ICSE Class 6 Physics Chapter 7 Magnetism

7 Magnetism

The force that a magnet exerts on iron is called magnetic force.

Substances that get attracted by a magnet are called magnetic substances. Iron, steel, cobalt and nickel are magnetic substances. Substances that do not get attracted by a magnet are called non-magnetic substances, e.g., wood, plastic, copper, paper, aluminium, rubber, stone, sand, ceramics, glass, aluminium, gold, silver, brass, etc. Magnetic substances are also known as FERRO MAGNETIC substances.

Syllabus

This chapter contains:

- Discovery of magnets, artificial and natural magnets, poles of a magnet, properties of magnetic poles.

- Types of magnet: Temporary magnet, permanent magnet, discovery of an electromagnet.

- Making magnets: Magnetic induction method, single touch method, double touch method, electrical method.

- Advantage and uses of electromagnets, demagnetising a magnet, properties of a bar magnet, uses of magnets.

Discovery Of Magnets

The first magnets were found from a naturally occurring mineral called magnetite around 800 B.C. There is a story about a shepherd named Magnes whose shoe nails and iron hook of his stick were stuck to a rock containing magnetite. The rock was later found to attract iron pieces. This rock was also called lodestone. The lodestone means stone which can indicate a correct direction. The word magnet originated from the name of town Magnesia, a district in Asia Minor where large deposits of magnetite were found.

What is a magnet?

A magnet is an object made of certain materials like iron which can create a magnetic field. The property of attracting iron by the magnet is called magnetic attraction. The force that a magnet exerts on iron is called magnetic force.

Artificial And Natural Magnets

Since magnetite is already magnetised when found in nature, it is called a natural magnet. Magnets that are made by man are called artificial magnets. These are generally made of steel in various shapes and sizes according to their uses. We have bar magnets, horseshoe magnets, U-shaped magnets, cylindrical magnets, magnetic needle, magnetic compass, etc.

Activity 1

To identify magnetic and non-magnetic substances.

Material required: Substances - coal, metals, wood, plastics, etc.

Procedure: Get some specimens of different substances, e.g., iron, steel, nickel, plastic, wood, copper, stainless-steel spoon, etc.

Use a magnet to test which ones are attracted by the magnet and which ones are not. Record your observations in the table given below:

Conclusion: The substances that are attracted by a magnet are called magnetic substances, whereas those that are not attracted by a magnet are called non-magnetic substances.

Teacher's Note

Magnetic substances like iron are used in everyday tools and appliances because their attraction to magnets makes them useful for fastening, holding, and movement. This principle is fundamental to devices like refrigerator door seals and industrial magnets used in manufacturing.

Activity 2

Searching a sewing needle in a box containing buttons, thread etc.

Material required: A bar magnet, a box containing buttons, thread etc. and a sewing needle.

Procedure: Drop the sewing needle into the box and reshuffle the contents. You may not be able to see the needle. Now move the bar magnet over the contents of the box.

The sewing needle gets attracted to the magnet and sticks to it.

You have found the sewing needle easily.

Teacher's Note

This activity demonstrates how magnets can help locate hidden magnetic objects, a principle used in metal detectors at security checkpoints and archaeological digs to find metal artifacts underground.

Poles Of A Magnet

If a bar magnet is suspended with a string tied at its mid-point such that it rests horizontally and can turn freely, it will come to rest pointing in a north-south direction. If it is disturbed and again allowed to come to rest, the same end of the bar will point to the north. The end of the magnet pointing geographical north is called the north pole of a magnet. This is an abbreviation for "a north-seeking pole". Similarly, the other end of the bar magnet is "a south-seeking pole", called the south pole.

Every magnet, no matter what is its shape, has both north and south poles. If you break a bar magnet into two pieces, each piece will again have a north pole and a south pole. The strength of the magnet is concentrated at these poles. This can be shown by dipping a bar magnet into a box of iron filings. It is seen that a large number of filings stick to the ends or poles of the magnet and only a very few are attracted towards its centre portion.

Activity 3

Take a bar magnet and suspend it by a string tied at its centre so that it may hang horizontally and freely as shown in Fig. 7.3. You will notice that the magnet points in the north-south direction when it comes to rest. You disturb the magnet any number of times, but when at rest, it will again point in the north-south direction.

