A small coll is intorduced between the poles of an electromagnent so that its axis coincides with the magnetic field direction. The cross sectional ara of the coil is equal to `S = 30 mm^(2)`, the its diameter a balistic galvanometer connected to the coil indicates a charge `q = 4.5 muC` flowing through it. Find the magnetic induction magnitude between the poles provided the total resistance of the electric circuit equals `R = 40 Omega`

Explanation

The flux through the coil changes sign. Initirally it is `BS` per turn.
Finally it is `-BS` per turn. Now if flux is `Phi` at an intermediate state then the current at that moment will be
`I = (-N (d Phi)/(dt))/(R)`
so charge that flows during a sudden turing of the coil is
`q = int i dt = -(N)/(R)[Phi -(-Phi)] = 2 N BS//R`
Hence, `B = (1)/(2) (q R)/(N S) = 0.5 T` on putting the values.