# Coulomb's Law

1. Jul 8, 2009

### define_normal

1. The problem statement, all variables and given/known data
Two small beads having positive charges 3q and q are fixed at the opposite ends of a horizontal, insulating rod, extending from the origin to the point x = d. A third small bead (in the middle) is free to slide on the rod. At what position is the third bead in equilibrium? Can it be stable in equilibrium?

2. Relevant equations
Coulomb's Law
F = ke (q1q2)/r^2

3. The attempt at a solution

2. Jul 8, 2009

### turin

You don't know any of the values, do you? Work symbollically. That is, just use q's and r's and such, instead of "actual numbers".

3. Jul 8, 2009

### define_normal

Hmm. So in order for me to do that, I don't need to know the charge of the third particle?

4. Jul 9, 2009

### turin

That's right. You would need to know the charge if you wanted to calculate a non-vanishing force. But, the problem stipulates equilibrium, so it doesn't matter. You are looking for the location where - something, regardless of the amount of charge that is placed there - will calculate to be zero. Even then, you have to calculate this location symbolically (i.e. in terms of d).