Millikan's Oil Drop Experiment

In summary, The chemistry professor discussed Milikan's oil drop experiment and how he determined the mass of the drop by measuring its speed as it fell. However, this raised questions about why air resistance does not appear in the equation and why the experiment was not conducted in a vacuum. Further research showed that Millikan used the terminal velocity technique to calculate the mass of the drop, but this method proved to be difficult and challenging. Additionally, trying to catch and measure the drops visually was also a challenging aspect of the experiment. Overall, the experiment was not an easy task and required a lot of patience and precision.
  • #1
mbrmbrg
496
2
My chemistry professor was discussing Milikan's oil drop experiment today, and she said that he determined the mass of the oil drop by measuring its speed as it fell. (I found this further description online) If that's the case, why doesn't air resistence appear in the equation Eq=mg from which e was derived?
I guess what's really bugging me is why, in the name of all that phyisicists hold dear, was this experiment not conducted in a vacuum?

~Malka
 
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  • #2
Oh ho! This site purports that Millikan used the approach I assumed would have been taken--get the volume of a drop and compute mass by using density. Is the terminal velocity thing an urban myth?
 
  • #3
He used the terminal velocity to calculate the mass of the drop - in fact the reason he got the wrong value was that he had the wrong viscosity of air.
The reason for not doing it in vacuum is simply that in vacuum the speed of the drop is independant of it's mass and so there is no way to measure the mass. The terminal velocity technique is a very clever method of measuring the mass of a very small object.

ps. Don't ever try and do this experiment as a lab project, it is even worse than Cavendish's G measurement to get right!
 
  • #4
mgb_phys said:
ps. Don't ever try and do this experiment as a lab project, it is even worse than Cavendish's G measurement to get right!

That sounds like the voice of experience speaking, and I agree (about both experiments).

The idea of measuring the diameter using the telescope seems far fetched. From my memory of doing it, the only drops that were "catchable" were so small they appeared as points in the telescope. And anyway it's impossible to get the drop perfectly stationary to measure it visually.

It would be impossible to do this is a vacuum. You have to "catch" a drop by adjusting the field as it drifts through your field of view. If you lose sight of it, you never get to see that particular drop again. Ideally having caught a drop, you can do repeated experiments on it before you lose it. You don't set the field to zero to measure the terminal velocity, just change its value by a small amount so the terminal velocity is slow enough to measure.

As amazon.com would say, "People who enjoyed pushing pieces of string uphill will also like repeating Millikan's experiment" :smile:
 
  • #5
whoa... scary. Thanks!
 

1. What is Millikan's Oil Drop Experiment?

Millikan's Oil Drop Experiment is a scientific experiment conducted by Robert Millikan in 1909 to determine the fundamental unit of electric charge, known as the electron. It involved measuring the rate at which charged oil droplets fall under the influence of an electric field.

2. How was the experiment conducted?

The experiment involved suspending oil droplets in a chamber filled with air. The droplets were then allowed to fall under the influence of gravity. By applying an electric field to the chamber, Millikan was able to measure the rate at which the droplets fell. He also measured the charge on the droplets by adjusting the strength of the electric field until the droplets were suspended in mid-air.

3. What were the results of the experiment?

Millikan's experiment showed that the charge on the oil droplets was always a multiple of a single, fundamental unit of charge. This unit of charge was later determined to be the charge of a single electron. The experiment also provided a more accurate value for the charge of an electron.

4. What was the significance of the experiment?

Millikan's Oil Drop Experiment was significant because it provided strong evidence for the existence of individual particles with discrete, quantized charges, such as electrons. It also helped to determine the value of the charge on an electron, which is a fundamental constant in physics.

5. How did Millikan's experiment contribute to our understanding of electricity?

Millikan's experiment was a major breakthrough in the field of electricity, as it provided evidence for the existence of individual, quantized charges. It also helped to solidify the concept of the electron and its role in electricity. The experiment also provided a more precise value for the charge of an electron, which is essential in many areas of physics and technology.

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