Question

A current $i\text{ampere}$ flows along an infinitely long straight thin walled tube of radius $R$, then the magnetic induction at any point inside the tube is

• A. $\infty$
• B. zero
• C. $\frac{2i}{R}\text{T}$
• D. $\frac{{\mu }_0}{4\pi }\frac{2i}{R}\text{T}$

Answer 1

The correct option is B zero

Let us consider the inside point as $\text{P}$ located at a distance $r$ from the axis as shown in the figure.

Using Ampere's law

$\oint \overrightarrow{B}\cdot \overrightarrow{dl}={\mu }_0{i}_{net}$

As we can see, current enclosed by the Amperian loop is zero.
$\therefore i_{enc}=0$ and $\oint dl=2\pi r$

So equation becomes

$B_P\left(2\pi r\right)={\mu }_0\left(0\right)$

$\therefore B_P=0$

Hence, option $\left(b\right)$ is correct.