Millikan's experiment A microscopic spherical oil droplet, of density ρ and unknown radius, carries an unknown electric charge. The droplet is observed to have terminal speed v1 when falling vertically in air of viscosity μ. When a uniform electric field E0 is applied in the vertically upwards direction, the same droplet was observed to move upwards with terminal speed v2. Find the charge on the droplet. [Use the low Reynolds number approximation for the drag.]