# Three point charges are arranged on a line.

Mdhiggenz

## Homework Statement

Three point charges are arranged on a line. Charge q3 = +5.00nC and is at the origin. Charge q2 = -4.00nC and is at x = 4.50cm . Charge q1 is at x = 2.00cm.

What is q1 (magnitude and sign) if the net force on q3 is zero?

## The Attempt at a Solution

So first thing I did was draw a diagram where q3 was located at (0,0) q1 at (2.00cm,0) and q2 was located at (4.50cm,0)

Then after looking at the problem for a while I know I must use coulomb's law however since the net for q3 is 0 that threw me off a bit.

So what I decided to do was make two separate coulomb's law equations and equal them to each other.

(q1)^2/r^2=(q2)^2/r^2

Ultimately solving for q1 gives.

q1= √[(q2)^2*(r1^2)/r2^2]

Not sure if I'm on the right track just brainstorming, much help is appreciated

Higgenz

voko
Why do you use the same r for (q1, q3) and (q2, q3)? What it should it really be in these two cases?

Mdhiggenz
Wouldn't the distance for q3 be 0 since it is at (0,0)?

Only thing else I can think of is maybe getting the distance of Q1 and Q2 and possibly use Pythagoras theorem to get the total distance?

Gold Member
You need to find the forces on charge $Q_3$ due to charges $Q_1$ and $Q_2$. What does $r$ in Coulombs Law actually represent?

voko
You need the distance between q1 and q3 and between q2 and q3. They are all on the same line. You are given their distances from the beginning of the line. Moreover, q3 is at the beginning of the line. If you still can't figure it out, take a piece of paper and pencil and sketch it.

Gold Member
I would suggest drawing a diagram, showing the directions of the forces on $Q_3$ due to $Q_1$ and $Q_2.$ Because you know the net force on $Q_3$ is 0, it should be quite obvious what direction the force on $Q_3$ due to $Q_1$ is and from this you can infer the polarity of $Q_1.$