In the early 1900s, robert millikan used small charged droplets of oil, suspended in an electric field, to make the first quantitative measurements of the electron’s charge. a 0.68-μm-diameter droplet of oil, having a charge of +e, is suspended in midair between two horizontal plates of a parallel-plate capacitor. the upward electric force on the droplet is exactly balanced by the downward force of gravity. the oil has a density of 860 kg/m3, and the capacitor plates are 4.0 mm apart. part a what must the potential difference between the plates be to hold the droplet in equilibrium?