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Millikan's oil drop experiment

  1. Feb 22, 2007 #1
    1. The problem statement, all variables and given/known data
    hello, our task was to go on to this site http://www.wcsyearbooks.com/millikan/experiment.html and then figure out the charge and the number of excess electrons on the oil drop using 5 electric fields.
    Electric field: 2.7x10^4, 3.1x10^4, 3.6x10^4, 4.0x10^4 and 5.2x10^4
    m=1.0 x10-15 kg
    g is -9.8 m/s2.

    2. Relevant equations
    for charge: q=mg/E
    found that q= 3.63x10^-19, 3.16x10^-19, 2.73x10^-19, 2.45x10^-19 and 31.89x10^-19

    for number of excess electrons: N=q/e
    found that e= 2.27e, 1.97e, 1.7e, 1.53e and 1.18e

    however this does not seem right but is is just simple calculations with most of the data given to me...

    3. The attempt at a solution
    i sort of did it up there i just didnt show the esact calcualtions because it is simple math but the thing i dont understand is whether or not the drops have an integer of excess electrons..if it did wouldnt N be a whole number?

    thanks for any help.
  2. jcsd
  3. Feb 22, 2007 #2
    Thats the supreme brilliance of the experiment--it should be an integer number. But given the difficulty of this experiment, I bet if you ran 100 oil droplets thru 5 voltages, they would start to cluster around n's. The one that outlies here is the 1.53. But 5 trials might be increased with better results. Can you do 5 for each 5 fields?
  4. Feb 22, 2007 #3
    im not sure if we are allowed to do that since she only asked for five but ill probably include a sources of error and put that down.

    I talked to my teacher this morning and she said she wasnt worried about the numbers that we discovered just as long as we are able to explain why it is incorrect. But thats where im confused..how is it possible to be wrong? If one of my numbers were an integer i couldnt ignore the other four...
    Last edited: Feb 22, 2007
  5. Feb 22, 2007 #4
    Now that would be the supreme mistake, you get one in 5 you like, ignore the rest. I understand the temptation. But you need to keep from imposing your wishes on the data set. You could do 5 trials and have one come up out to
    .99 or 1.02 and sieze on that when the true value is 1.3. Mendel who figured out the rudiments of genes did exactly that.

    And your teachers answer shows great insight as well--don't throw out the beby with the bathwater. All measurements and sampling are prone to error. Understand the size and source.
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