HYSTERESIS CURVE: Brief discussion.

Published by: Junaid Rashid. student at central university of Kashmir department of physics.

• Hysteresis is a property of ferromagnetic substances.
• Hysteresis loop is a curve between magnetising field H and magnetisation ( I or M) .Sometimes it is drawn between H and B (induced flux density which gives the measure of M)
• Hysteresis Loop is generated by measuring magnetic flux coming out of ferromagnetic substance while changing the external magnetising field.
• Initially, the external magnetising field is zero hence magnetisation is also equal to zero.
• Now with the increase in magnitude of H, magnetisation M also increases though not proportionally and attains it’s maximum value at point “a” Called as saturation point. Beyond saturation point, increase in H has no effect on the magnitude of M.
• Now, on decreasing the value of H, magnetisation also decreases. However when H=0 magnetisation is not equal to zero implying that material has retained some magnetism even after the removal of external magnetising field called as residual magnetism or remnant magnetism denoted by “ob”.
• This property of ferromagnetic substances to retain magnetism even when external magnetising field ceases is called as retentivity.
• With increase in external magnetising field towards negative side(decrement),M decreases and becomes zero at a particular value of H.
• The amount of magnetising field required to destroy the residual magnetism of ferromagnetic substance is called coercivity or coercive force denoted by “oc” In hysteresis loop.
• With further increase in H towards negative side(decrement), material is magnetised in opposite direction and magnetisation attains maximum value at point “f”
• At point “e”, H=0 but material has some residual magnetism(opposite direction) denoted by “oe”
• Further when H is increased towards positive side(increment), magnetism decreases and becomes zero at point “d” Such that “od” represents positive coercive force.
• With further increase in H, material gets magnetised in same direction as it was magnetised initially and attains maximum value at point “a” called as saturation point and the cycle is completed called hysteresis loop.
• At every point of the loop, M lags behind H called hysteresis.
• Summary of hysteresis loop: magnetisation (a), de-magnetisation (c) , magnetisation in opposite direction (f), de-magnetisation (d) and magnetisation (a)
• It is worth mentioning that for a unique value of H, M is not unique because it depends on the history of the given sample.
• When a ferromagnetic substance undergoes a cycle of magnetisation, energy is required to align the molecular dipoles in the direction of external magnetising field. On the contrary, when magnetising field is removed all the spent energy is not recovered back due to the fact that M doesn’t fall to zero when H=0. Hence there is energy loss.
• Energy lost per unit volume of material in each cycle of hysteresis loop is directly proportional to area under hysteresis loop.Wider the hysteresis loop, more the energy loss and vice versa
• Applications
• Hysteresis loop is useful in practical world in the sense that it is used to differentiate between electromagnets and permanent magnets.
• Electromagnets lose magnetism as soon as the external magnetising field is removed e.g soft iron. Magnetisation is temporary.
• Permanent magnets on the other hand retain magnetism even after the removal of external magnetising field which means that magnetisation is permanent in this case. e.g Alnico(Al, Ni, Co, Cu) , Steel.
• Use of soft iron in generators and transformers is based on the fact that it magnetize and de-magnetize fastly due to low value of Coercivity, and less area under hysteresis loop.

The End..