A metal ball of radius 4.0 cm is charged in dry air…

In a conductor at rest the field inside is zero and the extra charge sits on the surface. Just outside, $E = \sigma/\varepsilon_0$, with $\varepsilon_0 = 8.85 \times 10^{-12}\ \text{C}^2/(\text{N m}^2)$. A ball of radius $R$ with charge $q$ has the potential $V = kq/R$ and the field $kq/R^2$ at its surface, with $k = 9.0 \times 10^{9}\ \text{N m}^2/\text{C}^2$. Joined spheres share the charge in the ratio of the radii. In a dielectric $E = E_0/K$ and the bound charge has $\sigma_b = \varepsilon_0(E_0 - E)$.