Physics307L:People/Gibson/E/M Ratio

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Measure of e/m (charge-to-mass ratio)

Lab materials can be found in the lab summary from last years course: Physics 307L

The collection of data is difficult to make on the apparatus. There is a mirror ruler behind the tube which you are suppose to use to measure the radius of the circle formed by the electrons flowing inside. Lining up the circle onto the ruler (measured in cm) can be difficult and eye straining due to lack of definition in the reflection.


SJK 23:53, 17 October 2007 (CDT)
23:53, 17 October 2007 (CDT)
Excellent job reporting your values with associated random uncertainty! You can make the results easier to read by using only the significant digits. For example:
(6.3 +/- 0.7) E+11
(3.39 +/- 0.03)E+11

We took three sets of data which each contained ten measurements. For each set we found a value for the charge-to-mass ratio, which are :

  e/m = 6.28343E+11 +/- .725E+11
  e/m = 3.39E+11 +/- .0272E+11
  e/m = 3.07111E+11 +/- .00840E+11

The accepted value of e/m we (Matt Gooden and I), found online was:

  e/m= 1.758 820 150(44)e+11 C/Kg

There is a large amount of difference in what we measured and what the accepted value is. Several sources of error could be attributed to the apparatus itself, where a knob known as the focus seemed to radically change what we observed. We found a steady constant setting for the focus and left it there. No numeric values are known for these.SJK 23:56, 17 October 2007 (CDT)
23:56, 17 October 2007 (CDT)
I know we spent a lot of time discussing possible sources of systematic should include these in your writeup somewhere! Also, you spent time creating graphs, and it would be nice to link the images somewhere so they can be seen without needing to use Excel.

NOTE: Another group who did this experiment as well obtained similar data which also leads us to believe the error is with the apparatus.

Additional Information

There were questions involved in the end of the lab summary from the previous semmester which are listed at the end of my lab report here: E/M questions

Koch comments

  1. I uploaded a new version of one of your excel files. You can access the file here: Media:E_over_m_zane_and_matt_SJK_comments.xlsx. It shows how to use the linear fit to calcualte the e/m value