Answer
All charges are same therefore force is equal but direction is different.
So diagram is as above
Force at above charge ( right above side corner)
F = 2 k q 2 a 2 + k q 2 2 a 2 F=\frac{\sqrt{2}kq^2}{a^2}+\frac{kq^2}{\sqrt{2}a^2} F = a 2 2 k q 2 + 2 a 2 k q 2
With direction and side of square 0.1m and q=6.15mC
So force is
F R = 728.12 × 1 0 5 ( i + j ) 2 F_R=728.12\times10^5\frac{(i+j)}{\sqrt{2}} F R = 728.12 × 1 0 5 2 ( i + j )
Similiarly force for above left corner
F R ′ = 728.12 × 1 0 5 ( − i + j ) 2 F'_R=728.12\times10^5\frac{(-i+j)}{\sqrt{2}} F R ′ = 728.12 × 1 0 5 2 ( − i + j )
And now bottom left charge
F R ′ ′ = 728.12 × 1 0 5 ( − i − j ) 2 F''_R=728.12\times10^5\frac{(-i-j)}{\sqrt{2}} F R ′′ = 728.12 × 1 0 5 2 ( − i − j )
Now right bottom charge
F R = 728.12 × 1 0 5 ( i − j ) 2 F_R=728.12\times10^5\frac{(i-j)}{\sqrt{2}} F R = 728.12 × 1 0 5 2 ( i − j )
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