The force on a charged particle travelling at right angles to a uniform magnetic field is given by:
According to Fleming’s left hand rule, the force experienced by the particle is always perpendicular to its direction of motion. This is the condition for circular motion, so charged particles moving in a uniform magnetic field are deflected in a circular path.
Charged particles moving in a region occupied by both magnetic and electric fields will also experience an electrostatic force. Velocity selectors use this property to separate our particles of a certain velocity from a stream of accelerated, charged particles moving at a range of speeds.
Particles are not deflected if:
Only particles which are not deflected can pass through the collimator (narrow gap).