# Magnetic field

1. Oct 4, 2014

### paulina

what is the technical meaning of refocusing paraxial electrons along the direction of magnetic field?

2. Oct 4, 2014

### Staff: Mentor

I'm not quite sure. It sounds like making the electrons bunch up as they travel through a magnetic field. In optics, paraxial rays are rays which travel nearly parallel to the optical axis, so I assume that paraxial electrons are electrons that are traveling nearly parallel to the axis of the system. I'm not quite sure what "along the direction of the magnetic field" means, as I'm not well versed in EM theory.

3. Oct 4, 2014

### Staff: Mentor

Can you show us the source of this comment? Either post a link or list where you have read it? Was it in the context of Cathode Ray Tube (CRT) displays, or in the context of particle accelerators?

4. Oct 5, 2014

### paulina

there was a question related to this which i although solved but could not find its true meaning...
"In a region of space a uniform magnetic field Bis along the positive X-axis.Electrons are emitted from the origin with a speed 'v' at different angles.Show that the paraxial electrons are refocused on the x axis at a distance (2.pi..m.v/Be). Here, m is the mass of electron and e the charge on it."
i found the answer by finding the pitch of an electron which has velocity v along x and y axis ( v i +v j)...
i have no idea if my attempt was right or wrong.

5. Oct 5, 2014

### Staff: Mentor

Can you post your work on this question? Did you use the Lorentz force in your calculatons?

6. Oct 5, 2014

### paulina

(mv^2)/r = B.e.v
r = m.v/B.e
so, T = 2.pi.m/B.e
now, p = v.T (p= pitch)
p = 2.pi.m.v/ B.e

i did not use Lorentz force
now answer to my question please... :(

7. Oct 6, 2014

### Staff: Mentor

I'm not tracking exactly what you did with those equations (they may be related to the Lorentz Force, but I'm not sure). In any case, I did a quick Google search on this:

refocusing paraxial electrons along the direction of magnetic field

and got this hit list: https://www.google.com/search?sourc...&gs_l=hp....0.0.0.427...........0.ZfAOzJjBBpA

The first 2 hits are to this PF thread of yours, but the rest look to be good explanations of using magnetic fields to focus electron beams. Please see if the first couple explanation hits are useful for you. :-)

8. Oct 6, 2014

### paulina

well there is no electric field, so no lorentz force...
anyway thanks for the link...

9. Oct 6, 2014

### Staff: Mentor

There are 2 parts to the Lorentz Force equation, after all... :-)

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