Velocity selector

Definition

A velocity selector is a device that uses perpendicular electric and magnetic fields to filter charged particles based on their velocities. It ensures that only particles with a specific velocity can pass through undeflected.

5 Must Know Facts For Your Next Test

1. A velocity selector uses the principle that the electric force $\vec{F}_E = q\vec{E}$ and magnetic force $\vec{F}_B = q\vec{v} \times \vec{B}$ act in opposite directions on a charged particle.
2. The condition for a particle to pass through the selector without deflection is when the electric force equals the magnetic force, i.e., $qE = qvB$ or $v = \frac{E}{B}$.
3. If the velocity of a charged particle does not match $\frac{E}{B}$, it will be deflected by either the electric or magnetic field.
4. Velocity selectors are commonly used in mass spectrometers to select ions of specific velocities before they enter an analyzer.
5. The effectiveness of a velocity selector depends on precise alignment and control of both electric and magnetic fields.

Review Questions

• What is the relationship between electric field ($E$), magnetic field ($B$), and velocity ($v$) for a particle to pass through a velocity selector without deflection?
• Explain how a charged particle behaves in a velocity selector if its velocity does not satisfy the condition $v = \frac{E}{B}$.
• In what type of instrument are velocity selectors particularly useful, and why?

Related terms

Electric Field: A region around a charged object where other charges experience an electrostatic force. Represented by $\vec{E}$.

Magnetic Field: A field produced by moving electric charges or intrinsic magnetic moments, exerting forces on other moving charges within the field. Represented by $\vec{B}$.

Lorentz Force: The force experienced by a charged particle due to electromagnetic fields, given by $\vec{F} = q(\vec{E} + \vec{v} \times \vec{B})$.