Four charges equal to $- Q$ are placed at the four corners of a square and a charge $q$ is at its centre. If the system is in equilibrium the value of $q$ is\\
(1) $- \frac { Q } { 4 } ( 1 + 2 \sqrt { 2 } )$\\
(2) $\frac { Q } { 4 } ( 1 + 2 \sqrt { 2 } )$\\
(3) $- \frac { Q } { 2 } ( 1 + 2 \sqrt { 2 } )$\\
(4) $\frac { Q } { 2 } ( 1 + 2 \sqrt { 2 } )$