MAGNETISM

A permanent magnet is a ferromagnetic material (alloy of iron, nickel and cobalt). When magnet is freely suspended, it come rest pointing north and south directions. The end, which points towards north, is known as North Pole and other towards south is called South Pole. The area around the magnet is called magnetic field, a magnetic field cannot be seen, felt, smell or heard and therefore it is difficult to represent it. Michael Faraday suggested that magnetic field could be represented pictorially by imaging the field to consist of lines of magnetic flux. Magnets are divided into two general classes
A)     Natural magnet
B)     Artificial magnet
Natural magnet – The natural magnets are found in nature and they are known as Lodestone or leading stone. The natural magnet has chemical compensation of Fe3O4
Artificial Magnet – The magnet prepared by artificial method is called artificial magnet. It is classified as 
a)      Permanent magnet – the magnet which retains the magnetic property for a long period
b)      Temporary magnet – the magnet which losses magnetic property as soon as magnetizing force is removed ( Electromagnet )
Properties
1)      Attracts alloys of iron, nickel and cobalt
2)      Like poles are repel and unlike poles are attract each other
3)      Magnetic property losses when it is heated, dropped from heights
4)      If a magnet broken into pieces, each piece become independent magnet

Magnetic flux
            Magnetic flux is the amount of magnetic field produced by a magnetic field. The symbol for magnetic flux is Φ. The unit of magnetic flux is weber
Magnetic flux density
            It is the amount of flux passes through a defined area that is perpendicular to the direction of flux.
Magnetic flux density (B) = magnetic flux / Area
 Magneto motive force
            The current flowing in an electric circuit is due to the existence of EMF, similarly magneto motive force (mmf) is required to drive the magnetic flux in the magnetic circuit. The SI unit of mmf is Ampere turns (AT)
Fm = NI
Magnetic field strength (magnetizing force) (H) = (N x I) / L
Reluctance (S)
            Reluctance is the magnetic resistance of a magnetic circuit to the presence of magnetic flux
S = Fm / Φ = (N x I)/ Φ
Permeability
            For air or any non-magnetic medium, the ratio of magnetic flux density to magnetizing force is a constant, i.e. B/H=a constant. This constant is μ0, the permeability of free space (or the magnetic space constant) and is equal to 4π×10−7H/m, i.e. for air, or any non-magnetic medium, the ratio
B / H =μ0
(Although all non-magnetic materials, including air, exhibit slight magnetic properties, these can effectively be neglected.) For all media other than free space,
B / H =μ0μr
Where μr is the relative permeability, and is defined as
μr = flux density in material / flux density in a vacuum
μr varies with the type of magnetic material and, since it is a ratio of flux densities, it has no unit. From its definition, μr for a vacuum is 1
μ0μr = μ, called the absolute permeability.
Hysteresis

A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). It is often referred to as the B-H loop.
The energy loss associated with hysteresis is proportional to the area of the hysteresis loop

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