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Lab Overview

  • This lab is a reproduction of the Milikan Oil Drop experiment done by Robert Milikan in the early 20th century to discover the charge of an electron. The main idea of the experiment is that using oil droplets, which are very very small and having them fall through an electric field we measure the relative strength of the field on the drops (a force) to determine how much charge is on each drop. Since the drops are incredibly tiny there will only be several electrons on each drop contributing to its total charge.SJK 03:26, 7 November 2007 (CST)
    03:26, 7 November 2007 (CST)
    It's not that they're tiny that they only have a few electrons surpluss or defecit. It's just because things tend to be neutral, and when they are sprayed, they gain a few electrons. The smallness of the drops is necessary for them to have a low terminal velocity due to Stoke's drag.


  • for this experiment we found that the elementary charge(i.e the charge of a single electron) to be
SJK 00:29, 13 November 2007 (CST)
00:29, 13 November 2007 (CST)
As we talked about today, you need uncertainty on your final result!

and looking at the relative error with the accepted value of e, we have an error of

Which is a pretty good result!

Experimental Analysis

SJK 00:30, 13 November 2007 (CST)
00:30, 13 November 2007 (CST)
Great discussion and ideas here...maybe someone doing their formal report on this will implement a better analysis method.
  • The value we found above is very near the accepted value of the elementary electron charge e, hence our small relative errror. However, when analyzing the data to determine what multiple of e our measurments were we used the fact we already knew what the correct value was. This has many draw backs because in a real laboratory setting you dont have the luxury of already knowing what the answer is, otherwise you wouldnt be doing the experiment. Due to time constraints we did not develope another method to find out what e was from our data without using the accepted value. The idea to do it that way was taken from other groups who used the same method. If we had more time to do the experiment we would like to come up with a better way of using our data to determine e.
  • Also, the experiment has a lot of room for error involved, because it relies heavily on the experimenters. One partner must be watching the droplets and then instruct the other to start/stop a stop watch to time the drops. There is a lot of error involved in that due to reaction times. Eye strain is also a factor because watching the drops over a period of time strains the eyes and causes it to be more difficult to watch the drops and maintain certain drops in your field of vision.
  • So with more time, we would like to have taken more data and to have developed a better way to use our data to determine e. Milikian managed to do it, so we should be able to also.