MAGNETISM

Introduction

Magnets are objects of stone, metal or other material which have the property of attracting metals like iron, cobalt and nickel. The attracting property of a magnet is called magnetism and it is either natural or induced. The branch of physics which deals with the property of a magnet is also called magnetism.

The earliest evidence for magnets are found in a region of Asia Minor called Magnesia. It is believed that the Chinese had known the property of magnet even before 200 B.C. They used a magnetic compass for navigation in 1200A.D. Use of magnets in compasses facilitated long-distance sailing.

After the discovery of magnets the world progressed into a new direction. Today magnets play an important role in our lives. Magnets are used in refrigerators, computers, car engines, elevators and many other devices.

1 Classification of Magnets

Magnets are classified into two types. They are:

i. Natural magnets ii. Artificial magnets

Natural Magnets

Magnets found in the nature are called natural magnets. They are permanent magnets i.e., they will never lose their magnetic power. These magnets are found in different places of the earth in the sandy deposits.

Lodestone called magnetite (Iron oxide) which is the ore of iron is the strongest natural magnet. Minerals like Pyrrhotite (Iron Sulphide), Ferrite and Coulumbite are also natural magnets.

Artificial Magnets

Magnets that are made by people in the laboratory or factory are called artificial magnets. These are also known as manmade magnets, which are stronger than the natural magnets.

Artificial magnets can be made in various shapes and dimensions. Bar magnets, U-shaped magnets, horseshoe magnets, cylindrical magnets, disc magnets, ring magnets and electromagnets are some examples of artificial magnets.

Artificial magnets are usually made up of iron, nickel, cobalt, steel, etc. Alloy of the metals Neodynium and Samarium are also used to make artificial magnets.

2 Magnetic Properties

The properties of a magnet can be explained under the following headings.

Ø Attractive property

Ø Reflective property

Ø Directive property

2.1 Attractive Property

A magnet always attracts materials like iron, cobalt and nickel. To understand the attractive property of a magnet let us do an experiment. Take some iron fi lings in a paper and place a magnet near them.

You can observe here that the iron fi lings are attracted near the ends of the magnet. These ends are called poles of a magnet. This shows that the attractive property of a magnet is more at the poles. One pole of the magnet is called the North Pole and the other pole is called the South Pole. Magnetic poles always exist in pairs.

When a bar magnet is broken into two pieces, each broken piece behaves like a separate bar magnet. When a magnet is split vertically, the length of the magnet is altered and each piece acts as a magnet. When a magnet is split horizontally, the length of the new pieces of magnet remains unaltered and there is no change in their polarity. In both cases the strength of the magnet is reduced.

2.2 Repulsive Property

Take a bar magnet and suspend it from a support. Hold another bar magnet in your hand. Bring the north pole of this magnet close to the north pole of the suspended magnet then the north pole of the suspended magnet will move away.

This activity explains another property of a magnet that like poles repel each other i.e., a north pole repels another north pole and a south pole repels another south pole. If you bring the south pole of the magnet close to the north pole of the suspended magnet you can see that the south pole of the suspended magnet is immediately attracted.

Thus we can conclude that unlike poles of a magnet attract each other. i.e., the north pole and the south pole of a magnet attract each other.

2.3 Directive Property

Suspend a bar magnet from a rigid support using a thread. Ensure that there are no magnetic substances placed near it. Gently disturb the suspended magnet. Wait for a moment, let it oscillate. In a short time it will come to rest. You can see that the north pole of the magnet is directed towards the geographic north. Repeat the procedure a number of times. You will observe that the magnet is oriented in the same direction.

This experiment shows that a freely suspended bar magnet always aligns itself in the geographic north-south direction. The property of a magnet, by which it aligns itself along the geographic north-south direction, when it is freely suspended, is known as the directive property of a magnet. The north pole of the magnet points towards the geographic north direction and the south pole of the magnet points towards the geographic south direction.

