# Force question using Couloumb's law

1. Sep 1, 2010

### J89

1. The problem statement, all variables and given/known data

A charged particle A exerts a force of 2.67 μN to the right on charged particle B when the particles are 13.9 mm apart. Particle B moves straight away from A to make the distance between them 18.6 mm. What vector force does particle B then exert on A?

2. Relevant equations

F = Ke*|q1||q2|/r^2, Ke=8.99*10^9

3. The attempt at a solution
I assumed that q1 = 2.67 micronewtons, and had an idea that we need to solve for for q2..I think, so I just took the equation and set Ke=|q1||q2|/r^2 and solved for q2. I got 1.34 for an answer..don't know if that is even right..can someone clarify?

2. Sep 1, 2010

### diazona

What does each of the variables in Coulomb's law mean?
F = ?
Ke = a constant
q1 = ?
q2 = ?
r = ?
You need to figure that out before you can hope to use the equation properly.

3. Sep 1, 2010

### J89

F = force
Ke=Couloumb's constant = 8.99*10^9
r=distance in meters
q1 and q2 stands for the charge..

4. Sep 1, 2010

### diazona

Right. Since q1 is a charge, the value you plug in for it has to have units of charge.

Now, what kind of quantity does the problem ask you to find?

5. Sep 2, 2010

### J89

It's asking to find vector force that particle A exerts on B..so just to get a head start, I assume you would just plug it into the formula...

F = Ke |q1|/r^2

when it is rearranged, I assume you would multiply the charge by the original length and divide by the new length when it was expanded..

=2.67*(.0139)^2/(.0186)^2...right?

Last edited: Sep 2, 2010
6. Sep 2, 2010

### Andrew Mason

You have the right method but you are not explaining it very well. You seem to be confusing charge with force. What units will your answer be in?

You don't need to know the value for the charges at all. All you need to do is write out the expression for the force in both scenarios. Then solve for unknown second force.

AM