If you know the net current enclosed in an amperian loop is zero, when can you conclude that the field at every point on the loop is zero? Here's my actual problem. Part a), I'm pretty sure I did fine (B= u0*I/(2*pi*r)). Part b) I'm guessing that B equals 0 at any radius r greater than c. The symmetry tells me intuitively that I'm correct, but I can't prove it to myself. What am I missing?