A long, straight cylindrical conductor of radius R carries a uniformly distributed current III in the z$-$direction. Surrounding this conductor is a hollow cylindrical shell with an inner radius $2$R and an outer radius $3$R$.$ The hollow shell carries a uniformly distributed current $$I $($in the opposite direction$).$ Assume both currents are distributed uniformly over the respective cross$-$sectional areas.

$($b$)$ Sketch the magnitude of the magnetic field $$B$$as a function of r$.$

$($c$)$ Discuss the physical implications of reversing the direction of the current in the outer hollow shell.