Problem 3.18

Find the electric field strength vector at the centre of a ball of radius $R$ with volume charge density $\rho=$ ar, where a is a constant vector, and $\mathbf{r}$ is a radius vector drawn from the ball's centre.

\mathbf{E}=-1 / 6 \mathbf{a} R^{2} / \varepsilon_{0}

Constant Electric Field in a Vacuum