An electric current i is flowing in a circular coil of radius a. At what distance from the center of the axis of the coil will the magnetic field be of its value at the centre?
A
в
c
D

Correct option is
B
Magnetic field due to the circular current (I) carrying coil of radius , at a distance from the center of the coil is given by:

Now the field at a point times the field at the center:
So,
or, for,