# Question about electric potential energy?

1. Sep 24, 2011

basically i don't understand the basic main idea of electric potential energy.
since by definition, energy is ability to do work( to move an object across a distance).
Negative source charge creates a uniform field around itself, and we place a positive test charge very near the source charge. why the electric potential energy so high? this confuses me because the test charge would just stick to source charge and do very little work.
Now for instance image same test charge is place at 10 times farther than its previous location. electric potential energy is very small as r is increase but the test charge did more work because it has moved a greater distance.
thanks

Last edited: Sep 24, 2011
2. Sep 24, 2011

### rcgldr

The confusing part is electric potential energy is a signed value (positive or negative) and it's not absolute but relative to some point of reference.

For a negative source, the field is considerd negative. For a positve charge in a negative field, the potential energy is a negative value. That would mean that potential energy at a point near the negative source charge would be more negative than a point farther away. If the positive charge was accelerating towards the negative plate (and you ignore issue related to the moving positive charge and the field it generates, and this was in a vacuum), then it's kinetic energy increases, and it's potential energy decreases by becoming more negative.

Note that gravitational potential energy is handled in the same way. For large distances, a common convention is to define a reference point at infinite distance from the gravitational source as zero potential energy, in which case gravitational potential energy becomes increasingly negative as you approach the center of the gravitational source.

3. Sep 24, 2011

this means that the conservatin of energy holds here. But what doesn't more negative electric potential signifies.

4. Sep 25, 2011

### Drakkith

Staff Emeritus
Hold on, did you know there is a difference between Electric Potential and Electric Potential Energy?

5. Sep 25, 2011

### Studiot

Good catch, Drakkith.

6. Sep 25, 2011