# Negative electrostatic potential energy

1. May 18, 2006

### endeavor

I have an example in my book showing 3 point charges each at the vertices of an equilateral triangle. q1 = +2 microC, q2 = -2 microC, and q3 = +1 microCoulombs. the distance between each is 0.30m. The example calculates the electrostatic potential energy of the charge configuration, obtaining -0.12 J. The example says that because the electrostatic potential is negative, if the charges were released from rest, they would fall together, not fly apart. "They would fall together, because this would lower their potential energy still further (a larger negative value)." I'm having trouble understanding this principle of negative potential energies. Can someone explain this to me? What does it mean exactly to have negative potential energy? I know there can be a negative change in potential energy, and that is easy to understand using the similar gravitational potential energy. And why must the charges fall together, and why must the potential energy be lowered by having a larger negative value?

2. May 18, 2006

### vsage

It might be easier to think of it in terms of the work done in order to bring those charges to the points that they are at from a point of rest. The problem with using gravitational potential as a source of experience is that there is really no terrestrial object that has a negative potential with respect to the earth, i.e. there is no terrestrial massive object that requires positive work to be done to move it from a point infinitely far away to near the earth.

Last edited by a moderator: May 18, 2006
3. May 18, 2006

### endeavor

Okay, I think I have an understanding about the negative electric potential energy. But why would the charges fall together if they were released from rest?

4. May 19, 2006

### Dr.Brain

Negative potential of any system just signifies that 'energy is stored in the system' , and if we want to extract the energy in this particular system of yours , we need to lower the energy stored in the configuration (by c.o.e) , this is done by making the Potential energy less negative , that is taking the charges as far apart as possible ...

BJ