# Velocity Selector

## What is velocity selector?

Velocity selector is also known as Wien filter is a device with a perpendicular arrangement of electric and magnetic fields which is used as a velocity filter for charged particles. Velocity selector exploits the principle of motion of a charge in a uniform magnetic field. According to this principle, the force experienced by a moving charge with speed v in a uniform magnetic field is given as:

F = Bqv

Where,

F: force experienced by the charge

B: magnetic field

q: moving charge

v: speed of the charge

The speed of charged particles in velocity selector is $v=\frac{E}{B}$ is the velocity required by the particles to pass through the equipment.

## Fields of velocity selector

• Uniform electric field: This field is generated by the top plate with negative charges and the bottom plate with the positive charges. These charges result in the formation of the field which is pointing in the upward direction in the figure.
• Uniform magnetic field: This field is present uniformly between the two charged plates such that it can be directed either inwards or outwards.

### Limitations of velocity selector

• Neither the mass nor the charge of the particles is considered before passing through the filter.
• All the uncharged particles pass through the filter.