**hysteresis**. Also Read: Properties of Paramagnetic & Diamagnetic Materials

**Important Points:**

- At point O the magnetising field (H) is zero and and intensity of magnetisation (I) or B is also zero.
- The part OA of curve shows that I(or B) increases with H. At point A ferromagnetic material acquires the state of magnetic saturation.
- 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. - 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. - 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).
- 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 $- H _{0}$. - 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) - The loss in energy appears as heat.
- The slope of B-H curve gives permeability of material while the slope of
**I-H**curve gives susceptibility. - 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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