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Velocity selector
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Crossed E and B fields select speed v = E/B.
v = E/BCrossed E and B fields allow only particles with v = E/B to pass undeflected (qE = qvB). It's used before mass spectrometers.
E = 10⁵ V/m, B = 0.1 T: v = 10⁶ m/s.
Independent of charge and mass.
In more detail
A velocity selector passes particles that have a specific speed by balancing electric and magnetic forces. Particles with v = E/B move straight through without deflection.
Real-world uses
- Mass spectrometers use velocity selectors to choose ion speeds.
- Electron microscopes use similar filters for beam control.
- Electron microscopes use a velocity selector to pick out particles with a single speed.
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Frequently asked questions
What speed passes through?
v equals E divided by B, regardless of the particle's mass or charge.
Why does this work?
The electric and magnetic forces cancel for that speed, so the particle is undeflected.
Why do the electric and magnetic forces cancel?
At the right speed, the electric force and the magnetic force on the particle are equal and opposite, so it goes straight through.
Related topics in Magnetism & Induction
Circular motion in B field · Cyclotron frequency · Helical motion · Mass spectrometer · Cyclotron · Force on a current-carrying wire