# Millikan oil-drop help

1. Jan 28, 2007

### candyq27

1. The problem statement, all variables and given/known data
About 100 years ago, the American physicist Robert Millikan carried out a famous experiment known as the Millikan oil-drop experiment, in which he measured the charge on tiny droplets of oil. Millikan found the charge on these droplets to be quantized, and thus came up with a measure of the charge on the electron. For this problem, use g=10m/s^2 and e=1.6x10^-19C.
(a) Part of Millikan's experiment involved balancing the downward force of gravity acting on a droplet with an upward force applied by a uniform electric field. If a particular oil droplet has a mass of 2x10^-14kg, and the droplet has 5 excess electrons on it, what is the magnitude and direction of the electric field necessaryto hold the droplet at rest?
(b) Millikan coud change the charge on the oil droplet by using a radioactive source to ionize some of the air molecules in the vicinity. If our oil droplet from above acquired a 6th excess electron, what would be the magnitude of the droplet's acceleration, assuming the electric field was set to the value you determined above?
(c) Assuming the droplet in part (b) was initially at rest, how long would it take to travel through a distance of 5cm? Neglect all air resistive forces.

2. Relevant equations
Magnitude of the Electric Field: E=(kq)/(r^2)

3. The attempt at a solution

I'm really confused about this problem and I have no idea where to even start. If I use the equation I know dealing with electric fields, I don't know what to use for the value of r and I don't know how to add in the electrons or where I'm supposed to add them in. Please help me work through this problem. I really don't understand it.

For part (a), I used the equation N=q/e to find the value of q...
N=q/e
5=q/e
5=q/(1.6x10^-19)
q=5x(1.6x10^-19)
q=8x10^-19

But then I'm not sure what to do with it
I could use the equation E=kq/r^2 but I dont know what r is...??

Last edited: Jan 28, 2007
2. Jan 29, 2007

### candyq27

I also know that F=kQ, and E=kq/r^2, but I don't know how to figure out the electric field that is needed to hold the droplet up.