Two equal positive paint charge are placed at two corners of a squares , the other two corners having negative charges of equal magnitude. Find the field strength in the center of the square in figures.
p.s ( the figures are two square that in one of them positive charge are located at the opposite corners and they are located at the adjacent corners in the other one)

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My guess is that the charges cancel each other and so, the resultant force at the centre of the square is zero.