ICSE Class 6 Physics Chapter 06 Magnetism

Magnetism

You may have noticed that when you shake a pin holder, many of the pins stick to the opening, as shown in Figure 6.1(b). This is because a magnetic ring holds them there. Pin holders and the colourful plastic stickers used to attach slips of paper to the wall of a refrigerator are common examples of the use of magnets in our daily lives. Magnetic stickers usually have a small button-shaped magnet at the back.

As early as 800 BC, the ancient Greeks probably knew that magnetite, an iron ore mined in the province of Magnesia, attracted iron. The philosopher-scientist Thales of Miletus, who lived nearby, may have been the first Greek to study magnetism. The term 'magnet' was derived either from Magnesia, or from Magnus, the name of the shepherd who supposedly discovered magnetism when the iron tip of his stick and the nails of his shoes stuck to some rocks containing magnetite.

Magnetic Materials

Only some natural substances show magnetic properties. These are called magnetic materials, and include the elements iron, cobalt and nickel and many of their compounds.

Bring a magnet close to objects such as pens, pencils, pins, clips, coins, containers, utensils and common tools. The objects that are attracted by the magnet are made of magnetic materials, while the others are nonmagnetic.

Many of us are used to thinking that all magnets are made of iron, and that magnets attract all objects made of iron. Neither of these beliefs is fully correct. Many magnets do not contain any iron at all. They are made of alloys (mixtures) of cobalt, nickel and other elements. One such alloy of aluminium, nickel and cobalt is called alnico. Other magnetic materials called ferrites also do not contain iron.

In the activity described earlier, you may have noticed that a magnet does not attract utensils made of stainless steel. Thus, stainless steel is not a magnetic material, although it contains a large proportion of iron.

Magnets And Their Properties

An object which attracts other objects made of magnetic materials is called a magnet. The properties of magnets and the manner in which they interact with each other and with other objects made of magnetic materials is collectively called magnetism. As we shall learn later, magnetism also includes those effects of electric currents, which are similar to and closely linked with the properties of magnets. Magnets are available in different shapes, sizes and strengths at stores for scientific supplies, hobby shops and some toy shops. The common shapes are shown in Figure 6.2.

Poles Of A Magnet

Spread some iron filings on a sheet of paper or board. Place a bar magnet on this and move it around. Alternatively, sprinkle iron filings over a bar magnet, pick up the magnet and tap it gently a few times. You will find that the iron filings stick mostly to the ends of the magnet.

This activity shows that a magnet's property of attracting iron is maximum at its two ends and almost nil elsewhere. It is, in fact, possible to identify two points near the ends of a magnet where all its magnetic properties appear to be concentrated. These two points are called the poles of a magnet. They lie slightly inside the two ends. The distance between them is approximately 0.85 times the length of the magnet.

You may have wondered at the unusual shape of a horseshoe magnet. The advantage of the shape is that the two poles lie next to each other. Hence, their combined attractive power can act on the same object.

Directional Property Of A Magnet

Suspend a bar magnet from a fixed support such that it remains horizontal and can rotate freely, as shown in Figure 6.4. You will find that after some time it comes to rest along the north-south direction. Make a chalk mark on the end that is pointing towards the north. Disturb the position of the magnet and then allow it to come to rest. You will find that the magnet again lies along the north-south direction, with the marked end pointing north. Repeat the activity a few times.

The property of a magnet that we have tested in this activity is called its directional property. This property makes it possible to distinguish between the two poles of a magnet. The pole which points to the north is called the north seeking pole, or simply the north pole. The other pole is called the south pole.

The directional property of magnets has been of great importance in history. In ancient times, it was difficult for sailors to find their way when they were far away from land. This made long sea voyages difficult and dangerous. About one thousand years ago, natural magnets began to be used for navigation. A piece of natural magnet, called a lodestone, would be suspended on board a ship to find the north direction. This gave rise to the magnetic compass, which is used even today to find the direction.

Forces Between Magnets

Any two magnets exert forces on each other. This can be seen with the help of a few simple experiments.

Use a paper strip to hang a magnet. Bring the north pole of another magnet close to its south pole. The two poles will attract each other. The same thing will happen if you bring the south pole of the magnet you are holding close to the north pole of the suspended magnet.

Next bring the south pole of the magnet you are holding close to the south pole of the suspended magnet. In this case the suspended magnet will move away. The same thing will happen if you bring the north poles of two magnets close to each other.

It is possible to draw the following conclusions from these observations.

1. Magnets exert forces on each other.

2. These forces may be attractive, i.e., tending to pull the magnets towards each other, or repulsive, i.e., tending to push the magnets away from each other.

3. Like poles repel each other. This means two north poles try to push each other away and two south poles try to push each other away.

4. Unlike poles attract each other. This means a north pole and a south pole attract each other.

Test For A Magnet

Suppose you have a piece of metal and you want to find out whether it is a magnet. It would appear that you can do this easily by checking whether it attracts a small piece of iron. However, even if there is some attraction between the two, you cannot be sure as to which of them is a magnet. A surer test would be to take the piece of metal you want to test close to a magnet. Place the two in contact in different positions. If there is repulsion between them at some position, you can be sure that the piece of metal you are testing is a magnet.

This is summarised as repulsion is a surer test for magnetism than attraction.

Magnetic Field

The region around a magnet, where its effects can be felt, is called its magnetic field. It is possible to get some idea about the nature of this field through a simple experiment.

Sprinkle some iron filings uniformly on a thin sheet of glass or transparent plastic. Place the sheet carefully on top of a bar magnet and tap it a few times. The iron filings will arrange themselves as shown in Figure 6.6. This provides something like a photograph of the magnetic field.

Magnetic Induction

Normally pieces of iron, such as paper clips and pins, do not attract each other. However, if a bar magnet is brought close to them, they get attached to each other to form a chain. Why does this happen?

When a piece of magnetic material comes close to a magnet, temporary magnetic poles are created at its ends. Therefore, it becomes a magnet for the time being. This process by which a piece of magnetic material starts behaving like a magnet when it is brought close to a magnet is called magnetic induction. In Figure 6.7, a south pole is created or induced at the end of the clip touching the north pole of the magnet. The attraction between these opposite poles is what we see as the attraction between the magnet and the clip. The same process is repeated in the other clips. We can now say that in the process of attraction of a piece of magnetic material by a magnet, induction precedes (comes before) attraction.

The effects of magnetic induction can be either temporary or permanent. In the example we have discussed, they are temporary. The clips behave as small magnets only as long as they are in contact with the magnet. They lose this property when they are separated from the magnet.

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