13. In a Millikan's oil drop experiment a drop is observed to fall with a terminal speed 1.4mm/s in the absence of electric field. When a vertical electric field of 4.9x10v/m is applied the droplet is observed to continue to move downward at lower terminal speed 1. 71 mm/s. calculate the charge on the drop (Density of oil = 750kg/m3, viscosity of air=1.81 x 10^-5 kg/ms ,density of air=1.29 kg/m3) ​