A charged oil droplet falls 5.0 mm in 20.0 s at terminal speed in the absence of an electric field. The specific gravity of air is 1.35 x10^(-3) and that of oil is 0.75. The viscosity of air is 1.80 x 10^(-5) N s/m2.
(a) What are the mass and radius of the drop?
(b) If the droplet carries two units of electric charge and is in an electric field of 2.5 x 10^5 V/m, what is the ratio of the electric force to the gravitational force on the droplet?