnet stronger by doing these things:
• wrapping the coil around an iron core
• adding more turns to the coil
• increasing the current flowing through the coil.
A Simple Electromagnet:
When an electric current passed through a wire
it generates a magnetic field around the wire.
The magnetic field around an electromagnet is just the same as the
one around a bar magnet. It can, however, be reversed by turning
the battery around. Unlike bar magnets, which are permanent
magnets, the magnetism of electromagnets can be turned on and off
just by closing or opening the switch.
If the wire is made into a coil and connected
into a circuit, a magnetic field is produced
around the coil.
Using Electromagnets
Electromagnets are very useful as they can
be switched on and off, have their strength
altered or their poles switched around. They
are used in many things, especially when
we want to create movement.
Examples are :
Motors
Bells
Loudspeakers
The shape of the field around an
electromagnet is similar to the field
around a bar magnet.
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Electric bells like the ones used in most
schools also contain an electromagnet.
› When the current flows through the circuit, the
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electromagnet makes a magnetic field.
The electromagnet attracts the springy metal
arm.
The arm hits the gong, which makes a sound and
the circuit is broken.
The electromagnet is turned off and the springy
metal arm moves back.
The circuit is complete again.
The Relay
A relay is a device that uses a small current to operate a switch
to control a high current.
This shows how a relay is used to operate a car starter motor.
When the switch is closed, current flows
through the coil and the iron core becomes
magnetised.
This attracts the iron armature.
The top of the armature is pulled towards the
electromagnet.
The armature turns on the pivot, and the
bottom part moves to the right, pushing the
contacts together.
The second circuit is now complete, and the
large current will make the motor turn.
The Loudspeaker
A loudspeaker produces sounds by vibrating quickly
backwards and forwards.
Signals fed to a loudspeaker pass through an amplifier
which makes the amplitude of the vibrations larger. This
makes the sound louder.
Inside the speaker there is a permanent magnet
and an electromagnet. The strength of the
electromagnet is changed by the signal, and so
the force between it and the permanent magnet
also changes.
One of the magnets is attached to the cone of
the speaker. As the strength of the
electromagnet changes, the cone vibrates and
makes a sound.