Physics307L F08:People/Martinez/e/m ratio for Electrons

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SJK 03:39, 22 October 2008 (EDT)
03:39, 22 October 2008 (EDT)
Since it's been so long since you guys did this lab, I'm not going to make a lot of comments. I was really happy with the way you explored the possible sources of systematic error and trying to understand the observation of the varied color spectrum during the "mistake." These were all very good experimental skills to develop.
  • For this lab we explored an old experiment originally conducted by J.J. Thompson. The laboratory preparation for this experiment was extensive requiring a good deal of electric equipment and power supplies for the Helmholtz coils, anode heater, and the electrodes. After a setup with some electrical equipment problems we followed out the experiment outlined in the lab manuallab manual, the actual data we collected left us with a considerable error, however we did explore somewhat outside the realms of the lab and discovered some interesting phenomenon. At low voltage and current we found that the arc created by the electron beam had a different color (purple) at the peak, from the base color (cyan). Furthermore we found that lowering these values more resulted in the disappearance all together of that peak. We postulated the reasons for this at the quantum level and examined helium (the substance in our bulb) under the spectroscope, but were still unsure of the reason. We also explored the original way the experiment was carried out, by correcting an electron beam deflected by the electric field by a magnetic one, but collected no data as the value for the electric field could not be known with much accuracy.
  • wiki page for this lab. User:Stephen K. Martinez/Notebook/Junior Lab/2008/09/17
  • SJK 03:37, 22 October 2008 (EDT)
    03:37, 22 October 2008 (EDT)
    Good reporting of the error and discussion of discrepancy from accepted value
    For our data we got the value of 4.7(4) * 10^10 C/kg which is the mean and the standard deviation of 4.0 * 10^9 C/kg within 68% accuracy which is approximately half the accepted value of this quantity. Our experiment had a lot of possible errors which could account for our large error from the accepted, notably systematic errors that occurred because of the non uniformity of the electric field and the decrease in energy of our field away from the electron gun. This seems to be the norm for this experiment to be so far off from the expected.
  • I learned a lot about how to use this experimental apparatus, I had originally thought that this quantity was discovered by the Millikan oil drop experiment. I learned a little about how the electron gun works by thermosomething, and a little about the spectroscopy of helium.
  • Suggestions are: tape the black hood down to the apparatus and give it a larger cape so the student can poke their head under.