An ideal coaxial cable has inner conductor carrying current +I and outer conductor carrying current -I. What is the magnetic field outside the outer conductor?
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Question

An ideal coaxial cable has inner conductor carrying current +I and outer conductor carrying current −I. What is the magnetic field outside the outer conductor?

G1 Physics · Accepted Answer

An external Amperian loop encloses net current I−I=0, so by symmetry B=0.