Q. A current $i\ $flows along the length of an infinitely long, straight, thin-walled pipe. Then
A) The magnetic field at all points inside the pipe is the same, but not zero
B) The magnetic field at any point inside the pipe is zero
C) The magnetic field is zero only on the axis of the pipe
D) The magnetic field is different at different points inside the pipe
Explanation
(b) Figure shows infinitely, long, straight, thin-walled pipe carrying current $I$
Let $P$ be any point at a distance $r$ from the axis $OO_{1}$ of the pipe. Let $B$ be magnetic field at $P.$ Consider a closed circular path passing through point $P$ as shown in figure. From Ampere’s circuital theorem, $\oint_{}^{}B\ dl = \mu_{0}i$
[image:4_image136.emf]
$i = \ $current through the closed path. Obviously, $i = 0$
$\therefore\ \ \ 2\pi RB = 0\ \ \ \ or\ \ \ \ \ B = 0.$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.