1. Magnetism
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Mike Turner, Feb. 2004

2. What is a magnet?
A magnet is something that has a magnetic field around it.
Permanent magnets are usually made of steel, as steel keeps its magnetism.
All magnets have two poles – a north pole and a south pole.
The shape of the field around a bar magnet is like this :
We say the field lines flow from north to south.
The closer together the field lines, the stronger the magnetic field.
The field is concentrated at the poles.
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Mike Turner, Feb. 2004

3. Testing for Magnetism
When two magnets are brought close to each other, there is a force between them.
N S
N S
attract
N S
S N
repel
S N
N S
repel
S N
S N
attract
The only test for magnetism is repulsion.
If a piece of metal is attracted by a magnetic, that only proves that the metal is made of a magnetic material. To prove that it is a magnet, it must REPEL another magnet.
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Mike Turner, Feb. 2004

4. Magnetic Materials Most materials are not magnetic.
A common mistake is to believe that metals are magnetic – only a very few are.
The magnetic materials you need to know are :
iron
steel (which is nearly all iron)
nickel
cobalt
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Mike Turner, Feb. 2004

5. The Earth as a Magnet
The Earth acts as a huge magnet. Compasses work because they are affected by the Earth’s magnetic field.
As we know, the Earth has two poles. However, the North pole of the Earth is actually a magnetic South pole!
If a magnet is suspended from a string, or floated, so that it is free to spin around, it will always end up pointing in a North – South direction. The north pole of the magnet will be attracted by the south pole at the top of the Earth. This is how a compass works.
If a two or more compasses are brought close to each other, their magnetic fields will affect each other and they will no longer point north – south.
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Mike Turner, Feb. 2004

6. Magnetism and Electricity
Whenever an electric current passes through a wire, a magnetic field appears around the wire. The shape of the field can be seen in the diagram.
The stronger the current is, the stronger the magnetic field.
Some people believe that living near strong magnetic fields, such as those produced by power lines, can cause problems such as cancer. Lots of scientists are working to try to find out if this is true. At the moment, like many things in science, the definite answer is not known.
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Mike Turner, Feb. 2004

7. Electromagnets
We can make an electromagnet by coiling a wire and passing a current through it. This is often called a solenoid.
The material that the coil is wrapped around is called the core.
To increase the strength of an electromagnet we can do three things :
Add more turns to the coil of wire
Use a bigger electric current
Wrap the coil around a soft iron core
If a steel core is used, the electromagnet will still be strong, but soon the steel will become permanently magnetised and the electromagnet will not be able to be switched off.
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Mike Turner, Feb. 2004

8. 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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Mike Turner, Feb. 2004

9. The Electric Bell
An electric bell uses an electromagnet together with an iron armature. The bell is cleverly designed to switch itself on and off rapidly.
When the switch is pressed, current flows through the coil.
The soft iron core of the electromagnet becomes magnetised.
The iron armature is attracted to the electromagnet.
The hammer hits the bell.
As the armature moves towards the electromagnet it moves away from the contact screw.
The circuit is broken.
The current stops flowing.
The iron core loses its magnetism.
The armature is no longer attracted to the electromagnet.
The springy metal strip pulls the armature back.
The armature touches the contact screw.
The circuit is complete again.
Current flows, and the process starts again.
This is repeated many times each second.
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Mike Turner, Feb. 2004

10. 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.
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Mike Turner, Feb. 2004

11. 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.
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Mike Turner, Feb. 2004

12. How to answer an electromagnet question
General principles
Switch pressed
Current flows
Coil becomes magnetic
Attracts armature
Armature moves
Something happens!
When current switched off
Coil loses magnetism
Something else happens!
Principles applied to diagram of door lock
Push switch pressed
Current flows through coil
Coil and iron core become magnetic
Iron bolt is attracted to electromagnet
Iron bolt moves left
Door is unlocked
Switch is let go
Current stops
Coil and iron core lose magnetism
Iron bolt is no longer attracted
Iron bolt is pushed back by spring
Door is locked again
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Mike Turner, Feb. 2004

13. Answering an electromagnet question – the Circuit Breaker
Circuit breakers are safety devices built into mains electricity circuits. They are designed to switch off the current if something goes wrong.
Often when something goes wrong with an electric circuit, a large current will flow. Describe how the circuit breaker shown in the diagram works.
See if you can answer this question, then move on to the next screen for an example answer.
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Mike Turner, Feb. 2004

14. Answering an electromagnet question – the Circuit Breaker
Here is the answer to the question on the previous screen.
When a high current flows, the strength of the electromagnet increases.
The iron rocker is attracted to the electromagnet more strongly.
The iron rocker is pulled down by the electromagnet.
The switch contacts are separated.
The circuit is broken.
The current is switched off.
The springy piece of metal stops the rocker from moving back up.
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Mike Turner, Feb. 2004

15. Thank you for looking at this work, and good luck in your test.
You have now reached the end of the revision presentation for the ‘Magnetism’ topic.
I hope you have found it useful. If you have any ideas to improve this presentation please let Mr. Turner know.
Thank you for looking at this work, and good luck in your test.
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Mike Turner, Feb. 2004