# Homework Help: Horizontal parallel

1. Feb 5, 2013

### lizaliiu

1. A proton is suspended in a vacuum between two oppositely charged horizontal parallel plates. The separation between the plates is 2.60mm

2. I assume it uses E=vd/d, but I dont really know

3. what is the sign of the charge on the lower plate?
what is the voltage across the plate?

Last edited by a moderator: Feb 5, 2013
2. Feb 5, 2013

### Staff: Mentor

What does that mean?

Some hints how to start:
Can you identify the forces acting on the proton?
If the proton stays at rest between the plates, can you identify the direction of those forces?

3. Feb 5, 2013

### Staff: Mentor

Welcome to the PF.

You are required to show your work toward a solution before we can offer much tutorial help.

4. Feb 5, 2013

### lizaliiu

since gravity pulls the proton downward so the lower plate should be the positive charge to repel the proton.
F=mg=qE
E=mg/q=v/d, m=1.673x10^-27kg, g=9.8m/s2, q=1.602x10^-19 and d=2.60x10^-3m
so we just need to solve q to get the voltage across the plates?

5. Feb 5, 2013

### Staff: Mentor

You know q, I think you mean V? Right.

6. Feb 5, 2013

### lizaliiu

yes, I solved V= 2.66x10^-10...not positive sure it's correct tho

7. Feb 6, 2013

### Staff: Mentor

Gravity is weak, and you can check your calculations in WolframAlpha, for example. If the units match, it should be right.