Particles moving in magnetic fields

When a charged particle such as an electron moves in a magnetic field there is a force on it. This force is affected by the size of the magnetic field, the charge on the particle and the speed and direction of motion of the particle. This fact is really important in CRT TV sets, oscilloscopes and in nuclear particle accelerators. It is also vital in the production of the aurora, those beautiful ‘curtains’ of light seen at high latitudes near the poles of the Earth.

The force is large when:
(a) the magnetic field is large
(b) the charge on the particle is large (either positive or negative)
(c) the speed of particle is large
(d) the direction of motion of the particle is at a large angle to the magnetic field.

You should understand from these facts that there is no force on a stationary charged particle in a magnetic field and that the force is greatest if the particle is moving at right angles to the magnetic field. There is also no force on an uncharged particle.


The two diagrams show a charged particle entering a magnetic field. (First in 2D and secondly in 3D). The force (F) acting between the field and the particle makes the particle move in a circular path with the force always acting towards the centre of the circle.