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I'm having a lot of trouble trying to solve this problem:

Three identical point charges of magnitude $+1.0$ $mC$ are located at the corners of a square that is $1.0$ $m$ on a side. A point charge of magnitude $-1.5$ $mC$ is placed at the remaining corner. What is the electric field strength at a point halfway between 2 adjacent positive charges?

My thinking was that you can just find the strength of one charge to the center and just double it because they're identical charges. Why is this method wrong and how do I solve this problem?

The correct answer is $8.63\times 10^6$ $N/C$, but I have no idea how to get this.

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closed as too localized by dmckee Mar 5 '13 at 15:56

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Welcome to Physics.SE. This is not a homework help site as such, and while we are happy to answer questions about very basic physics we do not work specific problems on your behalf. –  dmckee Mar 5 '13 at 15:56

1 Answer 1

up vote 0 down vote accepted

The electric field is a vector. You just added the magnitudes of the two vector, without taking the direction into account.

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Then wouldn't the answer be 0 because the two charges are pushing each other apart in opposite directions? –  Andrew Mar 5 '13 at 6:46
    
For two charges, yes. But you have four charges. –  Bernhard Mar 5 '13 at 6:51
    
oh so the top two charges cancel each other out, but I have to add the bottom two charges using vectors. –  Andrew Mar 5 '13 at 6:53

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