Question

The magnetic fields at two points on the axis of a circular coil at distance of 0.05 m and 0.2 m from the centre are in the ratio 8 :1 . The radius of the coil is

• A. 0.2 m
• B. 0.15 m
• C. 0.1 m
• D. 0.01 m.

Answer 1

Answer: (C)

0.1 m

Answer 2

$\frac{B_1}{B_2} \frac{(r^2 + x_2^2)^{3/2}}{(r^2 + x_1^2)^{3/2}} \, i.e. \frac{8}{1} = \frac{(r^2 + 20^2)^{3/2}}{(r^2 + 5^2)^{3/2}} \, i.e. \, r$ = 0.1 m