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)?

$A$ point charge $q_1$ exerts a force $F$ on a point charge $q_2$. If a third charge $q_3$ is brought near the charge $q_2$,then the force exerted by $q_1$ on $q_2$ will be:

Two charged spheres are suspended by strings of equal length. The strings make an angle of $30^{\circ}$ with each other. When suspended in a liquid of density $0.8 \ g/cm^3$,the angle remains the same. The dielectric constant of the liquid is ........ (The density of the material of the sphere is $1.6 \ g/cm^3$.)

Three charges $+5 q, Q$ and $-2 q$ are kept along a straight line in the same order such that $+5 q$ and $-2 q$ charges are at a distance of $\frac{2 r}{3}$ and $\frac{r}{3}$ from the charge $Q$ respectively. If the net force on the charge $-2 q$ is zero,then $Q$ is

When air is replaced by a dielectric medium of constant $k$,the maximum force of attraction between two charges separated by a distance $r$:

Two copper balls,each weighing $10\,g$,are kept in air $10\,cm$ apart. If one electron from every $10^6$ atoms is transferred from one ball to the other,the Coulomb force between them is (atomic weight of copper is $63.5$).