Question

A current i ampere flows along an inifinitely long straight thin walled tube, then the magnetic induction at any point inside the tube is

• A. $\frac{{\mu }_0}{4\pi }\cdot \frac{2i}{r}tesla$
• B. zero
• C. infinite
• D. $\frac{2i}{r}tesla$

Answer 1

Answer: (B)

zero

Using Ampere's law at a distance r from axis, B is same from symmetry.
$\int B.dl=\mu i$
$B\times 2\pi r={\mu }_{0}i$
Here i is zero since current enclosed by the loop is zero, for r < R, whereas R is radius
$\therefore B=0$