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Question 6: Describe Hysteresis loop for a ferromagnetic material by drawing its curve for iron.

Answer

Hysteresis loop for ferromagnetic material

(Hysteresis means ‘remaining behind or lagging behind’. In the B-H curve, B remains behind H, therefore, it is called hysteresis curve).

“The phenomena of lagging flux density (B) behind the magnetizing force H in a magnetic material over a cycle of magnetization is called magnetic hysteresis.”

Explanation

Let a ferromagnetic material like soft iron is placed in an alternating current coil. When the current I is increased in the positive direction, H also increases around the soft iron specimen. When the current reaches its peak value, the magnetic domains of the iron piece are lined up parallel to the field. This increases the magnetic flux density in the region, B.

This stage, when all the domains are aligned parallel, is called saturation point. In the figure, the process is shown by ‘oa’. Here ‘a’ is the saturation point.

Now, when the current reverses its direction towards zero, H also gradually decreases.

However, B does not trace back the same path. Rather it follows a different path ‘ab’. This is because the residual (residual means left behind or lingering) magnetism present in the iron specimen. Actually, some domains in the iron still remains lined up and do not reverse easily. This is called retentivity (retaining itself by the magnetic domains of the iron). So when H reaches zero, B is at some other point b of the graph. Portion of magnetic flux Ob is called retentivity.

Now to reduce B to zero (i-e, when the specimen material iron is fully demagnetized), we have to reverse the direction of the current flow. The reversed direction of the current flow, and hence for that matter of H, now forces the retained magnetic induction at ‘b’ to bring to zero. This is called ‘Coercive force’. (Coercive means تشدّد کرنے والا). This process of coercion is depicted by the portion ‘bc’ of the graph.

Further increasing the current in the

reversed direction, magnetizes the specimen material to the saturation point ‘d’ in the opposite direction. See the graph. The specimen iron was first saturated at point ‘a’ and now at point ‘b’ in the opposite direction). This means all the magnetic domain are now aligned in the opposite direction.

Now if the magnetizing current is again reduced to zero, the residual magnetism (left behind magnetism) in the material is equal in magnitude to the previous value (the previous value was at point ‘b’) but now in the opposite direction at ‘e’.

Similarly, when the magnetizing current is reversed again (i-e, in the positive direction as before), the residual or retained magnetic flux is reduced to zero at point ‘f’.

By increasing the current more, the process repeats itself and the material reaches to the saturation point ‘a’ again. Therefore, as the alternating current varies between +Im and –Im, the B-H curve follows the loop ‘abcdef’. This path is called magnetic hysteresis loop.

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