# Homework Help: The Electric Potential Difference Created by Point Charges

1. May 13, 2007

### eclecticist

1. The problem statement, all variables and given/known data
One particle has a mass of 3.00 10-3 kg and a charge of +7.50 µC. A second particle has a mass of 6.00 10-3 kg and the same charge. The two particles are initially held in place and then released. The particles fly apart, and when the separation between them is 0.100 m, the speed of the 3.00 10-3 kg particle is 130 m/s. Find the initial separation between the particles.

2. Relevant equations
V = kQ/d
Vq=W
W=KE=1/2mv^2

3. The attempt at a solution

I don't really know where to start. I haven't seen any problems asking for initial separation before so I would appreciate it if someone could point me in the right direction.

2. May 13, 2007

### arunbg

The basic principle is the conservation of energy. Start by finding the potential difference between the two states, one at separation d(say) and the other at separation 0.1m. This difference manifests as the kinetic energy of the particles. Can you proceed?

3. May 13, 2007

### eclecticist

I understand that, but in order to calculate the kinetic energy of the particles, I need to know the speed of the other particle as well, because energy is conserved, which isn't given, so I'm not sure what to do after that. Also, if I'm not mistaken, mass doesn't affect the potential difference of a charge, right?

4. May 13, 2007

### Edward G

What about conservation of momentum, there is no external force so linear momentum is conserved.