# Homework Help: Lab help (Millikan oil drop experiment)

1. Jul 18, 2009

### Matt1234

Hello,
We did a lab in class yesterday and we are expected to write a report and graph our results. The lab was a simulation of the Millikan oil drop experiment. We used graphing calculators to do the simulation and took down 15 results. One thing i dont understand is how to come up with the charge for any given value.

1. The problem statement, all variables and given/known data

2. Relevant equations

q= (m g r) / V
Where
m = mass
g = 9.8
r = distance OR plate seperation (i am not sure, i have 2 values that could be R)
V = voltage

The other equation he gave us was m = (4/3) Pi r^3 P
pi = 3.14...
P = density of oil

I looked up the density of oil to be:
vegetable oil
(0.923 grams) per (cubic centimeter) = 923 kilograms per (cubic meter)

3. The attempt at a solution

I tried some calculations i kept comming up with different values for M (which i think is the case since no two oil droplets would be the same mass. ) Yet the equation he gave us for calculating mass using the density confuses me since m is NOT going to be constant.

I am required to:
Show a sample calculation for determining the mass of the droplet and the charge on the droplet, along with the rest of my report.

Thank you.

Last edited: Jul 18, 2009
2. Jul 18, 2009

### Gib Z

Well, we know that E= V/d, where V is the voltage applied and d is the distance between the plates, and that the electric force is given by F=Eq = Vq/d.

This must be equal to our weight force, F=mg, so that mg=Vq/d, giving q= mgd/V.

Your value R in the second column in the radius of each oil drop. Hence, the mass of the drop is just its volume times its density, or the formula you have above. The next thing you should be doing is drawing up another column, and write up the corresponding masses for each sample of different radius. Yes, you will get many different masses, each corresponding to a different Voltage.

You took 15 samples for a reason, you knew you would get different answers, all with some error. The point of taking 15 is to reduce your error, as some will lie above the right value, some less. To incorporate all your samples information, you have to now plot a graph of Voltage against Mass for your 15 samples. Your points should look at least approximately linear.

The next step is to take a good judgment and draw a line of best fit - a straight line that best approximates the data points. A good line of best fit should have approximately the same number of sample points above the line as there are below it. Once you have a good line of best fit, it is easy to determine that lines gradient. But remember what this gradient value represents, it is a "good" value for V/d, by good I mean that it sort of incorporates the information of all your data points. Now that we have a good value of V/d, call this value T, we know from before that q= mgd/V = gd/T. As we know the separation distance of the plates and gravitational acceleration, we can calculate q.

3. Jul 18, 2009

### Matt1234

Thank you so much, that helped alot. I used the formulas and compared to my results so i could find P to be 855 which is a reasonable number for oil.

Thank you sir!