In a co-axial straight cable, the central conductor and the outer conductor carry equal currents in opposite directions. The magnetic field is zero.
JEE MAIN 2024, Diffcult
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$\oint \overrightarrow{\mathrm{B}} \cdot \overrightarrow{\mathrm{d} \ell}=\mu_0 \mathrm{i}_{\mathrm{enc}}=0$
$\dot{\triangle} \mathrm{B}=0$ outside the cable
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