# Homework Help: HELP! Potential energy of proton in capacitor

1. Feb 8, 2010

### csimon1

1. The problem statement, all variables and given/known data

I have attached the image. The question is --

What is the potential energy of a proton at the midpoint of the capacitor?

I know this is an extremely easy problem, but I can't seem to get the formula right...

2. Relevant equations

I tried U = qV, using 1.6*10^-19 * 150V (because it is in the middle). What am I doing wrong? Is this not the right formula?

3. The attempt at a solution

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2. Feb 8, 2010

### fluidistic

Hi and welcome to PF!
I know a hard way to solve the question. I'm not sure it is of help in your case, but in case you've taken calculus 3 then it's ok.
$$\phi (r)= \int _{\infty}^r \vec E \cdot d \vec l$$. Thus the almost only difficulty would be to calculate the E field between the plates of the capacitor.

There might be of course a much faster way to solve it. So if anyone can help, welcome!