Problem

Consider a coaxial cable which consists of an inner wire of radius

a

surrounded by an outer shell of inner and outer radii

b

and

c

, respectively. The inner wire carries a current

I

and outer shell carries an equal and opposite current. The magnetic field at a distance

x

from the axis where

b < x < c

is (a)

\frac{\mu_0 I(c^2 - b^2)}{2\pi x(c^2 - a^2)}

(b)

\frac{\mu_0 I(c^2 - x^2)}{2\pi x(c^2 - a^2)}

(c)

\frac{\mu_0 I(c^2 - x^2)}{2\pi x(c^2 - b^2)}

(d) zero