A thin ring of $10 \mathrm{c}\mathrm{m}$ radius carries a uniformly distributed charge. The ring rotates at a constant angular speed of $40\mathrm{\pi }\mathrm{ }\mathrm{r}\mathrm{a}\mathrm{d}{\mathrm{s}}^{-1}$ about its axis, perpendicular to its plane. Is the magnetic field its centre is $3.8\times {10}^{-9}\mathrm{T}$ , then the charge carried by the ring is close to $\left({\mathrm{\mu }}_0=4\mathrm{\pi }\times {10}^{-7}\mathrm{N}/{\mathrm{A}}^2\right).$