Force on a Charged Particle



The Cyclotron

A cyclotron is a particle accelerator. It consists of two hollow D-shaped electrodes (called ‘dees’) that are attached to an alternating p.d. supply. The dees are placed in vacuum chamber and a magnetic field which acts at right angles to them.

A particle will move in a circle because of the magnetic field.

When it reaches the gap between the dees the alternating supply has made the other dee have the opposite charge to the particle. This causes the particle to accelerate across the gap and enter the second dee. This continues to happen until the particle is moving at the required speed. At this point it leaves the cyclotron.

The Mass Spectrometer

A mass spectrometer is used to analyse the types of atom that are in a sample. The atoms are given a charge, accelerated and sent into a magnetic field. If we look at the radius equation above we can see that atoms travelling at the same speed in the same magnetic field given the same charge will be deflected based on their mass. Heavy atoms will move in bigger circles than lighter ones.

Pair Production

If we think back to Unit 1 we saw this phenomenon in action. Pair production is when a photon of energy is converted into a particle and an antiparticle, such as an electron and a positron. If this happens in a magnetic field the electron will move in a circle in one direction and the positron will move in a circle in the other direction.