User:Joseph Frye/Notebook/Physics Junior Lab 307L/FormalReport/notes

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DAY 1 (October 11, 2010):

We ran two trials today. The first trial we kept the current to the coils constant at 1.5A (constant B field) and varied the accelerating voltage from about 175V to about 225V in steps of 25V. We then measured and recorded the radius of curvature in a google docs spreadsheet. The second trial we kept the accelerating voltage constant at 200V and varied the current to the coils from 1.5A to 2.0A and again measured and recorded the radius of curvature. our data is hereImage:Benedict Frye E M ratio.ods

after looking at the data we had a relative error of about 23% which according to Dr. Koch is normal for this experiment with this e/m apparatus. We spent the rest of day 1 discussing why there would be so much error in the experiment. Thats to say why is 23% relative error normal for this experiment? In an attempt to gain insight into this question we varied the voltage and current some more and noticed some strange things.

  • In the lab manual the beam is supposed to be violet, ours was green. however at low accelerating voltages we are able to get portions of the beam to be violet.
  • The filament voltage should not affect the radius of curvature but it does change the radius quite a bit







DAY 2(October 18, 2010):

we spent day 2 taking photos for the lab report and again trying to figure out why there is so much error in this experiment. We also tried to figure out why the beam is not violet but is instead green. Our observations are recorded on tab 2 of the included spreadsheet. Here are some of the anomalies that we observed.


The electron beam will form a helix when the velocity of the electrons is not perpendicular to the magnetic field shown here:



When the accelerating voltage is low and the current in the coils is high we are able to get the violet color described in the lab manual in part of the beam.


Follow up (December 6, 2010):

One of our thoughts about why the beam was a different color than the lab manual says is that perhaps the bulb contained a different gas than helium. To test this we used helium lab and a spectroscope. Using the equipment and technique similar to another lab we did(Balmer Series Lab). We found that the spectral lines from the electron beam are consistent with helium. There were no lines present that are not present in the helium spectrum. This means that the gas in the bulb is indeed helium. We also found that the violet light is present at normal experimental conditions but is not visible to the eye because of the brightness of the green light. Another interesting note is that I could see no other lines besides the violet and green. I had expected to see other faint lines from the helium spectrum. This does not mean they are not there just that I could not see them in the scope. We were unable to come to a satisfactory explanation for this phenomena.

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