How does a magnetic focusing coil in a CRT tv work?

Question

I have found a lot of diagrams of the components of a CRT tv, and one of the components is a "magnetic focus coil" or "focus coil". However, what i can't find, is how they work. All i know so far is that the electron beam travels through the coil and somehow, it is focused.I'm doing this for a physics assignment, so a detailed explanation of how the magnetic field interacts with the electron beam would be great.

Answer 1

Interesting question that a surprising amount of scientists will often get totally wrong. (Whilst researching this be careful what you search for as there are different types of magnetic lens.)

As to how a focus coil works. You can think of the individual electrons in a beam "entering" a magnetic coil along the magnetic lines (through the centre of the coil). The electrons will circle the field lines very tightly. So tightly they will appear to be "stuck" on the field lines.  When the field line bunch together the electrons are focused. If the bunched electrons are pulled out the other side by an electric field they don't have time to spread out. Thus the electrons will be focused.

Magnetic fields are also used to "steer" the beam in a CRT. Plus there are other types of magnetic lens that are often used in particle accelerators. However both these use slightly different magnetic configurations to focusing coils in a CRT. (As it is the multiple types that cause confusion I won't explain the other configurations).

I gave you as a source a machine from a physics lab in Germany that is basically a massive focus coil. You can see on their website a diagram on how the magnetic field squeezes the electrons. However unlike a CRT focus coil their machine also defocuses at the other end.

EDIT

The answer below is considering a steering (perpendicular) magnetic field if this is what you meant by focus he is totally correct.  However for a true magnetic lens the electrons go along the magnetic axis (axially). Google magnetic compression if this is confusing.

See I warned you it was complex.

Answer 2

A magnetic field exerts a force on a moving electron in the direction perpendicular to both. The only component of velocity not effected by the field is the one in the direction of the field, so that ends up being the direction the electrons follow over all. By varying the magnetic field direction and strength with position, this (Lorentz) force can be used to direct all electrons passing through it from the gun to the same point on the screen. That's what focusing is.