Magnetic Hysteresis – Class 12 Magnetism and Matter
Magnetic hysteresis occurs when an external magnetic field is applied to a ferromagnet such as iron and the atomic dipoles align themselves with it. Even when the field is removed, part of the alignment will be retained: the material has become magnetized. Once magnetized, the magnet will stay magnetized indefinitely. The lagging of intensity of magnetisation (I) or magnetic induction (B) behind the magnetising field (H) during the process of magnetisation and demagnetisation of a ferromagnetic material is called hysteresis. Also Read: Properties of Paramagnetic & Diamagnetic Materials   Important Points:
1. At point O the magnetising field (H) is zero and and intensity of magnetisation (I) or B is also zero.
2. The part OA of curve shows that I(or B) increases with H. At point A ferromagnetic material acquires the state of magnetic saturation.
3. When H decreases I(or B) also decreases along AB. At point B magnetising field H becomes zero but I(or B) is non zero. Retentivity : The value of I(or B) of a material when the magnetising field is reduced to zero is called retentivity or residual magnetism of the material. Retentivity is measured by part OB of curve.
4. Now H is increased in reverse direction to make I(or B) zero. I(or B) now decreases along BC and becomes zero at C.    Coercivity : The value of reverse magnetising field required to reduce residual magnetism to zero is called coercivity of the material. This is measured by part OC of curve.
5. When H is further increased I(or B) increases along CD. At D material acquires state of magnetic saturation. (D is symmetrical to point A).
6. Here magnetising field H becomes zero before I(or B). The intensity of magnetisation I(or B) always lags behind H. This is called hysteresis. The entire hysteresis loop is traced if H is repeatedly changed between $H _{0}$ and $- H _{0}$.
7. The area of hysteresis loop is a measure of energy dissipated per cycle per unit volume of the the specimen and depends on nature of material.                                                                                                                                                                                  In SI system area of B-H loop = $\mu_{0}$ (area of I-H loop)  In CGS System area of B H loop = $4 \pi$ (area of I-H loop)
8. The loss in energy appears as heat.
9. The slope of B-H curve gives permeability of material while the slope of I-H curve gives susceptibility.
10. If A is area of loop, V is volume of material, n is frequency then energy lost in magnetising and demagnetising specimen for time t is E = AVnt Joule and Heat produced $=\frac{\text { AVnt }}{ J }$ calorie

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