A small oil drop of radius $0.70\ \mu\text{m}$ and density $850\ \text{kg/m}^3$ hangs at rest in an upward electric field of $2.5 \times 10^{4}\ \text{N/C}$. How many excess electrons does it carry? Take $g = 9.8\ \text{m/s}^2$ and $e = 1.6 \times 10^{-19}\ \text{C}$.