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