Answer
$\sigma = -1.43\times 10^{-12}~C/m^3$
Work Step by Step
We can find the volume of the sphere:
$A = \frac{4}{3}~\pi~R^3$
$A = \frac{4}{3}(\pi)~(0.020~m)^3$
$A = 3.351\times 10^{-5}~m^3$
We can find the volume charge density:
$\sigma = \frac{q}{V}$
$\sigma = \frac{-300e}{V}$
$\sigma = \frac{(-300)(1.6\times 10^{-19}~C)}{3.351\times 10^{-5}~m^3}$
$\sigma = -1.43\times 10^{-12}~C/m^3$
