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Velocity Selector

A Velocity Selector

A velocity selector is a region in which there is a uniform electric field and a uniform magnetic field. The fields are perpendicular to one another, and perpendicular to the initial velocity of the charged particles that are passing through the region.

The force exerted on a charged particle by the electric field is given by:

F = qE

The magnitude of the force exerted by the magnetic field is F = qvB, as long as the velocity is perpendicular to the field.

The idea is that, if the two forces are equal and opposite, the net force is zero, and the particle passes through the region without changing direction. With the magnetic force being speed dependent, however, any charges traveling faster or slower than the ones that go straight through will be deflected one way or another out of the beam. You can investigate these concepts with this simulation.

In this simulation, the electric field is directed in the z-direction (upward/ downward which is represented by the red arrow) and the magnetic field is directed in the y-direction (into the plane/ out of the plane of the screen which is represented by the orange arrow).

Note also that when the charged particle is inside the region, the force acting on the charged particle will be shown acting on the particle. The arrow in green represents the electric force and the arrow in cyan represents the magnetic force.


For Teachers


Software Requirements


Android iOS Windows MacOS
with best with Chrome Chrome Chrome Chrome
support fullscreen? Yes. Chrome/Opera No. Firefox/ Sumsung Internet Not yet Yes Yes
cannot work on some mobile browser that don't understand JavaScript such as..... cannot work on Internet Explorer 9 and below



Fu-Kwun Hwang; lookang; Chee Wah Ong

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