Question

A small cylindrical magnet M (Fig. 3.95) is placed in the centre of a thin coil of radius a consisting of N turns. The coil is connected to a ballistic galvanometer. The active resistance of the whole circuit is equal to R. Find the magnetic moment of the magnet if its removal from the coil results in a charge q flowing through the galvanometer.

Answer 1

Let be the magnetic moment of the magnet M. Then the magnetic field due to this magnet is,

The flux associated with this, when the magnet is along the axis at a distance x from the centre, is

When the flux changes, an e.m.f. induced and a current flows. The total charge q, flowing, as the magnet is removed to infinity from x = 0 is,