Magnetism
Introduction

Magnets can be found in many everyday objects. Refrigerator doors have magnetic strips on them to keep them closed, computers use magnetic disks to store information, credit cards have magnetic strips on the back of them. Even loudspeakers and toasters can have magnets in them.

Attract or repel?
The two ends or poles of a magnet behave differently. They are called the north pole (N) and the south pole (S) of a magnet.

The two magnets in Fig.1 below are pointing in the same direction. They both have their north poles towards the right.

Figure 1.   Magnets pointing the same way.
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Drag the left-hand magnet to the right until its north pole approaches the south pole of the other magnet, then let it go.

What happens to the magnets?
  • Well done! The magnets are attracted.
  • Are you sure?
  • Move them closer together.
  • Click here to mark the question


Drag the magnets around to see if this is always the case.

In the experiment below, the right-hand magnet has been turned around.

Figure 2.   Magnets pointing in opposite directions.
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Drag the north pole of one magnet near to the north pole of the other, then let it go.

What happens to the magnets in Fig.?
  • Are you sure?
  • Well done!
  • Move them closer together.
  • Click here to mark the question


Now bring the south poles near each other. You will have to drag one magnet over the top and past the other one.

What happens to the magnets now?
  • Are you sure?
  • Well done! The magnets repel.
  • Move them closer together.
  • Click here to mark the question
Select the correct words to complete the sentences below.

  • Opposite or unlike magnetic poles experience a force between them that is. Like magnetic poles experience a force that is .
  • Click here to mark the question


In both cases, the
force
A force is a push or a pull. A force can lift an object, change its shape, or accelerate it.
force
between the poles is called the
magnetic force
The magnetic force is the force that occurs between magnetic objects. The magnetic force can be either attractive or repulsive depending on the two objects.
magnetic force
.

Magnetic fields
Around every magnet is a magnetic field. The
magnetic field
Around every magnet there is an invisible magnetic field.
magnetic field
will have an effect on any other magnet that comes into the field.

The needle of a compass always points along the direction of the magnetic field. Drag the plotting compass in Fig.3 below around the magnet to investigate the field. Observe how the direction of the field changes from point to point.

Figure 3.   Plotting the magnetic field.
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A common way to show the magnetic field is by drawing field lines or lines of force. The red (north) end of the compass points in the direction of the lines of force.

Which of these pictures shows the magnetic field of the magnet? Be sure to look at the arrows on the field lines.


The magnetic field is strongest where there are most field lines. This occurs near the poles of the magnet.

Boats, aeroplanes, and walkers use compasses to help them know which way to travel in the dark. The red (north) end of the needle on the compass always points towards geographic north. This is because the needle is influenced by the magnetic field of the earth. The place we call the North Pole of the earth is in fact its magnetic south pole.

Figure 4.   The Earth's magnetic field extends far into space.
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Magnets also attract non-magnetized objects, such as paper clips, iron filings, and fridge doors. Objects which are attracted to a magnet, but are not themselves magnets, are made of magnetic materials. Iron and steel are two common magnetic materials. Wood, paper, and plastic are not magnetic materials. They are not affected by magnetic fields.

Figure 5.   Metallic paper clips are attracted to a magnet.
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Drinks cans are made of aluminium and are not magnetic. Soup cans and baked bean cans are often made of steel.

How would you decide if a can is made of aluminium?
Why are there often magnets at can recycling centres?

Further fields
When two magnets are placed near each other, the two magnetic fields interact with each other. By just looking at the field patterns you can often tell whether the force between them will be attractive or repulsive. When the force is attractive, the field lines seem to be reaching from one magnet to the other. When the force is repulsive the field lines bend away from each other.
The diagrams below show the magnetic fields between pairs of magnets. For each one, decide whether the magnets will attract or repel.
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Figure 6.  
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In Fig.6 above, decide whether each pole is either north or south:
  • Pole 1 is:
    Pole 2 is:
    Pole 3 is:
    Pole 4 is:
    Pole 5 is:
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Summary


The two ends or poles of a magnet behave differently. One is called the north pole and the other is called the south pole.

Opposite or unlike magnetic poles attract each other. Similar or like magnetic poles repel each other.

Around every magnet there is a magnetic field.

Exercises
1. Choose the correct words to complete the table below describing the force between two magnetic poles.

  • Pole 1 Pole 2 Force
    North North
    North South
    South Attractive
    South Repulsive

  • Click here to mark the question
2. Select the correct words to complete the sentence below.

  • A magnet is suspended using a thin string. When it stops swinging, the north end of the magnet always points towards the geographic Pole of the Earth. This pole is in fact the magnetic pole of the Earth.
  • Click here to mark the question
Figure 7.  
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3. Decide which compass below shows the correct reading for each of the positions marked on Fig.7 above.
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Figure 8.  
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4. Decide whether the force between the magnets in Fig.8 above is attractive or repulsive.
  • Between magnets 1 and 2
    Between magnets 2 and 3
    Between magnets 3 and 4
    Between magnets 4 and 5
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Well done!
Try again!