# Charges: Electirc forces

## Homework Statement

Three point charges lie in a straight line along the y-axis. A charge of q1 is -9 microcouloumbs at y = 6 m and a charge of q2 is -8 microcoulombs at y = -4m. The net electric force on the third point charge is zero. Where is this charge located.

## Homework Equations

Fe = k (q1*q2/r^2)

## The Attempt at a Solution

Fe = k (q1*q2/r^2)
= (9*10^9)((9*10^-6)(8*10^-6)/(10m)^2))
= 6.5*10^-3 N

Then i attempted to plug in this value for Fe to receive the r, and the location, but
i don't know which two charges i had to use.

Thanks,

SLM

## Answers and Replies

ideasrule
Homework Helper
What's the equation for the electric field? Do you know how to add electric fields induced by different charges?

I know how to add different electrical forces, but the problem i have is finding the location of it, instead of the force.

collinsmark
Homework Helper
Gold Member
I know how to add different electrical forces, but the problem i have is finding the location of it, instead of the force.

So you've added the two equations for the electric fields of the first two charges together, giving you an equation for the overall electric field E as a function of y (considering both charges). What location along y produces a zero electric field? (Hint: you need to set something to zero and solve for y.)

If a charge is in a zero electric field, what can you say about the net electric force acting on that charge? 