A long staright wire of radius `a` carries a steady current `I`. The curent is unifromly distributed over its cross-section. The ratio of the magnetic fields `B` and `B'` , at radial distances `(a)/(2)` and `2a` respectively from the axis of the wire is:

Options

A. `1/4 `
B. `1/2`
C. `1`
D. `4`

Answer

C

Explanation

`B_("inside") = (mu_0 Ix)/(2pi a^2)`
`x = a/2 , B_1 = (mu_0I)/(4pi a)`
`B_("out side") = (mu_0 I)/(2pi x)`
x = 2a
` B_2 = (mu_0 I)/(4pi a)`