Five balls numbered $1$ to $5$ are suspended using separate threads. Pairs $(1, 2), (2, 4)$ and $(4, 1)$ show electrostatic attraction,while pairs $(2, 3)$ and $(4, 5)$ show repulsion. Ball $1$ must be:

Assertion: Consider two identical charges placed at a distance $2d$ apart along the $x-$axis. The equilibrium of a positive test charge placed at the point $O$ midway between them is stable for displacements along the $x-$axis.
Reason: The net force on the test charge at point $O$ is zero.

Four charges are arranged at the corners of a square $ABCD$,as shown. The force on a $+ve$ charge kept at the centre of the square is

Two spherical conductors $B$ and $C$ having equal radii and carrying equal charges repel each other with a force $F$ when kept apart at some distance. $A$ third spherical conductor $A$ having the same radius as that of $B$ but uncharged,is brought in contact with $B$,then brought in contact with $C$,and finally removed away from both. The new force of repulsion between $B$ and $C$ is-

Four electric charges $+Q, -Q, +Q$,and $-Q$ are placed at the four corners of a square. What are the electric field $(E)$ and electric potential $(V)$ at the center of the square?

$A$ point charge '$Q$' is placed at the centre of the line joining two equal charges '$+q$' and '$+q$'. The value of '$Q$' when the system is in equilibrium is