E-Potiential between 2 charges

[IvaNMK]

Homework Statement

I do not understand why the PE is positive when 2 charges are the same sign, and PE is negative when 2 charges are opposite signs

Picture

Homework Equations

W = - Delta PE

The Attempt at a Solution

I don't know how the how the forces go. What it means by +, - PE

Basically I don't understand the ppt

Plz help

http://img222.imageshack.us/img222/7026/9661766768304be98ca8f23oj9.png

 

[LowlyPion]

I do not understand why the PE is positive when 2 charges are the same sign, and PE is negative when 2 charges are opposite signs

Generally, like gravity, electrical potential energy is determined by the idea that you are bringing a charge from ∞.

If you had a backpack full of charges and you were walking in from say Topeka Kansas to another backpack full of charges on your table and they were the same sign, then as you get closer and closer you are going to feel the force more and more. That means that you are doing work against the E-Field of the charges sitting on the table. It is a force against the direction you are moving and we have defined work as the dot product of Force against the distance it moves.

If it takes positive work to get there, that means that it is stored as "potential" and if it were released, it would be accelerated away by the force back toward Topeka. So the convention is that it's positive PE for like charges.

For attractive forces the opposite is true. As you stroll in with your backpack of charges and you begin to get close to the table you will feel a pull in the direction you are moving. That pull is something that you have to resist pulling you closer. The work you have to do then is actually to keep it from going closer. And that's called -PE.