Positive point charges are placed at the vertices of a star shape as shown in the figure. The direction of the electrostatic force on a negative point charge at the centre $O$ of the star is

Two point charges $+3 \mu C$ and $+8 \mu C$ repel each other with a force of $40 \ N$. If a charge of $-5 \mu C$ is added to each of them,then the force between them will become: (in $N$)

$A$ charge $Q$ is divided into two charges $q$ and $Q-q$. The value of $q$ such that the force between them is maximum,is

The force between two point charges $A$ and $B$ is $F$. If $75\%$ of the charge of $A$ is transferred to $B$,then the new force between $A$ and $B$ is:

The electrostatic force on a small sphere of charge $0.4 \; \mu C$ due to another small sphere of charge $-0.8 \; \mu C$ in air is $0.2 \; N$.
$(a)$ What is the distance between the two spheres?
$(b)$ What is the force on the second sphere due to the first?

Two charged spheres separated at a distance $d$ exert a force $F$ on each other. If they are immersed in a liquid of dielectric constant $2$,then what is the force (if all conditions are same)?