A solid nonconducting sphere has a positive charge $q$ spread uniformly throughout its volume. The charge density or charge per unit volume, therefore, is $\frac{q}{\frac{4}{3} \pi R^{3}}$ . Use Gauss' law to show that the electric field at a point within the sphere at a radius $r$ has a magnitude of $\frac{q r}{4 \pi \epsilon_{0} R^{3}}$