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

Options

A. 0.15m
B. 0.2m
C. 1.0m
D. 0.1m

Answer

D

Explanation

`|B| = (mu_(0))/(4pi) (2pi N ir^(2))/((r^(2) + x^(2))^(3//2))`
`:. (B_(1))/(B_(2)) = ((r^(2) + 0.2^(2))/(r^(2) + 0.05^(2)))^(3//2) = (8)/(1)` This gives r= 0.1m