3 Magnetic Field

Spread some iron fi lings which are collected from the sand uniformly on a sheet of white paper placed on a table. Place a bar magnet below the white sheet. Gently tap the table. You can see the pattern as shown in the figure.

You can observe from this experiment that the iron fi lings are arranged in the form of curved patterns around the magnet. The space around the bar magnet where the arrangement of iron fi lings exists, represents the field of influence of the bar magnet. It is called the magnetic field. Magnetic field is defined as the space around a magnet in which its magnetic effect or influence is observed. It is measured by the unit tesla or gauss (1 tesla =10,000 gauss).

3.1 Tracing the magnetic field

We can trace the magnetic field with the help of a compass needle. A white sheet of paper is fastened on the drawing board using the board pins or cello tape. A small plotting compass needle is placed near the edge of the paper and the board is rotated until the edge of the paper is parallel to the magnetic needle. The compass needle is then placed at the centre of the paper and the ends of the needle, i.e., the new positions of the north and south pole are marked when the needle comes to rest. These points are joined and a straight line is obtained. This line represents the magnetic meridian. Cardinal directions N-E-S-W are drawn near the corner of the paper.

We can observe here that the compass needle gets deflected to a large extent, when it is closer to the magnet. When the distance is large, the deflection of the needle is gradually decreased. At one particular position there is no deflection because there is no magnetic force at this position. This shows that each magnet exhibits its magnetic influence around its specific region.

4 Magnetic Materials

Materials which are attracted by magnets are called magnetic materials and those materials which are not attracted by magnets are called non-magnetic materials. There are a number of materials that can be attracted by magnets. These can be magnetised to create permanent magnets. Magnetic materials can be categorised as magnetically hard or magnetically soft materials. Magnetically soft materials are easily magnetised. Magnetically hard materials also can be magnetised but they require a strong magnetic field to be magnetised.

Based on their behaviour in a magnetic field they can be classified as below.

Ø Diamagnetic

Ø Paramagnetic

Ø Ferromagnetic

4.1 Diamagnetic materials

Diamagnetic materials have the following characteristics.

Ø When suspended in an external uniform magnetic field they will align themselves perpendicular to the direction of the magnetic field.

Ø They have a tendency to move away from the stronger part to the weaker part when suspended in a non-uniform magnetic field.

Ø They get magnetised in a direction opposite to the magnetic field.

Ø Examples for diamagnetic substances are bismuth, copper, mercury, gold, water, alcohol, air and hydrogen.

4.2 Paramagnetic materials

The following are the characteristics of paramagnetic materials.

Ø When suspended in an external uniform magnetic field they will align themselves parallel to the direction of the magnetic field.

Ø They have a tendency to move from the weaker part to the stronger part when suspended in a non-uniform magnetic field.

Ø They get magnetised in the direction of the field.

Ø Examples for paramagnetic substances are aluminium, platinum, chromium, oxygen, manganese, solutions of salts of nickel and iron.

4.3 Ferromagnetic materials

The characteristics of ferromagnetic materials are given below.

Ø When suspended in an external uniform magnetic field they will align themselves parallel to the direction of the magnetic field.

Ø It has a tendency to move quickly from the weaker part to the stronger part when suspended in a non-uniform magnetic field.

Ø They get strongly magnetised in the direction of the field.

Ø Examples for ferromagnetic substances are iron, cobalt, nickel, steel and their alloys.

5 Artificial Magnets

Ø Artificial magnets are produced from magnetic materials. These are generally made by magnitising iron or steel alloys electrically.

Ø These magnets are also produced by stroking a magnetic material with magnetite or with other artificial magnets.

Ø Depending on their ability to retain their magnetic property, artificial magnets are classified as permanent magnets or temporary magnets.

5.1 Temporary Magnets

Ø Temporary magnets are produced with the help of an external magnetic field. They lose their magnetic property as soon as the external magnetic field is removed.

