What is hysteresis? Define the terms 'coercivity' and 'retentivity' of a ferromagnetic material.
Hysteresis: The phenomenon in which the magnetic induction (→B) lags behind with respect to the magnetizing field →H is called hysteresis.
It 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.
Retentivity: The property of the magnetic material to retain magnetism even in the absence of the magnetizing field is known as retentivity or remanence.
Coercivity: The magnetizing field (H) needed to demagnetize the magnetic material completely is known as its coercivity.