SJK 17:59, 27 November 2010 (EST)
Millikan Oil Drop
Experimentalists: Nathan Giannini & Richard T. Meyers
Junior Lab, Department of Physics and Astronomy, University of New Mexico
Albuquerque, NM email@example.com
In 1909, Robert A. Millikan embarked on the quest to discover the charge of an electron, and to convince those that electron theory was sound. In a Nobel talk in 1924, Millikan described his experiment, named the Millikan Oil Drop for simplicity. This experiment used an electric field to suspend oil droplet inside a small chamber which could be viewed by a microscope. These oil droplets needed to have very small diameters, so that the warping in their dimensions as they accelerated upwards or downwards would be kept to a minimum. By applying an electric field to these droplets, you could measure their rise and fall times and determine the charge necessary to create these rise and fall times. From there, a pattern is formed, as it is noticeable that the droplets had integer multiples of a value. This value has since been designated the elementary charge e = 1.6 *10^-19 Coulombs. In this experiment, which followed this method, we found that:
IntroductionSJK 23:13, 27 November 2010 (EST)
- In 1750, Benjamin Franklin stated that, "The electrical matter consists of particles extremely subtle since it can permeate common matter, even the densest, with such freedom and ease as not to receive any appreciable resistance." This was the first conception of atomic particles that had a charge.SJK 18:14, 27 November 2010 (EST)
- In this experiment, I will be using a set-up close to the one that can be found to be used by Robert A. Millikan, but with particles of diameter near a micro-meter. This will give us much better results than those obtained by Millikan in his own experiment, due to less surface for charges to populate on, allowing for the observation of smaller charges. SJK 23:10, 27 November 2010 (EST)
MethodsSJK 23:29, 27 November 2010 (EST)
Calibration of the Millikan Apparatus
- Before we started the experiment, it was necessary to calibrate the Millikan Apparatus. To begin with, we adjusted the leveling of our apparatus until the bubble was located in the middle of a circle located on top of the bubbles holder, which indicates where the bubble should be for the apparatus to be at 90 degrees to the gravitational field vector. From here, we cleaned the chamber of the Millikan Apparatus, which contained a removable capacitor plate, spacer, and a cap with a hole, who's purpose it was to reduce the amount of oil drops which enter our aparatus' chamber. Next, we inserted a wire into the hole located in the removable capacitor to aid us in the calibration of the light source. We first adjusted the microscope to bring the wire into focus. Afterward, we began to adjust our light source so that the right side of our wire was brighter then the left side . Then, we removed the wire and replaced the cap on the capacitor. The final step in calibrating our apparatus occurred when we were viewing our droplets. We adjusted the "Droplet Focus" on our microscope so that we could see the droplets as well-defined (not blurry) pinpoints of light.SJK 23:21, 27 November 2010 (EST)
Measuring the Rise and Fall Time of the Oil DropsSJK 23:25, 27 November 2010 (EST)
- To measure the rise and fall times of our oil droplets, it was necessary to go through a certain procedure to limit the interactions between our droplets. We, first, applied a potential difference to our capacitor plates, to accelerate most of our particles out of the chamber. Afterward, we searched for a particle that would fall .5mm(one major line on our grid) in approximately 10-15 seconds. Next, we tested our found particles to make sure that they did not have a charge of 5e or more (by accelerating them upwards and timing how long it took to get to one of the major lines). Finally, we would then begin measuring the rise and fall rates of our particles. Our method was to wait until the oil droplet was on one of the major lines and then begin timing it as it either rose (with an applied potential) or fell. We tried to do this 10 times for each particle, but we found that our particles had a tendency of dying before this point. Luckily, we were able to obtain a few particles that would last long enough. For one of the particles we were able to get over 10 runes with, we exposed it to Thorium for 10 seconds and then proceeded to measure the droplets rise and fall times again. It was apparent that the Thorium changed the charge on our particles, due to the fact that they accelerated upwards at a highly increased rate (about 3 times as fast by our own estimates).
Particle Average Fall Time (s) Average Rise Time (s) 3 12.80 5.76 4 16.47 4.41 5 16.26 4.88 7 13.91 3.93 8 15.87 12.99
Analysis and ResultsSJK 23:31, 27 November 2010 (EST)
- Our results for this experiment can be seen in Table 2 below. "a" refers to the radius of our droplet, "m" refers to the mass of our droplet, and "q" refers to the charge on our droplet.
- The charges shown above were calculated using the following equations:
- We separately determined the results for our data. I decided to use circular logic to determine the charge of 1 electron. I did so by using the already readily available quantity for the value to determine the charge on each droplet. The values I determined are shown in Table 3 below. The charges are in 10^-19C.SJK 00:42, 28 November 2010 (EST)
Particle Charge of one Electron 3 1.672 +/- 0.199 4 1.635 +/- 0.382 5 1.636 +/- 0.194 7 2.067 +/- 0.578 8 1.635 +/- 0/202
- This data shows that our data has high precision, with the exception of particle 7. This also means that there is a high probability that there must have been some kind of systematic error in our experiment that would cause our results to be off, as the human error represented in the measurements would to, at least, be minimal.
Discussion and Conclusion
- To give a good comparison as to the accuracy of our results, it is good to consider the accepted value for the charge of an electron, 1.6*10^-19C.SJK 00:40, 28 November 2010 (EST)
AcknowledgmentsSJK 00:46, 28 November 2010 (EST)
- To Richard T. Meyers for having patience when we repeatedly obtained an entire chamber full of neutral particles.
- To Steve Koch for allowing us to redo the Millikan Lab so that we could obtain better data.
- To Alex Andrego for the format of this Rough Draft.
ReferencesSJK 00:47, 28 November 2010 (EST)
-  Millikan Oil Apparatus, PASCO Scientific, used at University of New Mexico Dept. of Physics and Astronomy PHYSICS 307L:'Image:Pasco millikan manual.pdf' http://openwetware.org/images/e/ea/Pasco_millikan_manual.pdf
-  Search for Free Fractional Electric Charge Elementary Particles Using an Automated Millikan Oil Drop, V. Halyo, P. Kim, E. R. Lee, I. T. Lee, D. Loomba, Stanford Linear Accelerator Center, Stanford, California 94309, 1999 and M. L. Perl Techniquehttp://prl.aps.org/abstract/PRL/v84/i12/p2576_1
-  The electron and the light-quant from the experimental point of view, Robert A. Millikan, Nobel Lecture, May 23, 1924 http://labdid.if.usp.br/~estrutura/Experimentos/Millikan/millikan-lecture.pdf
Overall Koch Comments
Steve Koch 00:51, 28 November 2010 (EST): This is a very good first draft! There is no really big thing that needs to be done, but a bunch of things throughout. Thanks for redoing the experiment and doing a great job! For the "extra data session" I want you to fight the urge to be sick of the experiment and see if you can get more data. I'd also like you to explore further analysis methods, a good example being John Callow's 2009 method. Finally, looking into the charge differences quantitatively when doing the thorium exposure would be very nice.