A tiny spherical oil drop carrying a net charge q is balanced in still air with a vertical uniform electric field of strength 81π/7 x 105 V m-1.When the field is switched **, the drop is observed to fall with terminal velocity 2 x 10-3 m -1.Given g = 9.8 m -2,viscosity of the air = 1.8 x 10-5 N m-2 and the density of oil = 900 kg m-3 , the magnitude of q is
(a) 1 . 6 x 10-19 C
(b) 3 . 2 x 10-19 C
(c) 4 . 8 x 10-19 C
(d) 8 . 0 x 10-19 C
