# Help with point charges and electric fields

1. Oct 2, 2008

### lyndan

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

So the question states: Four point charges, each of magnitude q, are located at the corners of a square with sides of length a. Two of the charges are +q and two are -q. The charges are arranged in one of the following two ways: (1) The charges alternate in sign (+q,-q,+q,-q) as you go around the square; (2) the top two corners of the square have positive charges (+q,+q) and the bottom two corners of the square have negative charges (-q,-q). (a) In which case will the electric field at the center of the square have the greatest magnitude? explain. (b) calculate the electric field at the center of the square for each of these two cases

3. The attempt at a solution

I already figured out that the electric field will have a greater magnitude in the second case, because the point charges in the first square will essentially cancel each other out? I can't figure out how to calculate the electric field at the center...I started using the E=klql/r^2 but im just having trouble getting the answer...the first answer is zero and the answer to the second on is -4 2^1/2 kq / a^2 but so far i've been unable to get that

any help would be great thank you so much!

2. Oct 3, 2008

### tiny-tim

Hi lyndan!

Show us how far you've got, and then we'll be able to see where the difficulty is.