# Homework Help: Electric Field and charge location

1. Feb 6, 2015

### jeremy de leon

1. The problem statement, all variables and given/known data
A 6.0mC charge experiences a force of 2.0x10^-3 N in the +x-direction at a certain point in space.
A. What was the electric field at that location before the charge was placed there
B. What would be the force if the 6mC charge were replaced by a -2.0mC charge?

2. Relevant equations
E=F/Q
F=(kQ1Q2)/r^2

3. The attempt at a solution
I'm actually quite clueless as I think that the given problem lacks information.

I've tried having to use E=F/Q to try and solve for

2. Feb 6, 2015

3. Feb 6, 2015

### jeremy de leon

It got cut off, sorry. :)
To solve for the R but it just wouldn't work. So I've got no more ideas for this. I couldn't think of any other solution.

4. Feb 6, 2015

### Staff: Mentor

You don't need R to find the field. You were on the right track with E = F/Q.

5. Feb 6, 2015

### jeremy de leon

Yeah I got the electric field but how do i get the electric field BEFORE the charge was placed there?

Sorry, this is really confusing me. :/

6. Feb 6, 2015

### Staff: Mentor

The value that you got will be the field before the new charge is placed. The new charge reacts to the field that is due to other charges, not to its own field. Of course, if another charge were to be brought into the picture, then the new charge would feel the effects of the previous field plus the that of the field due to the original Q, and Q would also feel the new charge.