Sample Calculation on Milikan's Data Analysis

[realslow]

Homework Statement

Find the charge on the oil drop in Coulombs, and the number of electrons it is "missing".
V_plates= 2400 V
D_plates = 0.020 m
Oil Density = 850 kg/m³
Oil Drop Radius = .000051 m (from Stokes’ Law)

Homework Equations

$$E$$ = V/d
Volume = 4/3πr³
m= density x volume

The Attempt at a Solution

$$\Sigma$$ = (2400 V)/(0.020 m) = 1.2 x 10⁵
Volume = 4/3 π(0.000051)³ = 1.7456 x 10^-12 m³
m = (850 kg/m³)(1.7456 x 10^-12 m³) = 1.4838 x 10^-19 kg
And this is where I got to... now I know that F_e = F_g and I was thinking I could find F_g, but am stuck :( I would appreciate it if you could help me out!

 

[realslow]

Sorry about the electric field equations.. they came out weird haha

 

[ehild]

How much is the electric force on the charged oil drop in terms of its charge? And how much is the force of gravity if the mass is m?

ehild

 

[realslow]

I know that F_e = Q$$\Sigma$$ so if I find F_g I can solve for Q. However, I am confused as to what values I use for F_g. I know the gravitational constant and the two masses (which are equal?), but am confused about the distance between them. Should I be using the radius of the drop or twice the radius?

 

[realslow]

Sorry about that.. Fe = QE

 

[realslow]

okay I realized I just need to use Fg = mg :)