Question

A current i ampere flows along the inner conductor of a coaxial cable and returns along the outer conductor of the cable, then the magnetic induction at any point outside the conductor at a distance r metre from the axis is

• A. Zero
• B. $\infty$
• C. $\frac{{\mu }_0}{4\pi }\frac{2i}{r}$
• D. $\frac{{\mu }_0}{4\pi }\frac{2\pi i}{r}$

Answer 1

The correct option is A Zero

By Ampere's law

$\oint \overrightarrow{B}.\overrightarrow{dl}={\mu }_0{i}_{en}$

According to ampere loop law, the current enclosed due to both the wires is zero.
$i_{en}=0$

That means,
$\Rightarrow \overrightarrow{B}.\overrightarrow{dl}=0$
$\Rightarrow \overrightarrow{B}=0$

Hence, net magnetic field at P will be zero.
Hence, answer will be option (B).