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## Homework Statement

As shown in the figure, a square has sides of d = 9.0 cm with a charge of Q = +18.0 µC at one corner (a) and with charges of q = -6.0 µC at the remaining three corners (b, c, and d).

Image: http://www.webassign.net/grrphys2/17-p-018-alt.gif" [Broken]

Find the electric field at the center of the square.

magnitude _____ N/C

## Homework Equations

E=kq/r^2

## The Attempt at a Solution

Since the charges at corners b and d are the same, they cancel each other out, they won't contribute to the field in the middle. So the only corners we have to worry about are corners a and c. Ea = 18 uC/(4 pi eps d^2)

From the charge in c, the field strength is Ec = -6 uC/(4 pi eps d^2)

So the total field (both are directed toward c) = (12 uC) / (4 pi eps0 d^2).

The only thing I don't know how to calculate is r. I tried 4.5 sqrt. 2. I tried 0.09. I tried 3 sqrt. 2. None of them worked. Can you please help me?

I only have one more answer to try. I just thought of something, what if I try 4 sqrt (0.09 m)? Will that work?

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