A circular coil of radius `R` carriers an electric current. The magnetic field due to the coil at a point on the axis of the coil located at a distance `r` from the centre of the coil, such that `rgt gtR`, varries as

Options

A. `1/r`
B. `1/(r^(3//2))`
C. `1/(r^(2))`
D. `1/(r^(3))`

Answer

D

Explanation

`B=(mu_(0)NiR^(2))/(2(R^(2)+x^(2))^(3//2))=(mu_(0)NiR^(2))/(2r^(3)((R^(2))/(r^(2))+1)^(3/2))`
Since `rgt gt R, (R^(2))/(r^(2))` is negligible
`B prop 1/(r^(3))`