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 magntic fields B and B' at radii distance `(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 i.e., `( r le R)`
Magnetic field `B = (mu_0Ir)/(2piR^2)`
For points outside the wire `(r geR)`
Magnetic field , `B. = (mu_0I)/(2pir)`
`:. B/(B.) =((mu_0I(a//2))/(2pia^2))/((mu_0I)/(2pi(2a)))1:1`