Physics307L F09:People/Phillips/Eoverm

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Charge-to-Mass Ratio for Electrons

SJK 03:33, 22 October 2008 (EDT)
03:33, 22 October 2008 (EDT)
OK, I know we had this problem where I asked you to redo the summary because you did the old one w/ Stephen. Well, that's probably why you forget to report your final value w/ uncertainty here! I'm not going to worry about it, because you're already to the next lab. But make sure to do this in your following labs from now on!
In this lab, Stephen and I attempted to determine the charge-to-mass ratio (e/m) for electrons using a deflected electron beam. First of all, you can view our data here, which include our procedure and all of our raw notes regarding this experiment. We had a very tough time getting the apparatus to work properly, though this primarily concerned setting up the power supplies in a strange way. One thing we noticed right away (through a strange "mistake" during our procedure) was that this method of finding an electron's charge-to-mass ratio would be very inaccurate. What we saw was our loop bending in on itself, as if sucked in by the accelerating potential of the electron gun.SJK 03:34, 22 October 2008 (EDT)
03:34, 22 October 2008 (EDT)
I really like how you guys noticed this mistake and then studied it to see what clues it gave you. Very good experimental technique
This, plus many other sources of systematic error (loss of energy due to collisions with Helium atoms, and the related ionization, a not truly uniform magnetic field, etc.) caused our results to have some 70% error.
our "mistake"-loop bends back into itself (Steve Koch:Awesome picture, I love those colors, but still don't understand them.)

Of course, this result is not particularly pleasing, but it seems to be somewhat normal for this lab and apparatus (after looking at past results and Paul's error). Despite the disappointment related to the final results, I think this was a very valuable lab for us, meaning at least Stephen and I, to do because it showed us how lab operations really work, and even though our results weren't optimal (far from it, in fact), we were at least less than 100% off from the accepted value for e/m, giving us hands on evidence of physical constants.