A long cable carries current I into the page (−ẑdirection) uniformly distributed over its circular cross- section of radius R. The same current I returns in the opposite direction (ẑdirection) along the surface of the cable (there is a very thin insulating layer separating the two currents). (a) What is the magnetic field B induced everywhere? (b) What is the energy stored in the cable per unit length? (c) What is the self-inductance L of the system per unit length? Hint: Use your answer from (b).