McGee 1
Lab E6
Determination of “e/m” for the electron
Allison McGee
Mr. Fedorchak
Thursday 2:00 4:50pm
Partners: Stevie Whitehead
Trey Smith
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McGee 2
Abstract
: The purpose of this lab was to measure the ratio of an electron’s charge to its
mass using the Lorentz Force. We observed an average e/m value of 1.918 x 10 11 C/kg,
compared to the accepted value of 1.7588 x 10 11 C/kg. This yielded a percent
discrepancy of 9%.
Introduction
: Towards the end of the 19
th
century, J.J. Thomson studied the cathode rays
extensively, finding that the rays were particlelike in nature. He also discovered that the
ratio of these particles’ charge to mass was the same no matter what gas was in the tube.
Thomson is credited with the proof of the existence of the electron. By using the equation
below, it is possible to solve for the e/m ratio:
e/m= 2V/(B2r2)
This equation comes from manipulating the Lorentz force equation and setting it equal to
the centripetal force equation and combining it with the equation for conservation of
energy.
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 Fall '08
 Fedorchak
 Electron, Charge, Mass, Magnetic Field, Lorentz force, Allison McGee, e/m value, average e/m value

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