Teacher's Note

The Earth itself acts as a huge magnet with its own magnetic poles, which is why suspended magnets align with the north-south direction - this principle is used in compass navigation for hiking, sailing, and geological surveys.

Properties Of Magnetic Poles

Like poles repel, unlike poles attract:

Bring the north pole of a bar magnet near the north pole of a freely suspended bar magnet. The pole of the suspended magnet will be repelled. If the south pole of the first magnet is brought near the south pole of the suspended magnet, the same repulsion will take place. However, if the south pole of the first magnet is brought near the north pole of the suspended magnet, we observe an attraction between the opposite poles. This proves that 'Like poles repel and unlike poles attract'.

Repulsion is the only true test for a magnet:

Two north poles or two south poles are called like poles. A north pole and a south pole are unlike poles. It should be noted that repulsion is the only true test for a magnet. A magnet will attract a magnetic substance and will also attract another magnet if two unlike poles are facing each other. However, repulsion indicates that both are like poles. When an object is repelled by a magnet, the object is definitely a magnet.

Activity 4

Take a bar magnet and suspend it from a stand with a thread. It would come to rest in the north-south direction. Take another magnet and bring its north pole close to the north pole of the suspended magnet. The north pole of the suspended magnet will be repelled. Similarly, the repulsion of south pole of the suspended magnet will be observed when the south pole of another magnet is brought close to it. Now bring the north pole of another magnet near the south pole of the suspended magnet. The south pole of the suspended magnet will be attracted towards the north pole of the second magnet. Similarly, the south pole of the second magnet will be attracted towards the north pole of the suspended magnet and vis-a-vis.

Teacher's Note

The repulsion between like poles is used in maglev trains, which use magnetic levitation to allow trains to float above tracks without friction, enabling faster and more efficient transportation.

Magnetic poles always occur in pairs:

Take a bar magnet and cut it into two pieces. You will have two new magnets each with its own north and south poles. No matter how many pieces you make out of these, each piece will behave as a magnet, i.e., having a north pole and a south pole. It is not possible to isolate one pole from a magnet, i.e., north and south poles cannot exist separately.

Hence, we conclude that a single magnetic pole can never exist; rather they always occur in pairs.

Types Of Magnets

Some magnets can retain the magnetic properties for a longer period of time while some other magnets show magnetism as long as they are being magnetised. Accordingly, magnets can be classified as permanent and temporary magnets. The third type of magnets are called electromagnets.

Temporary Magnets

These magnets are usually made of soft (pure) iron. They can act as magnets only for a short duration. They lose their magnetism as soon as the magnetising force is removed. For example, paper clips, iron nails and other soft iron items.

Permanent Magnets

These magnets are made of steel, cobalt and nickel and are capable of retaining magnetism for a longer period of time even when the magnetising force is removed.

Differences between temporary and permanent magnets

Teacher's Note

Permanent magnets in steel are used in electric guitars and speakers, where sustained magnetism is needed to convert electrical vibrations into sound waves, whereas temporary electromagnets in doorbells can be switched on and off as needed.

Knowledge Bank

Iron is called a 'soft' magnetic material because it loses its magnetism easily. Iron is used in making electromagnets. It acts as a magnet as long as you are magnetising it. The moment you stop the process of magnetising, it loses the magnetic property.

Steel is called a 'permanent' magnetic material because it does not lose its magnetism easily. It remains a magnet even after you stop the process of magnetising it. This is the reason why it is used to make a permanent magnet.

Discovery of an Electromagnet

A Danish Physicist, Hans Christian Oersted, in 1819, discovered that there is a magnetic field around every wire carrying an electric current. This discovery has been one of the most fruitful achievements in the history of physical sciences. This was the discovery of electromagnets which are widely used. Whenever we ring an electric bell, run a motor, talk over a telephone or listen to a transistor, we make use of the magnetic field that surrounds current-carrying wires. Electromagnets are also used in fans, motors, mixers, airconditioners, etc. They are used for lifting heavy iron loads. You will learn more about them later in this lesson.

Teacher's Note

Electromagnets power many devices we use daily, from the starter motor in cars to the magnetic brakes in trains, making them one of the most important applications of magnetism in modern technology.

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