# Stable equilibrium

1. Jan 21, 2007

### physgirl

"stable equilibrium"

1. The problem statement, all variables and given/known data
Is there a difference b/w stable equilibrium and just equilibrium (of charged particles)? My hw problem exactly is:
2 small beads with charge +3q and +q each is fixed at opposite ends of a horizontal insulating rod. If a third charge is placed in between the two charges on the rod, at what position is the third bead in equilibrium? Can it be in stable equilibrium?

3. The attempt at a solution
I was able to calculate stuff for the first question and found the position... but I don't really know what the second question is asking.

2. Jan 21, 2007

### tim_lou

equilibrium means net force is zero. if you move an object slightly off the equilibrium, the object can either go further away from the equilibrium (unstable equilibrium) or get back to the equilibrium (stable equilibrium).

in your second question, you need to consider the sign of the third charge. if it's negative, what is the direction of the forces, and what if it's positive? (at equilibrium)

3. Jan 21, 2007

### physgirl

So if the third charge is negative, it'll feel a net attractive force from either side of the rod/charges and it'll end up moving away from the equilibrium somehwere in the middle... and if it's positive, it'll feel a net repulsive force (once its off equil) so it'll end back up in equilibrium?

4. Jan 21, 2007