Velocity Selector
Definition and meaning of Velocity Selector in chemistry.
A velocity selector is a specialized analytical device that filters moving charged particles. It uses crossed electric and magnetic fields to achieve this highly specific filtering task. Only particles traveling at one exact speed can pass through the entire device undeflected.
In more detail
Inside this specialized device, a beam of charged particles enters a sealed vacuum chamber. An electric field and a magnetic field sit perfectly perpendicular to each other. They are also both perpendicular to the forward motion of the traveling particles.
The electric field pushes the charged particles in one specific sideways direction. The magnetic field pushes the charged particles in the exact opposite sideways direction. For a particle to travel straight through, these two opposing forces must balance exactly.
This perfect physical balance only happens at a very specific and mathematically defined speed. We find this exact speed by dividing the electric field by the magnetic field. Particles moving faster than this ideal speed feel a stronger magnetic force and deflect.
Particles moving slower feel a stronger electric force and also deflect away from center. Surrounding physical barriers or slits absorb all of the deflected, unwanted charged particles. Students often incorrectly assume that a heavier particle needs a different magnetic field. However, the successfully selected speed is completely independent of particle mass or particle charge.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Operating Principle | Balances electric and magnetic forces |
| Selected Velocity Formula | v = E/B |
| Force Orientation | Electric and magnetic fields are perpendicular |
| Particle Dependence | Independent of particle mass and charge |
| Common Application | Filtering ions for mass spectrometry |
A modern mass spectrometer frequently uses a velocity selector as a critical initial filter. Scientists first create a beam containing many different ions moving at wildly different speeds. The velocity selector strictly forces all the exiting ions to travel at one uniform speed. This uniform speed allows the next section of the machine to separate them by mass. Without this starting uniform speed, the final mass measurements would be completely mixed up. If scientists want to select faster ions, they simply increase the electric field strength.
Frequently asked questions
Does the mass or electrical charge of the particle affect the selected velocity?
No, the specifically selected speed depends entirely on the ratio of the two fields. It is completely independent of the moving particle's mass or internal electrical charge.
What happens to particles that are traveling too fast?
Particles moving too fast experience a much stronger magnetic force than electric force. This imbalance pushes them into a physical wall where they are completely absorbed.
Why must the electric and magnetic fields be perpendicular?
They must be perpendicular so they can push the particle in exactly opposite directions. If they were parallel, the particle would speed up instead of traveling straight.