4 charges each equal to Q ARE PLACED AT THE 4 CORNERS OF the square and a charge q is placed at the centre of the square. if the system is in eqilibrium then the value of q is

F = kq² / 2l² * ( 1 + 2 root two)

the centre charge q will always be in equilb.. as always net force is zero.

just for euilibr... of 4 Q charges , equate net force on a Q by other 3Q and q.

thus you get the condition for finding q.

automatically other will be also in euilbr..

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answer is-    -Q/4 *[ 1+2 root2] i want detailed answer