Question: A basic property of the electric field E→ is that its divergence is zero at points where there is no charge. Suppose that the only charge is along the z-axis, and that the electric field E points radially out from the z-axis and its magnitude depends only on the distance r from the z-axis. Use the Divergence Theorem to show that the magnitude of the field is proportional to 1/r. [Hint: Consider a solid region consisting of a cylinder of finite length whose axis is the z-axis, and with a smaller concentric cylinder removed.]