Question

In a co-axial straight cable, the central conductor and the outer conductor carry equal currents in opposite directions. The magnetic field is zero.

• A. inside the outer conductor
• B. in between the two conductors
• C. outside the cable
• D. inside the inner conductor

Answer 1

Answer: (C)

outside the cable

Answer 2

By applying Ampère's law:

$\oint \vec{B} \cdot d\vec{\ell} = \mu_0 i_{\text{enc}} = 0$

Since the net enclosed current outside the cable is zero (equal and opposite currents in the central and outer conductors), the magnetic field outside the cable is zero.