Magnetism and Matter

•    Magnetism is the branch of Physics which deals with magnetic phenomena. Magnetism is also defined as the property due to which materials exert attractive or repulsive force on other materials. •    Magnetic field is defined as the space surrounding a magnet, in which a magnet’s magnetic force is exerted. The strength and direction of magnetic field is shown by imaginary lines called magnetic field lines. Magnetic field lines of a bar magnet can be simply traced using a compass needle. The magnetic fields produced by a bar magnet and a solenoid carrying current are same. •    A magnetic dipole is a closed circulation of electric current. A torque is experienced by a wire when a single loop of the wire with a constant current flowing through it is placed in a uniform magnetic field. The nature of a magnetic dipole and electric dipole is analogous to each other. •    Gauss’s law of magnetism states that the net magnetic flux through any closed surface is zero. Gauss’s law is applicable in both the electrostatics and magnetism with the basic difference that isolated magnetic poles do not exist but a single charge exists. A dipole or a current loop is the simplest magnetic element. •    The earth also has a magnetic field. Terrestrial magnetism or geomagnetism is the branch of Physics dealing with study of earth’s magnetism. The electrical currents produced by convective motion of metallic fluids in the outer core of the earth are responsible for earth’s magnetism.Earth’s magnetic field has three elements which are needed to specify to describe the magnetic field of the earth at a point on its surface. These elements are the angle of declination, angle of inclination, and the horizontal component of earth’s field. •    Magnetisation is defined as the net magnetic moment per unit volume of a substance. 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. •    The complex relationship between magnetic field (B) and magnetic intensity (H) in a material is shown by a B-H curve. B-h curve helps in explaining the phenomena of retentivity, coercivity and hysteresis.

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