PHY222, Elements of Physics II
Electron's Charge to Mass Ratio, 29 February 2008

Theory Requirements:

1. Qualitatively describe the motion of a charged particle in a magnetic field. Describe what happens when the particle's motion is parallel to the magnetic field, and when it is perpendicular.
2. State the expected value for the e/m ratio. State the experimental relationships that will be used to determine this quantity.

Comments on the Procedure:
As usual, summarize the major steps in the procedure - make an effort to avoid tangents. Do not blindly refer to a figure in the lab manual - where appropriate, give a brief description (such as wire placement, whatever will help you).

As a general warning, there are bulbs in the inventory that aren't at the peak of their condition. One possible fix occurs in the procedure's step 7. Where the lab manual calls for a voltage setting of 250V, it can be helpful to crank the voltage somewhat higher (until you see a clear trail), then gradually bring the voltage back down, as low as you can while the trail remains visible. Even then, some of the tubes will require a voltage setting closer to 350V - not a crisis, but it will take some willingness to improvise in order to obtain a sufficient number of measurements.

Comments on Data: (half the class can be reading this with an angry voice!)
DESCRIBE THE DATA TABLES and/or GRAPHS YOU PRESENT. This requires barely a sentence to state what the reader gets to find in each table or graph. Consider it some small evidence that you have some idea of what you were measuring and recording.
(Many of you are doing very well with this; others are neglecting it, and the grades reflect it.)

Results Checklist:

• Report the mean of your calculated e/m values, along with extremes and standard deviation like the good old days.
• Report the percent difference, in comparing your mean e/m value with the theoretical value. As a style tip - do not just blankly say, "The percent difference was_____." That can mean anything. The percent difference has to be in your measurement. It is better to say, "The percent difference in e/m was _________."
• Give a sample calculation for the e/m ratio, using any one of your measurements.

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