User:Emran M. Qassem/Millikan

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In this lab we measured the charge on an individual oil droplet using the Millikan oil drop apparatus. The idea is to atomize oil into droplets and let them fall through a lit viewing chamber, pick a droplet, measure it's falling speed, apply ionization to it using a radioactive source, and then apply a charge to see how fast it rises. If the droplet is charged with an electron, it will rise at a speed based on how many electrons there are.


The setup was very difficult and time consuming for this lab, and the procedure was even more difficult, straining, and frustrating. In the procedure we located a good candidate oil droplet by seeing how fast it fell, looking for one that falls between 7 and 15 seconds between major gratings. Once we located one, we ionized it and applied charge to it, letting it rise between two major gratings, then letting it fall between two major gratings, keeping track of the rise and fall times, for as many data points as we could before the particle either started behaving strangely or vanished.


The accepted value is:

 1.6 * 10^{-19} C \!

We were able to get data points for three oil drops and calculated the charges for them:

 1.4(1) * 10^{-19} C \!
 1.2 * 10^{-19} C \!
 2(2) * 10^{-19} C \!


Besides human fatigue, error, and pure sloppiness, the oil drops themselves were behaving quite strangely at some points. We had droplets vanish, start rising faster, start falling slower, start moving side to side, and even staying in the same place, all without any electric field. To explain the vanishing, I believe at some point, the oil droplets evaporate. This also explains the slower fall times and faster rise times. As evaporation occurs, the mass of the drop reduces, and it falls more slowly as it is more susceptible to air resistance. It will also start to rise more quickly as an effect of evaporation. This could also explain droplets moving side to side and even staying in the same place as the mass is reduced. As evaporation occurs, the air in the chamber becomes more saturated, and will start sticking to the lenses, reducing visibility. I believe having the chamber clean and only using the device for half an hour at a time between cleanings will give much better results.


This lab was very difficult to do well and requires a lot of patience and experience in doing the experiment. It would have been an interesting final report to do, as there was so much left to be desired from it, and the lab seemed to have so much potential to be done well. A way to improve this lab would be to have the viewer hold the stopwatch and click start and stop then let the partner read the stopwatch. I learned that experiments done by these great scientists were very difficult, especially that they didn't have a manual or guide on how to do it, and had to come up with the equipment, calculations, and methods. We are really standing on the shoulders of giants.

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