E/m

e/m lab

Objective: To measure the charge to mass ratio of an electron with a thorough error analysis (the error is not the difference between your measurement and the most recent published value). Other groups who make this measurement should get the same results as you within the error you specify.

Model: The e/m apparatus bends an electron beam in a somewhat uniform magnetic field as the electron travels through somewhat of a vacuum to form a circle. By measuring the radius of this circle, knowing the speed of the electron beam, and knowing the magnetic field this ratio can be calculated.

Method: Read sections 1, 3, the introduction to section 4, and then 4.1, 4.2 of the following link https://en.wikipedia.org/wiki/Mass-to-charge_ratio

Procedure: Use the apparatus described in the following link: https://www.pasco.com/file_downloads/Downloads_Manuals/Electron-Charge-to-Mass-Ratio-SE-9629.pdf

Questions to think about and ask the TA: How is the electron beam generated? -read the following link https://en.wikipedia.org/wiki/Thermionic_emission -What mechanism that you know about also emits electrons from a metal? -Why might there be enough thermal energy for an electron to overcome this work function energy? -These electrons strike He atoms at low pressure in the tube, which then fluoresce to make the beam visible. About how many atoms emit photons per electron if the electron is accelerated to 100V? -What effect does this have on the trajectory of the electron beam? -100 V is enough to generate current in your body. It is dangerous? Is there a simple way to estimate the danger? If you know what the dangers are then you should be safer than not knowing. If you didn't know a rattlesnake was dangerous you might try to pick it up. -Why wouldn't the electrons move in a circle? -How can you find B? Can you use your cell phone? -Some things you might want to explore in this lab exercise are (the purpose of the lab is to): determine e/m, see if e/m varies with accelerating voltage, etc.

Team projects to explore during the second lab meeting: -if your cell phone has an app to measure B see if the results are consistent with your calculation. -Bring the large permanent magnet up near the e/m tube. Does the beam deflect as you expect it to using the Lorentz force and your understanding the B field from such a magnet?