Homework Help: Were is the electric field zero?

1. Mar 15, 2010

blackblanx

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

What is the net electric field intensity at the origin due to two charges Q1= 2$$\mu$$C at (x=3,y=4) and Q2 = -10$$\mu$$C at (x=6,y=-8). And where should a third charge of magnitude 5 $$\mu$$C be placed so the net electric field at the origin is zero?

2. Relevant equations
E=KQ/R$$^{2}$$ $$\widehat{r}$$

3. The attempt at a solution

I have found the first part of the question to be E= 3236$$\widehat{i}$$-10069$$\widehat{j}$$ but I am not sure how you would find out were to place the third charge. Please help

2. Mar 15, 2010

tiny-tim

Hi blackblanx!

(have a mu: µ )
The third charge will have to supply a field of minus E.

3. Mar 15, 2010

blackblanx

but what are the coordinates of where that charge should be placed?

4. Mar 15, 2010

blackblanx

oh wait never mind i think i got it know thanks it would just be the negative of the electric field vector divided by k(5$$\mu$$C)

5. Mar 15, 2010

blackblanx

Forget I realized that does not work any ideas of how to find the coordinates?

6. Mar 16, 2010

tiny-tim

It'll be on the line in the direction of E, at the distance appropriate to the given strength.

7. Mar 18, 2010

ok thanks