Question

A velocity selector is used to separate charged particles moving in the negative x-direction. The magnetic field points in the positive y-direction. In which direction does the electric field point?

A velocity selector is used to separate charged particles moving in the negative y-direction. The magnetic field points in the negative z-direction. In which direction does the electric field point?

Answer 1

A velocity selector or Wein filter is a device in which magnetic and electric fields are arranged perpendicular to each other. It utilizes the principle of motion of charge in a uniform magnetic field.

The magnetic force (F) experienced by a charge (q), moving with velocity (v), in a uniform magnetic field (B) is given by

In a velocity selector charge enters perpendicular to the magnetic field. Hence

The electric force is given by

When both the electric and magnetic forces become equal, then only the charge will move undeflected.

Hence for proper velocity selection

1. Magnetic and electric fields must be perpendicular

2. Velocity vector and magnetic field must be perpendicular.

Hence magnetic field vector, electric field vector and velocity vector must be mutually perpendicular to each other

According to question,

Direction of velocity vector - along negative x direction

Direction of Magnetic field vector - along positive y direction

Hence, Direction of Magnetic field vector - along positive z direction