Point charges in a square

[kimlett]

http://eastvan.bc.ca/~kim/phys.jpg

Four identical charges Q+ occupy the corners of a square with sides of length a. A fifth charge, q, can be placed at any desired location. Find the location of the fifth charge, and the value of q, such that the net electric force acting on the original four charges, Q+, is zero.

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So, I don't know if I'm thinking this out right, but should the 5th charge be in the middle like that? And if so, should it be positive? Or would that just all fly apart, and it should be negative?

I know I should probably take the components and make them all equal to each other, but I wanted to make sure I started this question off right.

Thanks :)

 

[vsage]

Yes dude to symmetry it would be exactly in the center (and its a negative charge I assume you want to get the point charges in static equilibrium). Solve out Newton's second law to find the charge q in the center though.

 

[kimlett]

thanks vsage
I'll try it out :)