# 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 :)