User:Alex G. Benedict/Notebook/Physics 307L: Junior Lab/Excitation Levels of Neon Lab
Excitation Levels of Neon
SJK 23:48, 12 October 2010 (EDT)
Overall this is a good primary lab notebook. I think future notebooks will probably have more notes about the data as you take it. Also, some of the future analyses will call for more discussion. Finally, your comparisons with accepted values will become more sophisticated as we learn more in class.
This lab was done on Mondays 8/13/2010 and 8/20/2010 at the UNM physics department with my lab partner Joseph Frye User:Joseph_Frye. We followed experiment 6 outlined in Dr. Gold's lab manual.
Links
Equipment
SJK 23:44, 12 October 2010 (EDT)
Same comment as on Joe's page: Excellent use of pictures and annotated schematics. Those will go a long way in helping future experimenters with this lab. My only criticism with this section is makes and model numbers are not recorded. This can probably be discerned from the photos, but it's good to type it out explicitly too.
Pictures taken by Joseph Frye:
- Hertz Critical Potentials tube filled with neon
- Tube stand
- Picoamplifier and Alarmed Meter
- 2 Power Supplies
- Digital Multimeter
- 1.5V AA Battery
- Cables
Set Up
Figure 6.2 from Dr. Golds Lab Manual
Thanks to Tomas Mondragon for redoing the text on this diagram the resolution of the original image made it almost unreadable. Mondragon's Notebook Fall '08
NOTE: The part labeled F5 in the above image is actually supposed to read F3!
We first found all of the equipment we needed according to the lab manual and then connected it according to figure 6.2, basically just following the circuit diagram above. You will also need one female to male cable to connect to the apparatus, since most standard cables in the lab won't fit well.
We measured the current using the pico-amplifier and the accelerating voltage (Va) using the DMM, the DMM was attached directly to the power supply since the voltmeter on the device was hard to read.
Data
SJK 23:46, 12 October 2010 (EDT)
Good use of nice embedded spreadsheet. Main suggestion is to add a "notes" column in your spreadsheets. My guess is that you spent most of your time with the Google docs window open, and thus this would have helped you in taking any notes about the data along the way (as you would in a regular hand written lab notebook).
{{#widget:Google Spreadsheet |key=0AsLE8iWrtjlSdGxVQ2w3TDNtZEowVVc5QTFuSkFSZkE |width=800 |height=600 }}
We did a rough scan of the voltages ranging from 0 to 30V in 1V increments on the accelerating voltage with the filament voltage set at 1.8V to determine where the regions of interest were.
Here is the graph of that data:
After examining the graph we then decided to to another scan from 15V to 22V using 0.1V increments to get better resolution of the area. We did this once with Vf at 1.8V.
Here is the graph:
We also did the same range with the same 0.1V increments but used Vf=2.1V, the peaks produced in this graph were a bit sharper than the Vf at 1.8V, but had an odd discontinuity in the current.
We thought the sharp rise in voltage seemed odd so we took another set of measurements over the area that the discontinuity occurred in, we did not see it again during this run.
We also flipped the battery and measured Va at 1V intervals from 0V to 25V, but found this data to be less useful. Although you can still see some of the peaks in the area between 16V and 22V, which show up in the uneven portions of the graph. This data just seemed to be very similar to the earlier data, and so we did not explore it further.
Observations
- Looking at the data from the measurements we took with Vf=1.8V, there are two major valleys with relative minimums. One at about 18.4V and the other at about 21.3V. There were also relative maxima at about 16V and 19.4V.
- Looking at the data from the measurements we took with Vf=2.1V, there are again two major valleys with relative minimums. One at about 18.0V and the other between 21.0V and 21.1V. There is a strange jump in the current from 15.8V to 15.9V, which was not observed when measurements in that region were retaken. There were also relative maxima again at about 16V and 19.4V.
- In each case the current grows linearly with voltage after 21.5V.
Calculations
According to the paper in appendix C of Gold's manual, the accepted values for the first three peaks are 16.70eV, 18.65eV, and 19.75V. The next peaks is at 20.10V, however only one of our sets of data had a fourth extrema. From the same paper, the accepted value for the ionization energy of neon is 21.56V which is consistent with our data because for voltages higher than that our current grew linearly with the voltage as we would expect.
Our results for the first peak using the average of the two runs are:SJK 23:41, 12 October 2010 (EDT)
Same comment as on Joe's page: As mentioned on your summary page, when averaging together values from different runs, you should always ask yourself whether you have reason to believe they represent estimates with the same parent distribution. If you have reason to think otherwise, is the average the best way to go? Or would you be better off figuring out why the runs are different. We'll talk about this more in future weeks. Along the way, we'll also learn how to do a weighted average and calculate the new uncertainty after the average. If your uncertanties here represented SEM, and you assumed normally distributed mean (which is not the case for you here, since you're estimating the uncertainty in another way), then there are methods for propagating the uncertainty. The uncertainty in the averaged value would be less than 0.1, since you have information from two independent measurements. In the case here, though, that would be suspicious since the two values are well separated compared with uncertainty.
(16.3 + 15.8)/2 = 16.1 +/-0.1
error = (16.7-16.1)/16.7 = 3.6% relative error
For the second peak:
(18.4+18.0)/2 = 18.2 +/-0.1
error = (18.65-18.2)/18.65 = 2.4% relative error
For the third peak:
(19.4 + 19.4)/2 = 19.4 +/-0.1
error = (19.75-19.4)/19.75 = 1.8% relative error
For the fourth peak we were only able to distinguish maxima from the 1.8V data.
19.6 +/- 0.1
error = (19.6-20.10)/20.10 = 2.5% relative error
For the ionization energy of neon:
(21.3 + 21.05 +/-.02)/2 = 21.18 +/-0.1
error = (21.56 - 21.18)/21.56 = 1.8% error
Conclusion
SJK 23:42, 12 October 2010 (EDT)
Same comment as on Joe's page: (we also talked about this with respect to the e/m experiment on Monday) We are always tempted to report the relative error when comparing measurements with an accepted value. However, how do we know if the relative error is small enough that our measurements are consistent with the accepted value? The answer is to do statistical comparisons of the accepted value to the confidence interval of your measurements. In your case, +/- 0.1 V is much smaller than the 0.6 V discrepancy. If your +/- represented a 68% confidence interval (which it doesn't in your case), then very likely there is systematic error. Your interval is probably much higher than 68%, and thus even more likely that there's significant systematic error. Again, we'll talk about this more in the next few weeks of class.
- We found the first peak to be at 16.1eV +/- 0.1eV with relative error compared to the accepted value of 3.6%
- The second peak at 18.2eV +/- 0.1eV with relative error compared to the accepted value of 2.4%
- The third peak at 19.4eV +/- 0.1eV with a relative error of 1.8%
- The fourth peak at 19.6eV +/- 0.1eV with a relative error of 2.5%
- The ionization energy of neon at 21.18eV +/- 0.1eV with relative error compared to the accepted value of 1.8%
- the +/- 0.1eV uncertainty here comes from our uncertainty in measuring the voltages since we only measured in 0.1V intervals.
