Particle Trajectories in a Wien Filter

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The Wein filter, also known as a velocity selector, can be used to filter particles according to their speeds. This device has many applications, including electron microscopes and spectrometers.


The filter employs perpendicular electric and magnetic fields. Particles enter the filter with a speed perpendicular to both fields, such that they will deviate from a linear trajectory if their speed does not match the ratio E/B. The charge-mass ratio increases the deviation of the particle, but is unaffected for the appropriate matching speed. Also, for particles of negative charge, the forces act in the opposite direction.

In this Demonstration, we show the plates creating the electric and magnetic fields and the vector field representing the direction and intensity of each field. The trajectory of the particles, shown as a blue curve, is determined by these conditions.


Contributed by: Alejandro Javier Perez Andres (June 2016)
Open content licensed under CC BY-NC-SA



For the basic theory of the Wien filter, see [1].


[1] Ludwig Maximilian University of Munich, Physics Department. "Velocity-Selector (Wein Filter)." (Jun 28, 2016)

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