Electromagnetism - Magnetic Field

A coaxial cable has the cross section shown in the figure above. The shaded region is insulated. The regions in which r  a and b  r  c are conducting. A uniform dc current density of total current I flows along the inner part of the cable (r  a) and returns along the outer part of the cable (b  r  c) in the direction shown. The radial dependence of the magnitude of the magnetic field, H, is shown by which of the following?

 

(GR9677 #88)

Solution:

For r = 0 → H = 0
→ D and E are FALSE

For r = c → H = 0 since I_in and I_out cancel each other
→ A and C are FALSE

Answer:  B