In the early 1900s, Robert Millikan used small charged droplets of oil, suspende
ID: 1329579 • 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 0.80-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 5.0 mmapart.
What must the potential difference between the plates be to hold the droplet in equilibrium?
Explanation / Answer
mass of oil droplet = volume x density
= (4 x pi x r^3 / 3) x oil density
= 44.19 x (0.40 x 10^-6)^3 x 860 = 2.306 x 10^-16 kg
So, electric force = Gravititational force
qE = mg
1.6 x 10^-19 x E = 2.306 x 10^-16 x 9.81
E = 14135.66 N/C
V = E.d = 14135.66 x 5 x 10^-3 = 70.68Volt