Magnetism

Introduction

Any body which posses the power of attracting pieces of iron (magnetic material) is known as a magnet and the property of the body by virtue of which this attraction takes place is known as magnetism.

            Since lode stone possesses the magnetism when it is taken out from the earth, it is called the natural magnet. Commercial magnets are made artificially from iron and steel or alloy materials and they are called artificial magnets. Artificial magnets can be made either by rubbing a piece of iron or steel with the load  stone or by passing a electric current through a coil over the piece of iron or steel. Magnets prepared by the second method are called electro magnets.

            Magnets can be classified as being permanent or temporary, depending on their ability to retain magnetism. The material retain their magnetism for a long time after removal of magnetization force are called permanent magnets( e.g.: alnico) they are used in small dc motors, measuring instruments, speedometers, speaker etc. the substances which loses most of their strength when the magnetizing force is removed is termed as temporary magnet material( soft iron materials)

Magnetic poles

    

        Magnets have two opposite kinds of magnetism or magnetic poles, which attract or repel each other. One of the magnetic poles is called North Pole and the other South Pole. Similar poles repel each other and opposite poles attract each other. The force between two magnetic poles is directly proportional to the product of their pole strength and inversely proportional to the square of the distance between them.

Classification of magnetic materials

1.      Para-magnetic materials: they are not strongly attracted by the magnet. E.g.: aluminium, tin, platinum, manganese etc. their relative permeability is small but positive

2.      Dia-magnetic materials: they are repelled by the magnet. E.g.: Zinc, Mercury Lead, Sulphur, and Copper etc. their relative permeability is slightly less than unity.

3.      Ferromagnetic materials: they are strongly attracted by the magnet. E.g.: Iron, Steel, Nickel some of their alloys etc. their relative permeability is very high. They are further classified in to two:

o      Soft magnetic materials: do not retrain their magnetism for any appreciable time after the magnetizing force has been removed. They have very high relative permeability. E.g.: soft Iron, silicon steel soft ferrites etc.

o      Hard magnetic materials retain their magnetism for a long time. They are used for making permanent magnets and hence called permanent magnetic materials. E.g.: carbon steel, cobalt steel, alnico, hard ferrite etc.

Magnetic Field and Properties

The space around the poles of a magnet is called the magnetic field and is represented by magnetic lines of force. The total number of lines of force surrounding a magnet is called the total magnetic flux (f). The SI unit of magnetic flux is Weber (Wb)

The flux passing through a material or a plane at right angle to the direction of flux per unit area is called magnetic flux density. The unit is Tesla.

B = f/A

Magnetic Field Strength designated by H at any point is defined as the force experienced by a u\nit north pole when placed at that point.

                                               

Magnetic potential at a point in the magnetic field is defined as the work done in moving a unit north pole from infinity to that point against magnetic force.

Magnetic permeability is the measure of the ability of a material to support the formation of a magnetic field within itself.

                                                  B = mH

and                                       m = m₀m_r

where, m₀ is called the permeability of free space, and is equal to 4px10^-7H/m. and m_r = m/m₀ is called the relative permeability.

The reciprocal of magnetic permeability is magnetic reluctivity.

Magnetic susceptibility is defined as the ratio of intensity of magnetization to the magnetizing force and is represented by c.

Review Questions

1. What is an electro-magnet?

2. Define magnetic flux.

3. Substances having permeability less than the permeability of free space, are known as

1. A. Dimagnetic
2. B. Bipolar
3. C. Paramagnetic
4. D. Ferromagnetic

4. The ratio of flux density to the magnetizing force is known as

1. A. Permeability
2. B. Susceptibility
3. C. Flux density
4. D. None of the above

5. A property of a material which opposes the creation of magnetic flux in it is known as

1. A. Magnetomotive force
2. B. Reluctance
3. C. Permeance
4. D. Reluctivity