# Homework Help: Electron's trajectory in positively charged hollow cylinder

1. Apr 12, 2015

### Mary001

How an electron moves when it is released from rest to a positively charged hollow cylinder? The hollow cylinder is finite and insulating. The electron is constrained to move only in z direction (into the center of the hollow cylinder). My thinking is that there is no electric field inside the hollow cylinder. Therefore, the electron will not be affected by the electric field. So what causes it to move should be gravitational force and it should travel towards the earth? But an electron's mass is tiny so gravity can be ignored here? Also, the next task asks me to calculate the work done by the electric field on the electron to make it travel. I think I'm not on the right track now.

Any help would be much appreciated.
Thank you.

2. Apr 12, 2015

### Staff: Mentor

Is this homework?
Edit: moved to homework section now.

There is, as the cylinder is finite.
Gravity can be ignored for electrons. Even the tiniest deviation from some exact geometry of electric fields will be orders of magnitude more important than gravity.

Can you find the electric potential as function of position?

Last edited: Apr 12, 2015
3. Apr 12, 2015

### Mary001

It is a question from my mid-exam sample

Oh really? I've read some documents and they say electric field in a hollow cylinder is zero. I can see that in this case, as the finite is finite, there is electric field. But why's that?

Yep, I can. Is it how I should start with to find the work done by the electric field?

4. Apr 12, 2015

### Staff: Mentor

Why should it be zero? A value of exactly zero requires something special, like a symmetry (a cylinder of infinite length).

If you can calculate the potential, the work done should be easy.