In the early 1900s, Robert Millikan used small charged droplets of oil, suspende
ID: 2230828 • Letter: I
Question
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 0.71-mu m-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 6.0 mm apart. What must the potential difference between the plates be to hold the droplet in equilibrium?Explanation / Answer
By qE = mg =>qE = Vdg =>qE = 4/3pr^3 x d x g =>E = [4 x p x r^3 x d x g]/[3 x q] =>E = [4 x 3.14 x (0.36 x 10^-6)^3 x 860 x 9.8]/[3 x 1.6 x 10^-19] =>E = 1.12 x 10^4 V/m By V = E x d =>V = 1.12 x 10^4 x 5.5 x 10^-3 =>V = 61.60 Volt