The mass of the electronPå svenska
|Actually we won't measure the mass of the electron in this experiment.
What we measure is the ratio of the charge and mass of the electron
(qe / m). If you have done Millikan's
oildrop experiment you have a value of the charge you can use to
calculate the mass of the electron.
As you might know charged particles are affected by a force when they move in magnetic fields. The force can be calculated with this expression:
F = q·v·B where q is the charge, v is the velocity and B is the strength of the magnetic field.
In this experiment the electrons are first accelerated by an electric field in the electron canon.
They get a velocity that we can calculate by comparing the kinetic
We get the speed as:
Since the electrons are making a circular motion in the magnetic field we can use the following expression for the force acting on a single electron:
Use your algebraic skills combined with expression  for v, and you will find that the ratio q / m can be expressed with just the accelerating voltage U, the magnetic field B and the radius r of the circle. You just have to measure the radius on the screen.
Enter the values in your expression for q/m and use the value of q from Millikan's experiment and you will get the mass of the electron.
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