Question: An infinite cylindrical wire with radius 2R caries a uniform current density J=Jz, except in an infinite cylindrical hole parallel to the wires axis. The hole has a radius of R and is tangent to the exterior of the wire. Calculate the magnetic induction B everywhere inside the hole. It would seem to me that according to Ampere's law B inside the cavity would just be 0 because there is no enclosed current. What am I missing? And the reason I doubt my answer so much is that he then wants us to plot B as a function of radius in the hole and I can't imagine plotting 0 was his intention.