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Four point charges of equal magnitude are held at the corners of a square. The length of each side of the square is 2a and the sign of the charges are the following: Two positive charges on the top two corners of the square and two negative charges on the bottom two corners of the square.

The point P is at the center of the square.

a) Deduce that the magnitude of the electric field strength at Point P due to one of the point charges is equal to (kQ)/2a

^{2}:

I managed to do this one by proving that r

^{2}, that is, the distance between the center of the charge and the point P is equal to 2a

^{2}by doing Pythagoras

The next two parts of the question however, ask me to draw an arrow showing the resultant force and then determine, in terms of Q, a and k the magnitude of the electric field at point P. I simply can't do it however because it seems to me that there will be no resultant electric field at P! There are two positive charges and two negative charges, all at the same distance from point P and all have the same magnitude!

Can anyone help me with this (but don't give out the answer, I want to to try and work it out )

Thanks,

Peter