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Homework Help: Millikan Oil Drop Lab

  1. May 2, 2013 #1
    1. The problem statement, all variables and given/known data
    Hi, I had to calculate the charge of an oil drop using a lab simulation

    2. Relevant equations
    q = mgd/v

    3. The attempt at a solution
    This is the result i came up with:

    The charges are not multiple of 1.6x10^-19 C. How can i fix this?

  2. jcsd
  3. May 2, 2013 #2

    rude man

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    You realze I hope that even for your smallest oil drop you had about 2.9e-12/1.6e-19 = 18,125,000 electrons on that oil drop! And you expect to resolve to within 1 electron?

    You need either a much smaller drop or, much more plausibly, you need to reduce the number of electrons on each drop. Millikan himself managed to put no more than about a dozen electrons on his drops.
  4. May 2, 2013 #3


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    You certainly use wrong units for the radius. Can you imagine a half-meter size drop of oil? How did you measure the size of the oil drops?

    Last edited: May 2, 2013
  5. May 3, 2013 #4
  6. May 3, 2013 #5


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    So it was simulation... Nice. Have you noticed that one scale is 0.5 mm? So the radii were wrong. Anyway, you can not get the electric charge with that simulation. It results too big charges, order of 10-12 C.

    We did that experiment in real life when I was a student (about 50 years ago). We got several tens on electrons on a single drop, and then subtracted the charge values in hope that the smallest difference would be the elementary charge, but it never came out. So we did a very modern thing: We simulated the measurement data :wink: and then evaluated them. And we also wondered if Millikan really succeeded to obtain the elementary charge from his experiment.

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