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.

 

[nlightner]

Hello,

I understand your confusion regarding the potential energy between two charges. Let me explain it further.

First, it is important to note that potential energy is a measure of the energy stored in a system due to its position or configuration. In the case of two charges, the potential energy between them is a result of their relative positions and their interaction with each other.

Now, let's consider two charges with the same sign, let's say both positive. This means that they repel each other, as like charges repel. When they are placed close to each other, they have a high potential energy because of their repulsion. As they move further apart, the potential energy decreases because the force of repulsion between them decreases. Therefore, in this case, the potential energy is positive because it represents the energy required to separate the two charges.

On the other hand, if we have two charges with opposite signs, let's say one positive and one negative, they will attract each other. When they are placed close to each other, they have a high potential energy due to their attraction. As they move further apart, the potential energy decreases because the force of attraction between them decreases. In this case, the potential energy is negative because it represents the energy released when the two charges are brought closer together.

In summary, the sign of the potential energy depends on the type of interaction between the two charges. Positive potential energy represents repulsion, while negative potential energy represents attraction. I hope this explanation helps you understand the concept better.

Best,