Question

If a long hollow thin copper pipe carries a direct current, the magnetic field associated with the current will be present

$[$0.77 Mark$]$

• A. Only inside the pipe
• B. Only outside the pipe
• C. Neither inside nor outside the pipe
• D. Both inside and outside the pipe

Answer 1

The correct option is B Only outside the pipe

For a long hollow pipe carrying a current $i$, the Magnetic field outside the pipe is given by
$B_{out}=\frac{{\mu }_{0}i}{2\pi R}$
According to Ampere's circuital law, if an Amperian loop is considered inside the hollow pipe, then current enclosed by the loop will be zero and hence, the magnetic field inside the pipe will be zero.
$B_{in}=0$