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KS3 3.2.3 Electromagnets

It is hard to overemphasise the importance of the discovery of electromagnetism for our modern world. It led to the invention of electric motors which are now all around us and to the ability to generate electricity upon which our civilization now depends.

Electromagnetism and electromagnets

Two hundred years ago a Danish Physicist called Hans Christian Oersted discovered that - if you passed an electric current through any piece of wire, a magnetic field appeared around the wire.

He placed a compass near the wire and saw it move when the current flowed, proving the presence of a magnetic field.

He had discovered a link between electricity and magnetism, which we call electromagnetism.

Furthermore, it was discovered that if you passed the current through a coil of wire wrapped around an iron core, the resulting magnetic field could be made significantly stronger; we call this "coil plus iron core" an electromagnet.

The strength of the magnetic field is increased if:

• the current is increased
• the number of turns on the coil is increased
• iron is used for the core rather than a metal such as steel.

If you are already familiar with permanent bar magnets, you will notice that the shape of the magnetic field around the electromagnet is exactly the same as that around a permanent bar magnet; this should make it easier to remember. Also you will notice that, like a bar magnet, the electromagnet has magnetic poles, North and South.

You can see from the diagram that the magnetic "field lines" (shown in red) get further and further apart with distance from the electromagnet.
This tells us that the strength of the electromagnet at any point decreases as distance from the electromagnet increases.

So, at either of the Poles, where there are a lot of field lines, quite closely packed together, the magnetic field strength is large.
Elsewhere where there are fewer field lines, it is weaker and gets weaker still as distance is increased.

Note: Two reasons NOT to use steel for the core of an electromagnet:
First - the resulting magnetic field would be significantly weaker.
Second - the magnetic field would not turn off immediately when the current is turned off, as it would with an iron core.
This is due to the fact that steel is a "hard" magnetic material, meaning it is hard to make it lose its magnetism.

Electromagnetism and solenoids

A solenoid is simply an electromagnet without its metal core, ie it is just a coil of wire.

But, when we pass a current through it we still generate a magnetic field like that around the electromagnet (and around a permanent bar magnet), however the resulting field is weaker; this is simply because there is no iron core.

Notice that, like in an electromagnet, the magnetic field runs all the way inside the coil, but unlike in an electromagnet where there is a metal core, we can "get at" the field inside the solenoid. This makes it suitable for certain uses.

Uses of solenoids/electromagnets

Using a solenoid as a door lock

In the following diagram an iron bar (which is magnetic) is placed inside the solenoid but it is being pushed outwards by a spring such that it goes through a hole in a door, locking the door.
When the switch is closed, however, the magnetic field inside the solenoid will pull the iron bar fully inside, even against the force of the spring, such that the bar/bolt no longer goes through the hole in the door, hence the door becomes unlocked!
When the switch is released and the magnetic field disappears, the spring pushes the iron bar back into its original position and the door beomes locked once again. Clever, isn't it?

Electromagnetic door locks like this are used widely in hotels and office buildings; the "switch", of course, would not be just a push switch; it would be controlled by a key pad or a key card to make it secure.

Using an electromagnet to pick up objects

The biggest advantage of an electromagnet over a permanent magnet is that its magnetic field can be easily turned on or off whenever the user wishes simply by allowing the current to flow or not to flow through the coil. This makes electromagnets very useful in situations such as in scrapyards where an electromagnet can be positioned over some scrap metal (or even over a whole car), then it can be turned on, attracting all the scrap metal. The scrap metal can then be moved around and repositioned by turning off the electromagnet.

To make the electromagnet stronger we would use a large number of turns of wire for its coil and/or we would use a large current, and we would always choose an iron core.

Keywords

Electromagnet: A non-permanent magnet turned on and off by controlling the current through it.

Solenoid: Wire wound into a tight coil, part of an electromagnet.

Core: Soft iron metal which the solenoid is wrapped around.

Now its time for you to have a go at a few questions.