# Magnetism and Matter

## The branch of Physics which deals with magnetic phenomena is known as magnetism. Magnetism is also defined as the property due to which materials exert attractive or repulsive force on other materials. The space surrounding a magnet, in which its magnetic force is exerted, is called the magnetic field. Imaginary lines that indicate the strength and direction of a magnetic field are called magnetic field lines. Use of a compass needle is a simple way to trace the magnetic field lines of a bar magnet. Bar magnet and a solenoid carrying current produce the same magnetic fields. A magnetic dipole is a closed circulation of electric current. When a single loop of wire with a constant current flowing through it is placed in a uniform magnetic field, the wire experiences a torque. The nature of a magnetic dipole is analogous to a electric dipole. Gauss’s law of magnetism states that the net magnetic flux through any closed surface is zero. The basic difference between the Gauss’s law in electrostatic and in magnetism lies in the fact that isolated magnetic poles do not exist but a single charge exists. The simplest magnetic element is a dipole or a current loop. Te earth also has a magnetic field. The branch of Physics dealing with study of magnetism of earth is called terrestrial magnetism or geomagnetism. The earth’s magnetism is due to the electrical currents produced by convective motion of metallic fluids in the outer core of the earth. Elements of earth’s magnetic field - the angle of declination, angle of inclination, and the horizontal component of earth’s field are needed to specify to describe the magnetic field of the earth at a point on its surface. The net magnetic moment per unit volume of a substance is called magnetisation. On the basis of magnetisation, materials found on earth are divided into three categories – diamagnetic, paramagnetic and ferromagnetic. Diamagnetic substances are those substances which are repelled by a magnet. Paramagnetic substances are the substances which are weakly magnetised when placed in an external magnetic field. Ferromagnetic substances are the substances which gets strongly magnetised when placed in an external magnetic field. B-H curve is a graphical representation to obtain a complex relationship between magnetic field (B) and magnetic intensity (H) in a material. The phenomena of retentivity, coercivity and hysteresis are explained on the basis of B-H curve. The B-H curve also helps in determining the materials used to make permanent magnets and electromagnets. Core of electromagnets are made of ferromagnetic materials which have high permeability and low retentivity. Soft iron is suitable material for electromagnets. Electromagnets are used in electric bells, loudspeakers and telephone diaphragms.

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