In the Millikan oil drop experiment, an atomizer (a sprayer with a fine nozzle) is used to introduce many tiny droplets of oil between two oppositely charged parallel metal plates. Some of the droplets pick up one or more excess electrons. The charge on the plates is adjusted so that the electric force on the excess electrons exactly balances the weight of the droplet. The idea is to look for a droplet that has the smallest electric force and assume that it has only one excess electron. This strategy lets the observer measure the charge on the electron. Suppose we are using an electric field of 3 × 104 N/C. The charge on one electron is about 1.6 × 10−19 C. Estimate the radius of an oil drop of density 858 kg/m3 for which its weight could be balanced by the electric force of this field on one electron. (Problem 36 is courtesy of E.F. Redish. For more problems of this type, visit www.physics.umd.edu/perg/.)