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) outside the cable (c) in between the two conductors (d) inside the inner conductor

Answer: (b). Apply Ampere's law B · d l = μ₀ Ienc on a circle outside the cable. It encloses I and -I, so Ienc = 0 ⇒ B = 0 outside. Inside the inner conductor and between the conductors Ienc ≠ 0, so B ≠ 0 there. So the answer is (b) outside the cable.

In a co-axial straight cable, the central conductor and the outer… | JEE Main 2024 (5 Apr, Shift 1)

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Ampere's Law, Cylinders, Solenoid & Toroid

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

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Answer: (b)

Apply Ampere's law $\oint \vec B \cdot d\vec l = \mu_0 I_{enc}$ on a circle outside the cable.
It encloses $I$ and $-I$ , so $I_{enc} = 0 \Rightarrow B = 0$ outside.
Inside the inner conductor and between the conductors $I_{enc} \ne 0$ , so $B \ne 0$ there.
So the answer is (b) outside the cable.

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