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Magnetic Hysteresis Loop (curve)

Now its time to know something extra in the field of magnetism, therefore I'm gonna introduce hysteresis curve of magnetic material.
B-H graph


 When a ferromagnetic material is placed under the influence of a magnetising force (H), a magnetic field (B) induced in it  and when a graph is plotted between B-H
the adjoining graph is obtained

  1.  When magnetising force is increased magnetic field induced in the material also increases and saturates at 'a' (oa)
  2. Now the magnetising force is reduced and become zero at O .some magnetism still remains in the body. this ability of a material to retain the magnetism when magnetising force become zero i.e., H=0 is called retentivity i.e., curve (ob)
  3. now the magnetising force is applied in reverse direction to demagnetise the sample completely, this ability of material is called coercivity (curve oc)
  4. When magnetising force is further increased, a reverse saturation is obtained ( cd curve)
  5. When magnetising force changes further a complete loop a,b,c,d, & e  obtain.
Note: For making permanent magnet we prefer a material with high permeability, high retentivity and high coercivity
That's why steel or AlNiCO is preferred for making a permanent magnet, but soft iron is prefered for making electromagnets.

 Area of hysteresis or B-H graph gives energy loss during a cycle of magnetisation and demagnetisation that's why in making the core of transformer soft iron is used instead of steel.
soft iron has higher retentivity that steel but low coercivity

use of electromagnets: 
1. use in an electric bell
2. cranes etc

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