In the Millikan oil-drop experiment, an atomizer (a sprayer with a fine nozzle)
ID: 1950346 • Letter: I
Question
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 excess elctrons exactly balances the weight of the droplet. The idea is to look for a droplet that has the smallest electric force and assume it has only one excess electron. Suppose we are using a potential of 3 x 104 V across the 1.0 meter distance between the plates. An oil drop of radius 5.15 X 10-7 m is the smallest droplet to float in the electric field. The density of the oil used is 858 kg/m3. What is the magnitude of the net charge on the plate?
Explanation / Answer
For the droplet to float, the electrostatic force resulting from the parallel plate capacitor must be equal and opposite to the gravitational force pulling the droplet downward.
FE is the force due to the electric field, E is the electric field, Fg is the force due to gravity, m is the mass of the droplet, g is the acceleration due to gravity (9.81 m/s2), is the density of the oil
FE=Eq=Vq/x=(3x104)q/1.0=3x104q (in Newtons when q is in Coulombs)
Fg=mg=(4/3)(5.15x10-7)3(9.81)=4.815x10-15 N
Fg=FE when 3x104q=4.815x10-15 N
q=1.605x10-19 C
This answer is close enough to the value of the elementary charge of an electron that I believe you are to conclude that this bead has grabbed a single electron.