A square with charges and an electric field

1. Feb 24, 2009

Melawrghk

1. The problem statement, all variables and given/known data
This is a problem from my physics midterm. I have no idea whether I got ANYTHING at all right in it, although I certainly hope so.

2. Relevant equations
E = kq/r^2

3. The attempt at a solution
Okey dokey. I was running out of time (majorly) so I figured I'd only look at charge 1's electric field at the origin. The main theory (or something like that) behind it was that charges 2 and 4 lay in a line, so their electric fields would coincide, so charge 4 could be used to cancel out 2's effects.
I then decided that charge 3 basically has to counterbalance charge 1's electric field. So I found 1's electric field at the origin:
E1 = (k*10e-9)/7.25 = -12.4V/C
I then split it into components, x & y, because that is what charge 3 would have to counteract.
E1 = -4.61i - 11.51j

Long story short, I arrived at the conclusion that q(3) should be -10e-9 C. But I'm not sure if that's at all right. Thoughts? Feedback?

2. Feb 24, 2009

Delphi51

So you can choose Q4 as well as Q3?
I think your choice for Q3 is right on. The combined E's due to Q1, Q2 and Q3 will be in the direction of Q4 so they can be cancelled by choosing just the right Q4.

3. Feb 24, 2009

Melawrghk

Yep, both q3 & q4 were unspecified. The question only asked about q3 though.
Thanks for the reply! :) That makes me feel a little better about my overall predicament.