ICSE Class 8 Physics Chapter 12 Magnetism

Magnetism

You have read in your earlier classes that magnet was first discovered in a small town Magnesia in Asia Minor, around 800 B.C. It was an ore of iron which showed the property of attracting iron objects. This ore was called Magnetite. It was found that freely suspended strips of this material always rest in north-south direction. For this characteristic property, it was called the lodestone. Through chemical investigations, it was found to be composed of oxides of iron (Fe3O4). Now they are called magnets. They show the property of attracting iron. This property of attracting iron is known as magnetism and the force that a magnet exerts on iron or on another magnet is called magnetic force.

Magnetic And Non-Magnetic Substances

Those substances which are attracted by a magnet are called magnetic substances. For example, iron, cobalt and nickel are called magnetic substances.

Those substances which are not attracted by a magnet are called non-magnetic substances. For example, aluminium, copper, wood, plastic, etc., are called non-magnetic substances.

Properties Of A Magnet

Attractive Property

Magnets have attracting property and they attract magnetic substances towards them. This attractive property is zero at the middle and maximum at the ends of a bar magnet.

Experiment: Spread some iron filings uniformly on a table top and then place a magnet (bar magnet) over them.

Now lift the magnet gently and slowly. You will observe that maximum number of iron filings are clinging near the two ends of the magnet and they keep on decreasing towards the centre of the magnet.

Thus, a bar magnet has maximum attractive property at its two ends and zero attractive property at its centre. The two ends of the magnet, at which it has maximum attractive property, are called poles of the magnet. A magnet has two poles, the north pole and the south pole. It must be noted here that the poles are not exactly at the ends of the magnet but they are slightly inside.

In case of a U-shaped or a horse-shoe magnet, the two open ends serve as the poles. When a U-shaped magnet is dipped in iron filings, the maximum number of iron filings cling at the two ends of it, as shown in Fig. 12.2.

Directive Property

Suspend a bar magnet freely with the help of a silk thread. Note the direction in which it comes to rest. Now turn the magnet to some other direction and leave it free. Again, note the direction in which it finally comes to rest.

You will observe that the bar magnet always comes to rest along the north-south direction of the earth (see Fig. 12.3). The end of the bar magnet which points towards the north pole of the earth is called the north pole of the magnet, while the end that points to the south pole of the earth is called the south pole of the magnet.

The property of a magnet to always point in the north-south direction of the earth is called the directive property.

On a magnet, north and south poles are represented by letters N and S respectively.

Like Poles Repel Each Other While Unlike Poles Attract

Take two bar magnets. Suspend one magnet freely with the help of a silk thread. Hold the other magnet in your hand. Take the north pole of the magnet you are holding near the north pole of the suspended magnet. You will find that they repel each other. This will be shown by the north pole of the suspended magnet moves away from the north pole held of the magnet in your hand.

Now take the south pole of the magnet in your hand near the north pole of the suspended magnet. You will find that the suspended magnet gets attracted to the magnet held by you. The same will happen if you direct the south pole of the magnet you are holding towards the north pole of the suspended magnet (see Fig. 12.4).

Thus we conclude that, like poles repel each other while unlike poles attract each other.

Poles Always Exist In Pairs

Take a bar magnet and break it into two pieces. It will be observed that each of the two pieces acts as an independent magnet with its own north and south poles. This can be verified by using attractive and directive properties of the magnet. Again, break each piece of the magnet into smaller pieces. It will again be observed that each small piece so obtained also behaves like an independent magnet, i.e., having a north pole and a south pole (see Fig. 12.5). It is not possible to isolate one pole from a magnet, i.e., the north and south poles can not exist separately.

Hence we conclude in a magnet a single pole can never exist or the poles always exist in pairs.

Magnetic Compass

A magnetic compass is a small magnetic needle kept in a brass box which has a glass top. The magnetic needle is pivoted at its centre i.e., the needle is free to move (rotate) about its centre. The end of the needle, which indicates its north pole, is painted red. When the magnetic compass is placed on the table, the needle rests in the north-south direction with the red end pointing towards north direction. Thus, a magnetic compass is used to locate the direction at a place. It is used by travellers, sailors and navigators to find direction when they sail through an unknown location.

Teacher's Note

Compasses have been used for centuries by explorers and sailors to navigate across oceans and find their way home, making magnetism one of humanity's most practical discoveries.

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