Milliken Oil Drop Experiment. .

[shikagami]

Can some one help me with this Milliken oil drop problem.

Data from a Milliken oil drop experiment are given below. Calculate the value of the charge on an electron from this data:

Voltage needed to suspend drop (V) Drop diameter (microns)
103------ 1.1
44.7----- 1.2
127------ 1.6
175------ 1.5
11.3----- 0.9

Given:
Plate separation: 0.5cm
Density of oil: 0.95 g/mL
Gravitational constant: 9.80 m/s^2

Ok... this is what I did: q=mg/E
1. I calculated for electric field first since: E= kV/m
2. One microns is equal to 10^-6 m
3. I changed volts into kV.
4. I took the average of all 5 of the electric fields and got 67896.71718 kV/m
5. Now I solved for the mass by cubing the plate separation and canceled it with the mL... to get grams then I changed grams into kilograms to get 1.1875 x 10^-4 kg.
6. And finally I plug everything back into the equation: q=mg/E and get the answer of 2.706 x 10^-8 coulombs.

Please tell me if I did it right.

 

[Gokul43201]

No, you made mistakes because you have not properly understood the concept.

1. The force on a charge q due to an electric field E is qE. The electric field between a pair of plates at a voltage difference V, is given by V/d, where d is the distance between the plates (and has nothing to do with the size of the drop).

2. The gravitational force on the oil-drop is its weight, mg. m is the mass of the drop which is the product of volume and density. You are given the density, and you can calculate the volume of each drop assuming it is a perfect sphere.

3. For the drop to be suspended between the plates, the upward and downward forces must match. So qE = mg or q = mg/E. Having found m of each drop and the corresponding E acting on it, you can find the value of q on each drop. Do not average all the values; find for each case separately.

4. Use the different values of q to find the answer.

 

[ravenprp]

Yes, it looks like you have correctly calculated the value of the charge on an electron using the Milliken oil drop experiment data. Your method of finding the electric field and mass is correct, and your final answer is in the correct units of coulombs. Good job!