Ø They are made from soft iron. Soft iron behaves as a magnet under the influence of an external magnetic field produced in a coil of wire carrying a current. But, it loses the magnetic properties as soon as the current is stopped in the circuit.

Ø Magnets used in electric bells and cranes are the examples of temporary magnets.

5.2 Permanent Magnets

Ø Permanent magnets are artificial magnets that retain their magnetic property even in the absence of an external magnetic field.

Ø These magnets are produced from substances like hardened steel and some alloys.

Ø The most commonly used permanent magnets are made of ALNICO (An alloy of aluminium, nickel and cobalt).

Ø Magnets used in refrigerator, bar magnet, speaker magnet, fridge magnet and magnetic compass are some familiar examples of a permanent magnet.

Ø Neodymium magnets are the strongest and the most powerful magnets on the Earth.

The magnetic properties of a magnet will be removed from it by the following ways.

Ø Placing the magnet idle for a long time.

Ø Continuous hammering of the magnetic substance.

Ø Dropping the magnet from a height.

Ø Heating a magnet to a high temperature.

6 Earth’s Magnetism

Earth has been assumed or imagined by the scientists as a huge magnetic dipole. However, the position of the Earth’s magnetic poles is not well defined in the Earth.

Ø The south pole of the imaginary magnet inside the Earth is located near the geographic north pole and the north pole of the earth’s magnet is located near the geographic south pole.

Ø The line joining these magnetic poles is called the magnetic axis.

Ø The magnetic axis intersects the geographic north pole at a point called the north geomagnetic pole or northern magnetic pole. It intersects the geographic south pole at a point called the south geomagnetic pole or southern magnetic pole.

Ø The magnetic axis and the geographical axis (axis of rotation) do not coincide with each other.

Ø The magnetic axis of the Earth is inclined at an angle of about 10° to 15° with the geographical axis.

The exact cause of the Earth’s magnetism is not known even today. However, some important factors, which may be the cause of the Earth’s magnetism, are as follows.

Ø Masses of magnetic substances in the Earth

Ø Radiations from the Sun

Ø Action of the Moon

However, it is believed that the Earth’s magnetic field is due to the molten charged metallic fluid inside the Earth’s surface with a core of radius of about 3500 km compared to the Earth’s radius of 6400 km.

6.1 Earth’s Magnetic Field

A freely suspended magnetic needle at a point on the Earth comes to rest approximately along the geographical north - south direction. This shows that the Earth behaves like a huge magnetic dipole with its magnetic poles located near its geographical poles.

The north pole of a magnetic needle approximately points towards the geographic north (NG). Thus, it is appropriate to say that the magnetic north pole of the needle is attracted by the magnetic south pole of the Earth (Sm), which is located at the geographic north NG.

Also, the magnetic south pole of the needle is attracted by the magnetic north pole of the Earth (Nm), which is located at the geographic south SG.

The magnitude of the magnetic field strength at the Earth’s surface ranges from 25 to 65 micro tesla.

7 Uses of Magnets

We come into contact with magnets offen in our daily life. They are used in wide range of devices. Some of the uses of magnets are given below.

Ø In ancient times the magnet in the form of ‘direction stone’ helped seamen to find the directions during a voyage.

Ø Nowadays, magnets are used to generate electricity in dynamos.

Ø Magnets, especially electromagnets are used in our day to day life.

Ø They are used in electric bells and electric motors.

Ø They are used in loudspeakers and microphones.

Ø An extremely powerful electromagnet is used in the fast moving Maglev train to remain floating above the tracks.

Ø In industries, magnetic conveyor belts are used to sort out magnetic substances from scraps mixed with non-magnetic substances.

Ø Magnets are used in computer in its storing devices such as hard disks.

Ø In banks, the magnets enable the computers to read the MICR numbers printed on a cheque.

Ø The tip of the screw drivers are made slightly magnetic so that the screws remain attached to the tip.

Ø At hospitals, extremely strong electro magnets are used in the MRI (Magnetic Resonance Imaging) to scan the specified internal organ.