It was first noticed 150 years ago that when some small compasses are placed near a wire carrying an electric current, the needles lie in the direction of rings around the wire. As soon as the current is turned off the needles move back to their original north-south positions. The electric current makes a force field around the wire in the same way that a magnet would do.
Electromagnets
| Electromagnet is a temporary magnet whose
magnetism is produced by an electric current. A single wire that has current passing through, has a magnetic force around
it. It becomes a temporary magnet as long as there is current flowing through
it. By holding the wire with your right hand and the thumb showing the direction
of the current, the direction of the magnetic force is as shown by the curving
of your fingers. See diagram below.
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A coil of wire called a solenoid can make a much
stronger magnetic field than a single piece of wire. It behaves like a bar
magnet, the pole of the magnetic field can be determined by the
right-hand grip rule. Imagine gripping the current-carrying solenoid
with your right hand so that your fingers curling round the solenoid in
the direction of the current. Your thumb then points towards the north
pole of the solenoid.
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| The poles of the current-carrying solenoid
can also be determined by looking at the direction of the current from
each end of the solenoid. If the direction of the current is clockwise,
the viewed end of the solenoid is the south pole. If the direction of
the current is anticlockwise, the viewed end of the solenoid is the
north pole.
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The strength of the magnetic field
increases:
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Make an electromagnet
You can make your own electromagnet with some wire, a battery and an iron nail. Wind the wire around the nail, keeping the coils very close together. The more coils there are, the stronger the magnet will be. Twist the ends of the wire around the two terminals of the battery to make the electricity from the battery flow around the wire, the nail becomes a magnet. You can test with some paper clips to see how strong it is. By undoing one of the terminals of the battery the nail stops being a magnet again.
Another way of making magnets
A magnet can sometimes make something else into a magnet without even touching it. Its lines of force stretch out across space and make the "magnet-molecules" in the object line up. This is called magnetic induction. Only temporary magnets are made by "induction".
Applications of Electromagnet
| The Electromagnetic Lifting Machine
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The Electric Bell
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| The Electromagnetic Relay
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The Telephone Earpiece
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| The Circuit Breaker
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