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

`rgta, beta_(1)=(mu_(0)Ir)/(2piR^(2))=(mu_(0)Ia//2)/(2pia^(2))=(mu_(0)I)/(4pia)`
`rgta, beta_(2)=(mu_(0)I)/(2pir)=(mu_(0)I)/(2pi.2a)=(mu_(0)I)/(4pia)`
`(B_(1))/(B_(2))=1`