Finding the voltage of an oil droplet, Millikan's experiment

[bobsmith76]

Homework Statement

The density of the oil used to form droplets in the Millikan experiment is 9.20 × 102 kg/m3 and the radius of a typical oil droplet is 2.00 μm. When the horizontal plates are placed 18.0 mm apart, an oil drop, later determined to have an excess of three electrons, is held in equilibrium. What potential difference must have been applied across the plates?

Homework Equations

V = (4/3)(3.14)r^3

q = (mgd)/V

where q = charge
V = voltage

The Attempt at a Solution

step 1. find the mass of the droplet.

4/3(3.14)(2*10^-6)

step 2
then multiply that by the viscosity which is 92 kg/m^3 which is .007

step 3
get V by itself

V = mgd/q

step 4
find the charge (q)

1.6 * 10^-19 * 3 = 4.8*10^-19

step 5
plug the numbers into the above equation

.007(9.8)(.018)/(4.8*10^-19)

the correct answer is 1.3 * 10^4, my answer is way off

 

[ehild]

How do you calculate the volume of a droplet from the radius?

ehild

 

[technician]

you have forgotten r^3
Sorry ehild... did not see your post in time

 

[bobsmith76]

Good, now I got it. Thanks for your help.