Consider a ????cable which consists of an inner wilre of radius `a` surrounded by an outer shell of inner and outer radii `b` and `c` respectively. The inner wire caries a current `I` and outer shell carries an equal and opposite current. The magnetic field at a distance `x` from the axis where `bltxltc` is

Options

A. `(mu_0I(c^2-b^2))/(2pix(c^2-a^2))`
B. `(mu_0I(c^2-x^2))/(2pix(c^2-a^2))`
C. `(mu_0I(c^2-x^2))/(2pix(c^2-b^2))`
D. zero

Answer

C

Explanation

`B_x=mu_0/(2pi) I_("in")/x`
where `I_("in")=I-[I/(pi(c^2-b^2)][pi(x^2-b^2)]`
`=(I(c^2-x^2))/((c^2-b^2))`