What is hysteresis? Define the terms 'coercivity' and 'retentivity' of a ferromagnetic material.

Hysteresis: The phenomenon of lagging behind the magnetic induction ( B → ) with respect to the magnetizing field ( h → ) is called hysteresis. This happens when an external magnetic field is applied to a ferromagnet such as iron and the atomic dipoles are aligned with it. Coercivity: The magnetizing field ( H ) needed to demagnetize the magnetic material completely is known as coercivity. In simple words, coercivity is the minimum value of magnetising intensity that is required to bring the material to its original state. In the case of a ferromagnetic substance, when the applied magnetic field is increased in the reverse direction, then at some point the substance completely loses its magnetization. This point is known as coercivity. Retentivity: The property of the magnetic material to retain magnetism even in the absence of the magnetizing field is known as retentivity or remanence. Or in other words when the applied magnetic field is decreased to zero after meeting the saturation point of magnetization, then the remaining magnetization shown by the substance is known as retentivity.

Define magnetic 'declination' and 'dip' at a place.

Magnetic declination: The angle between geographic meridian and magnetic meridian at a place is referred to be magnetic declination at a place. Magnetic dip: This is also referred as magnetic inclination. Magnetic dip at a place is the angle which the direction of total intensity of the earth's magnetic field makes with the horizontal.