Question

A current I flows along the length of an inifinitely 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 is zero only on the axis of the pipe
• C. the magnetic field is different at different points inside the pipe
• D. the magnetic field at any point inside the pipe is zero

Answer 1

Answer: (D)

the magnetic field at any point inside the pipe is zero

There is no current inside the pipe. Therefore
$\oint \overrightarrow{B}.\overrightarrow{dt}={\mu }_{0}I$
$I=0$ $\therefore B=0$