Electromagnetic question

1. Oct 7, 2015

max1995

1. The problem statement, all variables and given/known data
. A charge 2Q is located at the origin while a second charge -Q is located at x = 1m. Find the position x where a third charge should be placed so that the net force on this charge is zero?

2. Relevant equations

3. The attempt at a solution
I know that the charge must be placed further away from the origin than -Q and net force of 0=k*2Q*Q3/(r1)2 + k*-Q*Q3/(r2)2
where
k=8.99x109
r1=distance from 2Q to Q3
r2= distance from -Q to Q3

and also r1=r2+1m

Thanks for any help

2. Oct 7, 2015

max1995

quick question, I have tried again, does the distance between the origin and the third charge equal 3.41m

3. Oct 7, 2015

RUber

Are you to assume that the third charge is also a -Q?
In that case, letting x denote the position of the third charge, your guiding equation simplifies to:
$0 = \frac{-2Q^2}{(x^2)} + \frac{Q^2}{(1-x)^2}$

4. Oct 7, 2015

max1995

I don't really know.

by solving that equation does that mean x=~2.41m?

5. Oct 7, 2015

ehild

That is correct for the distance of the third charge from the origin. But show your work in detail.

6. Oct 7, 2015

RUber

That (2.41) doesn't seem to solve the equation. I get about 1 instead of zero when I plug that in. 3.41 is good.

7. Oct 7, 2015

ehild

No need to assume anything about the third charge.

8. Oct 7, 2015

RUber

Got it. It can factor out just like k.
Thanks.

9. Oct 7, 2015

max1995

Thank you all very much :)