Physics
Cambridge IGCSE

TOPIC 4B: MAGNETISM

# 4.1 Magnets

Maglev trains can travel at extremely high speeds. The record - at the time of writing - is 603 km/h! This is the fastest train in the world. But how do maglev trains work?

YouTube - How a maglev train works
(Nathan Nagle)

In the video shown here, you can see that the train 'floats' above the track so that there is no friction between the surfaces. This relies on the basic properties of magnets - that they can attract (pull together) and repel (push apart).

## Attraction and repulsion

The diagram below shows two magnets put close together. Magnets have 2 poles called the north and south poles. These are typically at opposite ends. If two poles are placed together that are the same type, they will repel. if the poles are different, they will attract.

Figure 1: The basic properties of magnets

## Compasses

A compass is a small bar magnet that can rotate freely. The Earth has a small magnetic field, and the compass poles will be attracted and repelled be the Earth's magnetic field until it settles, and points to the Earth's north and south poles.

They are used for navigation, but beware as there are some famous areas on the Earth where large deposits of iron make a compass point in the wrong direction!

Figure 2: A magnetic compass

The fact that a compass points towards the Earth's north pole is evidence that the Earth must contain magnetic materials. Further research has shown that these materials must be in the Earth's core, and must contain a large mass of magnetic material, probably iron.

Extension:

Why does the north pole of a compass point north if two north poles repel? This is a famous source of confusion!
Can you solve it yourself an make a prediction? Or do some research?
If you get stuck, the solution is here:

The answer is that the 'north pole' of the Earth in the arctic must actually be a magnetic south for the north pole of a compass to point towards it. This is shown in this image.

Note that the Earth's magnetic field is not actually made by any permanent magnetism inside the Earth, but by electrical currents within the core.

## Magnetic materials

You can make magnets out of certain magnetic substances. These include iron, cobalt and nickel, and some other very rare metals. Steel is mostly made of iron, so is also magnetic. Nearly every other common metal or non metal is not magnetic.
If you bring a magnet near to a magnetic material (like a steel can) that has not been magnetised, it will always be attracted to the magnet. Other substances like an aluminium drinks can are not magnetic, so will not be attracted or repelled.

Magnetic materials are further subdivided into 2 classes:

1. Permanent 'Hard' Magnetic materials (e.g. steel):-

• make permanent magnets
• retain their magnetism over long periods of time
• are hard to demagnetise.

2. Temporary 'Soft' magnetic materials (e.g. 'soft' iron):-

• Make temporary magnets
• Quickly gain and lose their magnetism.

The easiest way to magnetise a soft or hard magnetic substance is to place it in a magnetic field - preferably a strong one. (See below for more on magnetic fields). We say that magnetism has been induced ('made to happen') in the substance by the external magnetic field. However, for a soft magnetic substance, this magnetism will quickly disappear once the external field has been removed.

If you take a magnetic material like a steel paper clip and place it in a strong field, it will induce magnetism in the clip. It will immediately be attracted to the source of the original magnetic field. Induced magnetism always causes a force of attraction. Why is this? In simple terms, the induced magnet follows the field lines of the magnetic field that created it. This means the two magnets are identical and aligned the same way, which means they will attract as shown in figure 3. Note that once magnetism is induced in the paper clip, the south pole of the clip is towards the north pole of the permanent magnet.

Figure 3: Induced magnetism in a paper clip

Questions:

1. The south pole of a magnet is used to test a range of materials. Explain what will happen when the south pole is placed near to:

• a) Another south pole.
• b) A copper wire.
• c) A steel paper clip.

a) Like poles repel, so the other south pole will repel.

b) Copper is a non-magnetic material, so it will not be attracted or repelled.

c) Steel is a magnetic material so will always be attracted to any magnetic pole (by induced magnetism in the paper clip).

2. A loudspeaker contains a permanent iron core.
Explain what is meant by a permanent magnetic material.

A permanent magnetic material retains the magnetic field for a long time and is hard to demagnetise.

## Magnetic fields

Figure 4 shows what is known as a magnetic field around a magnet. A magnetic field is just a region in which a
magnetic pole experiences a force
. The lines we draw on diagrams like this show the direction of the force. These lines help us to understand magnetism and are not real! The field is all around the magnet and not just where we draw the lines.

By definition, a magnetic field line shows the direction of the force on a small north pole. You can see this in the diagram - the red north poles of the compasses are pointing in the same direction as the field lines.

Figure 4: The magnetic field around a bar magnet

Field lines always point from north to south. You should add arrows to show this when asked to draw field lines in exam questions.

The lines near the poles are close together, indicating a strong magnetic field. Generally, the further the distance from the poles, the weaker the field is.

The forces between magnets - and all magnetic forces - can be explained in terms of the interactions between magnetic fields.

## Investigating field lines

This is a standard experiment to show hidden magnetic fields, but is not a 'required practical'. However, it is still a core part of the course!

You will need:

• A permanent bar magnet (or two).
• Some iron filings in a salt shaker.
• A piece of paper or plastic (to keep the iron filings away from the bar magnet).

Method

Place the bar magnet on a desk and put the paper on top. You may need some other non-magnetic materials around the magnet to keep the paper flat. Then carefully sprinkle iron filings all around the magnet.

Results

The iron filings act as induced magnets. They magnetise in the field of the bar magnet, and line up with the magnetic field, showing the shape as shown in figure 5.

Figure 5: Investigating magnetic field lines
(Wikimedia - public domain image)

Questions:

3. A compass is used to investigate the magnetic field around a strong bar magnet.
Which of the following statements best describes the direction of the compass needle?

• a) The north pole of the compass points along the direction of the field lines.
• b) The south pole of the compass points along the direction of the field lines.
• c) The north pole of the compass points directly towards the south pole.
• d) The north pole of the compass points directly towards the north pole.

The answer is (a) The north pole of the compass points along the direction of the field lines.

This is because by definition, field lines show the direction a north pole points.

4. A student tries to demonstrate the magnetic field lines around a permanent magnet by sprinkling salt crystals on a table mat placed over a strong permanent magnet.

• a) Explain why no magnetic field pattern is seen.
• b) Suggest an improvement to this investigation, so that field lines can be seen.

a) Salt is not a magnetic material. The crystals will not be affected by the magnetic field.

b) Iron filings should be used instead, and sprinkled slowly on the region around the magnet.

Tier

## Please choose a tier of entry

 Extended Tier (Core and Supplementary content, Grades A* to G) Core Tier (Core content only, grades C to G) Remember my choice