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

• Correct answer
  A

  outside the cable
• B

  inside the outer conductor
• C

  inside the inner conductor
• D

  in between the two conductors

Solution

$\oint \overrightarrow{\mathrm{B}} \cdot \mathrm{d} \vec{\ell}=\mu_0 \mathrm{i}_{\mathrm{enc}}=0$ $\therefore \mathrm{B}=0$ outside the cable

Hence, the correct answer is option: 'A'.