# Potential energy charges

1. Jul 17, 2010

### pat666

1. The problem statement, all variables and given/known data
Two charges, one positive and one negative, are separated by a fixed distance.
The sign of the potential energy of this pair
is always positive;
is always negative;
is sometimes positive and sometimes negative;
depends on the distance.

2. Relevant equations

3. The attempt at a solution
I think that it is sometimes positive and sometimes negative; since it would depend on the magnitude of the individual charges????

2. Jul 18, 2010

### graphene

what is the expression for potential energy of two charges?

Last edited: Jul 18, 2010
3. Jul 18, 2010

### Pythagorean

No superposition requred here. Potential energy is a property of the two particle system. A single particle would have none.

4. Jul 18, 2010

### pat666

apparently not then- can something have negative potential energy?????? I am now thinking always positive??

5. Jul 18, 2010

### Pythagorean

Sure. It's just a matter of your coordinate system, really. Consider gravitational potential energy.

6. Jul 18, 2010

### pat666

yeah i see that... so would the answer be "depends on the distance"??? (i know im just stabbing in the dark now but i need the answer and cant find anything useful in my textbook)

7. Jul 18, 2010

### Pythagorean

let's take graphene's advice in post #2:

everything on the bottom you can hold constant (i.e. you can ignore it for this question)

r = distance. Distance is always positive in this case. It's the distance between the two charges (there's no way to make it negative, because the question of distance between two objects isn't concerned about the object's position relating to any coordinate system, just each other.)

So everything on the bottom is constant and positive. Now think of the different cases for q1 and q2 and what U they'll result in. Then use what you discover to answer the question.

8. Jul 18, 2010

### pat666

if im only concerned about the top then it has to be negative for this case doesnt it??

9. Jul 18, 2010

### Pythagorean

Yup, like gravity, it will always be attractive. Opposite charges attract.