A tiny spherical oil drop carrying a net charge q is balanced in still air with a vertical uniform electric field of strength `(81pi)/7 xx 10^5 Vm^-1`. When the field is switched off, the drop is observed to fall with terminal velocity 2 `xx` 10-3 ms-1.
Given g = 9.8 ms-2, viscosity of the air = 1.8 `xx` 10-5 Ns m-2 and density of oil = 900 kg m-3, the magnitude of q is