If a long hollow copper pipe carries a current, then magnetic field is produced

Inside a hollow pipe carrying current, the magnetic field is zero, since according to Ampere‟s law,

$B_{\mathrm{i}}.2\pi \mathrm{r}={\mu }_0\times 0\Rightarrow B_{\mathrm{i}}=0$

But for external points, the current behaves as if it was concentrated at the axis only; so, outside, $B_0=\frac{{\mu }_{0}i}{2\pi r}$Thus, the magnetic field is produced outside the pipe only.