User:Ryan P. Long/Notebook/Physics 307L/2009/09/28
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Note: Tom and I have identical lab notebooks except for 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:
My value below for q is calculated using the equations on pages 2 and 9 of the pasco manual, my partner, Tom deserves credit for this great spreadsheet below.
Equation for q from page 10:
According to our calculations with our recorded data, our mean charges for each drop are as follows:
Drop 1: 1.78(2)E-15 C
Drop 2: 1.24(3)E-15 C
Drop 3: 2.01(2)E-15 C
Drop 4: 6.58(3)E-16 C
Drop 5: 5.81(2)E-16 C
Drop 6: 8.16(4)E-16 C
Drop 7: 6.68(3)E-16 C
Drop 8: 3.40(2)E-16 C
In comparing our results to the accepted value (1.6E-19 C) given by the Pasco manual, we have some seriously bad results. We are off by order of three to four magnitudes, our results have a very high charge! My calculations are a little closer to the accepted value than Tom's because I used meters in the separation of the plates, I'm not sure why he used centimeters.
Possible causes of error
While going over the manual for the second time, post data collection, we realized that our procedure was incorrect, we believe we were using the voltage controller improperly. Every time we would time the fall of each droplet, we never switched off the voltage controller as we should have. Other human error could be due to the actual act of trying to watch the droplets and simultaneously use the stopwatch (it was rough).
This lab was a real eye opener for me, in the sense that our values were terribly off from the accepted value, and had we not known the charge value, we would have been reporting some seriously wrong calculations to the rest of the world. But I guess thats why experiments are carried out numerous times with different procedures. Next time we are going to scrutinize the manual more thoroughly in order to insure we are doing the experiment properly. As far as improving this experiment for future labs, we talked about maybe having a camera mounted to the eyepiece of the apparatus so that one's eyes don't get too strained while watching droplets. Watching the droplets on a monitor would be far easier, and most likely would result in better measurements.
Despite our sub-par results, we worked hard taking the measurements, and I'd like to thank Tom for being an awesome partner and keeping spirits high during some arduous droplet watching! Also thanks to Dr. Koch, his teaching assistant Pranav, and our peer, Xander for helping us with setup.