Q. A direct current $I$ flows along the length of an infinitely long straight thin walled pipe, then the magnetic field is
A) Uniform throughout the pipe but not zero
B) Zero only along the axis of the pipe
C) Zero at any point inside the pipe
D) Maximum at the centre and minimum at the edge
Explanation
(c) Required arrangement is shown in figure. According to Ampere’s circuital law
$B_{out} = \frac{m_{0}}{4p}\frac{2I}{r}$
[image:4_image99.emf]
For an internal point, $r < R$
$B_{internal} = \frac{\mu_{0}(0)}{2\pi r} = 0$
For a point on the pipe, $r = R$
$B = \frac{\mu_{0}I}{2\pi r}$
For an external point, $r < R$
$B_{external} = \frac{\mu_{0}I}{2\pi r}$
Therefore, option (c) is correct.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.