User:Thomas S. Mahony/Notebook/Physics 307L/2009/09/28
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Note: My partner Ryan and I share the same lab notebook up to the Analysis section
For our experiment, we followed the instructions listed in the Pasco Manual.
We plugged the PSU into the top voltage banana connectors on the Millikan oil drop apparatus. We then took a measurement of the spacing of the capacitor plates using the calipers. The capacitor spacing was 7.6 mm. We also measured the voltage across capacitor plates to be 501.3(1) V using the multimeter (Please note that the .1 V uncertainty corresponds not to the SEM of the voltage but the total range of measurements registered by the multimeter). We then and placed the multimeter probes in the thermistor holes on the oil drop apparatus to measure its resistance throughout the experiment.
Before starting the experiment, we had to calibrate the device. First we removed the capacitor plates and rinsed them thoroughly with water. We dried them and replaced them. Then we inserted the focusing wire into the top hole of the capacitor plate. We adjusted the light so that the wire was clearly visible through the scope. We also turned the droplet focusing ring and reticle focusing ring so that both the background grid and the wire were in focus. The device was now calibrated and ready to take data.
The procedure of taking data consisted of a few parts. First we turned the ionization source lever to the spray droplets setting. Then we inserted the tip of the squeezer into the top of the chamber and squeezed oil droplets in. Once we were content with the number of droplets in between the capacitor plates, we switched the lever back to off and started taking data. We picked a droplet and timed how long it took to cross 6 sets of 5 major grid lines, with a line spacing of 0.5 mm. This means each droplet traveled a total of 3 mm. To change the direction of the droplet, we used the polarity switch.
Raw data sheet:
The pressure was not measured but was calculated using a pressure calculator and some conversion factors given the fact that UNM is located at about 5335 feet above sea level according to UNM's soccer page.
The charge was calculated using the formula on page 9 in the Pasco Manual.
After doing the analysis I have arrived at my mean values for charge of 7 different droplets:
Though normally a graph would be helpful to see how these values compare with the accepted value of the charge of a single electron (from wikipedia):
e = C
there would be little point since my charge values are off by 5 to 6 orders or magnitude. I seriously doubt each drop I saw had ~10,000 electrons.
Clearly there is a huge systematic error here. In fact, it is so large I really cant say for certain what influence random error had on my experiment; I can only speculate.
After looking over Boleszek's data and the manual, I believe Ryan and I did the experiment wrong and this may have lead to our charge values being so huge. Instead of turning the capacitor off and timing the droplets falling, we left the capacitor plates charged in the opposite polarity. I wish I had noticed this when I was in the lab. Other systematic errors might include human error in timing the droplets, error in using the calipers to measure the capacitor plate separation, and using a calculated pressure instead of measuring it in the lab. My last possible guess as to why our answers are so large is that I simply did the charge calculation wrong. Despite having poured over the analysis section for a while, I simply cannot see where I may have made a mistake.
Conclusions and suggestions
As I mentioned in the Error section, I believe that despite Ryan's and my best efforts, we did the experiment wrong. This may have been the principal cause of our unexpected results, or I could have just made a mistake in doing my charge calculation. Nevertheless, this lab taught me the importance of reading ahead and double checking to make sure I am following the procedure correctly. I imagine the earlier you catch a mistake, the better you are off.
I do, however, have a couple suggestions for how to improve on this lab. I think anyone who has performed this experiment will agree with me that it is a long and arduous process which could be greatly alleviated by using a CCD and computer screen to watch the droplets. Also, I think the lamp on the Millikan apparatus could be much brighter, and this would make it a lot easier to see the droplets.
In doing my analysis, I checked with Boleszek's page to get ideas and see if his data was consistent with my own. Thanks Boleszek.