A coaxial cable consists of a solid inner conductor of radius R1, surrounded by a concentric cylindrical tube of inner radius R2 and outer radius R3. The conductors carry equal and opposite currents I distributed uniformly across their cross-sections. Determine the magnetic field at a distance R from the axis for:
a) R < R1,
d) R> R3.
A coaxial cable carrying a current through it, the magnetic field at all positions is calculated using Ampere's law.