User:David K. O'Hara/Notebook/physics 307 lab/E/M ratio summary

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SJK Incomplete Feedback Notice
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My feedback is incomplete on this page for two reasons. First, the value of the feedback to the students is low, given that the course is over. Second, I'm running out of time to finish grading!

The objective of this lab was to measure the ratio of the electron charge over its mass. This quantity was determined by subjecting an electron beam to a magnetic field generated by a pair of helmholtz coils. The current was adjusted so that the electron beam was "bent" around in a circle. Given the size of the circle, the current, the accelerating voltage, and the magnitude of the magnetic field generated I was able to calculate a value for the e/m ratio.


The value I claculated for e/m was 2.426e11 +/- 4.65e10 C/Kg. Compared to the accepted value of e/m of 1.759*10^11 C/kg gave me a percent error of about 38%.

This value is in the ballpark of the accepted value in the idea that it is of the correct magnitude, but not truly close to the expected value.

SJK 15:44, 19 December 2009 (EST)
15:44, 19 December 2009 (EST)
Again, I think you're using the standard deviation, not standard error of the mean for uncertainty.


The largest contributor to error in this experiment is the way we had to take the measurements. The parallax involved paired with the dimness of the beam created a lack of precision that showed through quite dramatically in final result.

SJK 15:42, 19 December 2009 (EST)
15:42, 19 December 2009 (EST)
It's not actually the parallax, but rather that the system is fundamentally flawed (helium and other factors). See comments on other students' labs.