Theory

Stationary electric charges do not experience any forces at all when they are inside magnetic fields. However, when electric charges are moving inside magnetic fields, they experience a sideways force which deflects them into a circular path.

The next bit is a bit complicated! The direction of the force is given by 'Fleming's Left Hand Rule'. Using your left hand, point your thumb up in the air, your first finger away from you, and your second finger towards your right hand side. You are now ready to work out the direction of the force experience by the electrons!

Keep your hand locked in that shape!

1. The first finger needs to point in the direction of motion of the magnetic field (in this simulation, that's into the screen).
2. The second finger needs to point in the direction of the electric current. Since electrons are negatively charged, point your second finger towards the left (where the electrons are coming from).
3. Now your thumb should be pointing to the floor (and your elbow should be at an uncomfortable angle!). The direction of your thumb is the direction in which the electrons should be pushed.

As soon as the electrons experience the downward force their direction of travel will change. To follow what happens next, you need to point your second finger towards the left and a little upwards (even more uncomfortable!). Notice that your thumb is now starting to point a little towards the left.It is this change of direction of force that causes the electrons to travel in a circular path until they escape from the magnetic field.

In a synchrotron the magnets need to be very strong because the electrons have a much larger mass than normal (due to relativistic effects). The electromagnets are not circular but rather kidney bean shaped. The field strength needs to be adjusted very precisely to keep the electrons travelling along the desired track!