A tiny spherical oil drop carrying a net charge q is balanced instill air with a vertical uniform electric field of strength $$\frac{81 \pi}{7} \times 10^5 Vm^{-1}$$. When the field switched off, the drop is observed to fall with terminal velocity $$2 \times 10^{-3} m s^{-1}$$. Given $$g = 9.8 m s^{-2}$$, viscosity of the air = $$1.8 \times 10^{-5} Ns m^{-2}$$ and the density of oil = $$900 kg m^{-3}$$, the magnitude of q is