A current I flows through an infinitely long, straight copper walled pipe. Then:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The magnetic field at any point inside the pipe is zero
Use Ampere’s circuital law with a circular loop of radius r drawn inside the pipe, coaxial with it. By symmetry the field has the same magnitude all along the loop and is tangential to it.
\( \displaystyle \oint \vec{B}\cdot d\vec{l} = B(2\pi r) = \mu_0 I_{\mathrm{enc}} \)
The current flows only in the copper wall of the pipe, so the loop inside the hollow encloses no current, \( I_{\mathrm{enc}} = 0 \), and hence \( B = 0 \) at every point inside, not only on the axis.