Physics307L F07:People/Le/e/m

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--Linh N Le 22:34, 28 October 2007 (CDT)


The purpose of this lab is to measure the ratio of the charge of an electron (e) to its mass.

We will send a beam of charged electrons perpendicularly through a magnetic field. The field will produce a force on the beam, bending it into a circle. By measuring the radius of the circle, we will be able to calculate the ratio of e/m.

For a more comprehensive look into this lab, I refer you to my notebook: Part 1 Part 2


I will forgo the tedium of reporting all my measurements (as that can be found in the lab) and get right to the meat of the problem.

4 sets of data were taken, and the respective e/m ratios are:

SJK 01:08, 13 November 2007 (CST)
01:08, 13 November 2007 (CST)
Where are the units on these numbers???? Also, see Cary's summary for suggestion about how to format final result.
  • 3.2116e11 +/- 6.3934e09 C/kg
  • 3.2246e11 +/- 7.7978e09 C/kg
  • 4.5411e11 +/- 1.2047e10 C/kg
  • 2.7461e11 +/- 8.7489e08 C/kg

The accepted value is: 1.76e11 C/kg


Comparing the values we got to the accepted values, I'm able to give relative % errors

%error set 1=82.5%

%error set 2=83.2%

%error set 3=158%

%error set 4=56.03%

Final Thoughts

SJK 01:10, 13 November 2007 (CST)
01:10, 13 November 2007 (CST)
Especially since the results have such severe systematic error, I would expect more discussion of the sources of this error and any ideas for how to mitigate this error and other ideas for how to make the experiment better in the future.
Later: Oh, now i found the discussion in your primary lab notebook. Great analysis! You should include this in the summary!

The data is precise, but very inaccurate. I would have to say that the apparatus has fundamental design flaws, although it itself if constructed properly.

The way of measuring the radius involves a high amount of error, since we must account for paralax.

Increasing the voltages supplied to the apparatus increases the accuracy of the data, but that has a cap before we overload the apparatus.