A current I flows through an infinitely long straight thin walled tube of radius R. The magnetic field strength at a distance r from the axis of tube (r < R) will be
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → zero
Apply Ampere’s circuital law to a circle of radius r (r < R) coaxial with the tube. By symmetry B has the same magnitude all round the circle and is tangential to it.
\( \displaystyle \oint \vec{B}\cdot d\vec{l} = \mu_0 I_{\mathrm{enc}} \)
\( \displaystyle B\,(2\pi r) = \mu_0 I_{\mathrm{enc}} \)
The current flows only in the thin wall at radius R, so a circle inside the tube encloses no current: \( I_{\mathrm{enc}} = 0 \).
\( \displaystyle B = 0 \)