Lesson 5: Magnets

Unit 1: Magnet and Magnetic Materials
A substance is said to be magnetic if it is attracted by a magnet. Objects pointing to same direction when freely suspended are said to be magnet.

Examples of magnetic materials are iron, cobalt, nickel and certain alloys. Substances such as brass, wood, copper and glass are not attracted by magnets. These are called non-magnetic materials,

Unit 2: Properties of magnets and patterns
(i) Experiments show that iron filings (small pieces of iron) cling mainly to the ends of a bar magnet. This shows that the ends of a magnet are regions where the attracting power is greatest. These ends are called poles.

(ii) It is observed that if a bar magnet is suspended so that it swings freely in a horizontal plane, it always comes to rest with its axis pointing approximately north-south (N.S). The pole which is attracted towards the North pole of the earth is called a north seeking pole, or simply a north pole, and the pole which is attracted towards the south pole is called a south seeking or just a south pole. A suspended or pivoted magnet used to find north and south is called a compass.

This is called the basic law of magnetism. Thus a north and a south pole attract each other, but two north or two south poles repel each other.

Test for polarity of a magnet
The polarity of a magnet may be tested by bringing both its poles in turn near to the known poles of a suspended magnet. Repulsion will indicate similar polarity. If attraction occurs, no firm conclusion can be drawn, since attraction would be obtained between either two unlike poles or a pole and a piece of unmagnetised magnetic material. Repulsion is therefore the only sure test for polarity.

Unit 3 Theory of magnetism
If we cut a magnet in two, an interesting thing happens. Instead of separating the north and south poles, we obtain two magnets. Extra north and south poles are formed and each of the two small magnets has half the strength of the original magnet.

This process can be continued, repeatedly dividing a magnet in two, and obtaining more smaller magnets. We might ask, ‘How long can this go on? What is the smallest magnet we can obtain?’ Clearly, the smallest magnet is one molecule, and the original magnet must have been made up of a very large number of tiny molecular magnets.

This explains the difference between iron (and the other ferromagnetic elements like nickel and cobalt) and non-magnetic material s. Their molecules are magnets, while those of non-magnetic materials are not.

When magnetic materials are unmagnetised, the molecular magnets are mixed up to point in random directions, and they cancel each other When the material is magnetised, the molecular magnets are all turned to point the same way and reinforce each other.

Magnetising a piece of iron thus consists of turning its molecules more in line, while demagnetising it consists of mixing up the molecules again. This is, of course, a simplified view of the situation.

Unit 4: Magnetisation and Demagnetisation of Magnetic Materials

Magnetisation
Magnetisation is the process of making magnets. This process can be effected by any of the following methods:

(i) Single Touch method
(ii) Divided Touch method
(iii) Hammering method
(iv) Electrical method

(i) Single touch method
In this method, the magnetic material to be magnetized is stroked from one end to the other several times with the pole of a permanent magnet. For example, if the material ¡s stroked from the start with the north pole end of a permanent magnet, the other end of the magnetic bar acquires south polarity.

(ii) Double stroking or divided touch
A steel bar can be magnetised by stroking the bar from the middle with opposite pole end of two permanent magnets towards the opposite ends. If a North Pole and South Pole ends of two permanent magnets are used in stroking a metal bar from middle to opposite ends several times, opposite polarity are then formed.

If similar pole ends of two permanent magnets are used in stroking, the end will have similar but opposite polarity to the stroking ends. Such polarity so produced at the ends is called consequent poles.

(iii) Hammering method
This is an obsolete method which was adopted by placing a red-hot steel bar along the earth N—S field and hammered repeatedly for some time. The bar is then allowed to cool in the same direction. On testing the lower end will be found to possess a weak north-polarity.

(iv) Electrical method:
The best method of magnetising is to insert it into a solenoid (a coil of wire with many turns) through which a steady direct current flows, as shown below:

After some time, the material is removed and it is found to have become a magnet. The polarity of the magnetised specimen depends on the direction of the current. If when we look at the end of the bar the current is flowing in a clockwise direction, that end will be a south pole.

If current is flowing anticlockwise, it will be a north pole. The material is stroked from one end to the other end several times in the same direction with one pole of a magnet. This pulls the opposite poles of the molecular magnets in one direction and leaves them pointing one way. A disadvantage of this is that it produces magnets in which one pole is nearer the end of the material than the other.

Demagnetisation
The process by which a magnet loses its magnetism is known as Demagnetisation. To demagnetise a magnet, the molecules have to be shaken out of their orderly arrangement. The best way of demagnetise a magnet is to place it inside a solenoid through which an alternating current is flowing as shown below:

The solenoid is placed with its axis pointing east to west. After a few seconds the magnet is slowly withdrawn from the solenoid and taken a Long distance away. The alternating current reverses every 0.01s and hence reverses the magnetism in the material 100 times per second. This has the effect of shaking up the molecules and making the material lose its magnetism.

Another method of destroying magnetism is by heating the magnet until it is red hot and then allowing it to cool while lying in an E-W direction. The molecules are shaken up by thermal agitation. This is not recommended as a practical method because the heat would spoil the steel.

It should be noted that any rough treatment of a magnet such as dropping it, hammering it in the east-west direction or disorderly arrangement during storage will cause weakening of the magnetism in a magnet.

Unit 5: The Magnetic Properties of Iron and Steel

(i) Iron is more easily magnetised than steel.
(ii) Iron is more easily demagnetised than steel.
(iii) In a solenoid bearing a set current, iron becomes more strongly magnetised than steel.
(iv) Steel keeps its magnetism much longer than iron.

Because of these differences in their magnetic properties, iron and steel are used for different things.

(a) Steel is used in making permanent magnets, such as compass needles, bar magnets, ball-ended magnets, and so on.
(b) Iron nails are often used for experiments in magnetisation and demagnetisation because they are easier to magnetise and demagnetise.
(c) Iron is used for making electromagnets where strong magnetism is required for a short time.
(d) Steel is used for magnets in vehicles where magnetism can be lost by vibration.

Induced Magnetism
When a piece of unmagnetised magnetic material touches or is brought near to the pole of a permanent magnet, it becomes a magnet itself. A bar magnet is fixed in a wooden clamp as shown below:

NB – (do not use a metal clamp for this experiment).

In the figure above, soft iron nails (unmagnetised) are placed at the north end of the magnet as shown. The nails can be placed one below the other. The number of nails that can be hung in a single chain depends on the strength of the magnet. The stronger the magnet the larger the number of nails. Experiment shows that if the magnet is gently moved, all the nails fall off.

Explanation
It is clear that the first nail is attracted by the magnet, hence it sticks to it and becomes a magnet; it has been magnetised by induction. In the same way the second, third and fourth nails have been magnetised by the first. They will remain attached to one another as long as the magnet is there. Once the magnet which induces magnetism is removed they will fall off.

What happens is that the pole of the magnet attracts the opposite poles of the molecular magnets in the magnetic material, partially turning the molecular magnets in line and magnetising the material.

Now suppose an experiment is performed with a magnet, a nail and iron filings, in which the magnet does not touch the nail but is brought near it, it is found that the iron filings jump up and become attached to the nail which has been magnetised by induction. This shows that actual contact is not needed for magnetic induction to take place.

Scroll Down to Select Page 7 for the next lesson – Lesson 6: Elastic Properties of Solids (Hooke’s Law)


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