Question

Show that the far field of a magnet is equal to the far field of the solenoid

Answer 1

Dear Student,

a=radius of the solenoid
2l = length of solenoid with centre O
n = number of turns per unit length
I = current passing through solenoid
OP = r

Consider a small element of thickness dx of solenoid at distance x from O. and number of turns in element = n dx.
We know magnetic field due to n turns coil at axis of solenoid is given by

from this the pic will help.