Well, I was going through some example problems to study for a test and came upon one I can't figure out. Here is the question: A long straight cylindrical tube has an inner radio Ri and an outer radius Ro. It carries a current i, uniformly distributed over its cross section. A wire which runs along the tube axis carries a current of the same magnitude but opposite in direction. The magnetic field created by these currents is: A. zero outside the tube, but non-zero elsewhere B. zero inside the tube, but non-zero elsewhere C. zero everywhere D. zero outside the tube and in the conducting material of the tube, but nonzero inside the tube E. non-zero everywhere I really should know this, but seem to be having a brain fart. Since the currents are opposite, the magnetic fields will be opposite in direction but equal in magnitude. Now, I would think this would mean that the field would be zero inside the tube and non-zero elsewhere, but something seems wrong with that. Any hints would be greatly appreciated!