A thin glass rod of radius R and length L carries a uniform surface charge σ. It is set spinning about its axis, at an angular velocity ω. Find the magnetic field at a distance s ≫R from the axis, in the xy plane (Fig. 5.66). [Hint: treat it as a stack of magnetic dipoles.] [Answer:\mu_{0} \omega \sigma L R^{3} / 4\left[s^{2}+(L / 2)^{2}\right]^{3 / 2}]