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## 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
(q / _{e}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: 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
energy m·v^{2} / 2,W = _{E}q·U.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 [2] 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. |