HYSTERESIS

 HYSTERESIS (B-H CURVE) :

          Consider a ferromagnetic substance (iron bar) is placed in a magnetizing field. As magnetizing field H is slowly increased, the intensity of magnetization also increases. The graphical representation of the extent of magnetization at different value of H. To start with H = 0 and magnetization B is also zero. As H increases B also increases along the curve OA. State of saturation is reached at A  and further increases of H will not increase B. 

         Now if we decrease H , the demagnetization of the sample does not take place along the curve AO. Thus  as H is decreased and becomes zero then it is seen that B does not become zero but has the value OG called retentivity. Hence demagnetisation lags behind the magnetizing field H. This phenomena in which B lags behind the magnetizing field H is called Hysteresis. The residual magnetism left behind in the magnetic substance when magnetic field is reduced to zero is called retentivity. It gives magnetization retained by the sample, when it behaves likes a permanent magnet. The value of B at point G is called remanent magnetic induction. Now when H is increased in negative direction , B also decrease along GC and at `c' B reduced to zero. The value of H given by OC is called coercivity. 

      The magnetizing field H in the negative direction required to make B zero is called coercivity. 

      This given the ability of the ferromagnetic substance to remain magnetized even when subjected to demagnetising field. Further increase of H in reverse direction magnetise the sample in the opposite direction attaining the saturation state of magnetization at D. The increase of H from this state takes the sample through the path DEFA to the saturated magnetization again in the initial direction. The variation of magnetization represented by AGCDEFA is called cycle of magnetization. The closed curve AGCDEFA is called Hysteresis loop.

     Thus lagging of magnetization B behind the magnetizing field H, when the ferromagnetic substance is taken through a complete cycle of magnetization is called Hysteresis.