Question

Consider a coaxial cable with 2Amps in the center conductor coming out of the page, and 1Amp in the outer conductor going into the page. The center conductor has a radius of 1mm, the outer conductor's inner radius is 2mm, and the outer conductor's outter radius is 3mm. What is the magnitude of the magnetic field at point P which is 5mm from the cable's axis?

Answer 1

Trivial problem, come on. The E effective for ammpers' law is 1am out-2amps in= -1 amp (inward)

Use ampere's law.

For a more difficult version, see this:
http://physics.bu.edu/~duffy/semester2/d14_ampere_cylinder.html