In a Millikan-type oil-drop experiment, the plates are 8 mm apart. An oil drop is found to remain at rest when the upper plate is at a potential 136 V higher than that of the lower one. When the electric field is switched off, the drop is found to fall a distance of 20 mm in 36 seconds with a uniform speed. Find (a) the charge on the drop and (b) the number of electrons attached to this drop. Density of oil = 880 kg m and coefficient of viscosity of air = 180 poise.