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4 charges with the same in sign are located at a square as this below figure. You have to determine the coulomb force at one of the charges, the length of square side is a meter.



Here is an alternative solution for this case, you might have your own. Choose one charge, in this solution q₃ is choosen. Sketch the all electrostatic forces that are acting on it cause of its interactions with the other charges. See the figure below

Find the magnitudes of each forces F₃₂, F₃₄ and F₃₁

The next step, you have to sum F₃₂ and F₃₄, of course in a vector operation, the angle is 90°. Keep in mind that kq²/a² named F in here to simplify the calculations.



Finally insert the values of F and k to get the final result. The same method and the same results can be applied for the three other charges.