Question

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

Answer 1

Explanation

1. For a loop inside the hollow portion of a conductor, the current enclosed is zero until the loop is on the surface or inside the material of the conductor.
   Therefore current $I=0$
2. We know that magnetic field $B=\frac{{\mu }_0}{2\mathrm{\pi }}·\frac{I}{r}$ where ${\mu }_0$ is the permeability of the free space, $I$ is the current, and $r$ is the radius.
   $\therefore$$B=\frac{{\mu }_0}{2\mathrm{\pi }}·\frac{I}{r}=0$
3. So, the magnetic field inside the pipe is zero.

Hence, the magnetic field associated with the current will be only outside the pipe.