A current $I$ flows along the circumference of the inner conductor of a co-axial cable and returns along the outer conductor of the cable. The radii of the inner and outer conductors are $a$ and $b$ respectively. Let $P$ be a point whose distance $r_1$ and such that $r_1>b$ and $Q$ is at distance $r_2$ from the axis such that $a<r_2< b$. The magnetic field at