# Homework Help: Electric Potential Energy question

1. Sep 20, 2008

### th3plan

This isnt really a homework problem. I just want to understand electric potention
$$\Delta$$V=Uf-Ui=-W

Uf= final
Ui=initial

Can someone explain to me how electric potential acts between two positive electrons, or two negative electrons or a + and a negative electron. In which cases would work be positive and why ?

Thank you

Last edited: Sep 20, 2008
2. Sep 20, 2008

### tiny-tim

Hi th3plan!

We (arbitrarily) define the potential to be 0 at infinite distance.

To bring a + charge near another + from infinity, you have to overcome the repulsion, so you have to do work on it.

To bring a - charge near a + to infinity, you have to overcome the attraction, so you have to do work on it.

3. Sep 20, 2008

### th3plan

so if work has to be done on it its considered -w , for a + to a + to be brought together, but for a + and a - u have to do work to bring together ? Why ? They naturally attract :S ?

4. Sep 20, 2008

### tiny-tim

Hi th3plan!

No, you have to do work to separate them, to bring them to infinity.

5. Sep 20, 2008

### tiny-tim

Hi th3plan! Thanks for the PM.
(Assuming the potential at infinity is 0?)

The electric field points outward for a same charge, and inward for an opposite charge.