A long straight wire of radius `a` carries a steady current `I`. The current is uniformly 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

For points inside the wire , `B = (mu_(0) Ir)/(2pi R^(2)) (r le R)`
For points outside the wire , `B = (mu_(0) I)/(2pi r) ( r ge R)`
According to the question , `(B)/(B.) = ((mu_(0)I (a//2))/(2pi a^(2)))/((mu_(0)I)/(2pi (2a))) = 1:1`