# Confusing Introductory Electricity Question

1. Jan 13, 2008

### jedjj

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

In the figure, particle 1 of charge +1.0 µC and particle 2 of charge -2.5 µC, are held at separation L = 7.0 cm on an x axis. If particle 3 of unknown charge q3 is to be located such that the net electrostatic force on it from particles 1 and 2 is zero, what must be the coordinates of particle 3?
x = ? cm
y = ? cm

the figure is not necessary for the question if you know that particle 2 is to the right of particle 1.

2. Relevant equations

$$F=\frac{kq_1q_2}{d^2}$$
$$k=8.99\times10^{9}$$

3. The attempt at a solution

Because both particles are on the x axis, to net zero force it must be on the x axis, therefore the y value is 0, and that is correct. I have attempted to use my force equation and found the value as 4.592N(in the negative direction), but that doesn't tell me anything. I also realize that the negative charge is stronger so particle 3 is going to be to the left of the two particles.

Without knowing the charge on particle 3 how can I find this distance?

2. Jan 13, 2008

### Feldoh

Principle of super position works

3. Jan 13, 2008

### jedjj

I haven't done a super position problem yet as this course just started a week ago. Can you explain how I would use it here?
EDIT: we covered the idea quickly, but I don't understand how to use it.

EDIT: nevermind. I wasn't looking hard enough through the notes. Sorry!

Last edited: Jan 13, 2008
4. Jan 14, 2008

### Feldoh

Basically if particle 3 is equally distant from particle 1 and particle 2, which are in opposite directions of particle the force of the two particles will "cancel out" on particle 3.

It's the same when you did gravity, where the force of gravity on a system of particles follows that same rule.