Physics307L F09:Winter Break Lab Fest/chargemass

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This is what the electron beam looks like when it's going well
This is what the electron beam looks like when it's going well

In this lab, you can estimate the ratio of the charge to the mass of an electron. However, the instrument is dying a bit, and even when it's working, the estimate is only within a factor of 2 or so. So, we should probably not do the careful measurements of the radius of the circle for this lab, but rather should just explore the cool effects of an electron beam traveling through dilute helium gas, in the presence of a magnetic field. Some principles:

  • Electrons are negatively charged
  • Moving, charged particles feel a force perpendicular to their direction of motion when traveling in a magnetic field.
    • Thus they travel in helical / circular paths!
  • The moving electrons will cause the helium to glow. This is similar to the glowing tubes in the Balmer series, and also similar to fluorescent light bulbs.

General procedure

  1. Discuss safety first!
  2. Make sure everything is wired correctly (see manual or student reports)
  3. Explore how changing the accelerating electric field and the magnetic field change the circle size.
  4. Explore making helical beam paths by rotating the bulb.
  5. How does changing the polarity of connections (switching plus and minus) change things?


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