Four charges are arranged at the corners of a square $ABCD$, as shown in the adjoining figure. The force on the charge kept at the centre $O$ is

A charge $Q$ is placed at each of the opposite corners of a square. A charge $q$ is placed at each of the other two corners. If the net electrical force on $Q$ is zero, then  $\frac{Q}{q}=$ ______

Two charges ${q_1}$ and ${q_2}$ are placed in vacuum at a distance $d$ and the force acting between them is $F$. If a medium of dielectric constant $4$ is introduced around them, the force now will be

The electrostatic force of interaction between an uniformly charged rod having total charge $Q$ and length $L$ and a point charge $q$ as shown in figure is

Two protons $A$ and $B$ are placed in space between plates of a parallel plate capacitor charged upto $V$ volts (See fig.) Forces on protons are ${F_A}$ and ${F_B}$, then

An isolated solid metallic sphere is given $ + Q$ charge. The charge will be distributed on the sphere