In the Millikan oil-drop experiment, an atomizer (a sprayer with afine nozzle) is used to introduce many tiny droplets of oil betweentwo oppositely charged parallel metal plates. Some of the dropletspick up one or more excess electrons. The charge on the plates isadjusted so that the electric force on the excess electrons exactlybalances the weight of the droplet. The idea is to look for adroplet that has the smallest electric force and assume that it hasonly one excess electron. This strategy lets the observer measurethe charge on the electron. Suppose we are using an electric fieldof 7.0 104 N/C. The charge on one electron isabout 1.6 10-19 C. Estimate the radius of an oil drop of density 839 kg/m3 for whichits weight could be balanced by the electric force of this field on one electron.