The ratio of the forces between two small spheres with constant charge $(a)$ in air and $(b)$ in a medium of dielectric constant $K$ is:

$A$ charge $q$ is placed at the centre of the line joining two equal charges $Q$. The system of the three charges will be in equilibrium if $q$ is equal to:

The ratio of the relative strengths of the gravitational force and the electromagnetic force between two charged particles is

$A$ force $F$ acts between sodium and chlorine ions of salt (sodium chloride) when placed $1 \ cm$ apart in air. The permittivity of air is $\varepsilon_0$ and the dielectric constant of water is $K$. When a piece of salt is placed in water,the electrical force acting between sodium and chlorine ions $1 \ cm$ apart is:

Two charges of equal magnitudes and at a distance $r$ exert a force $F$ on each other. If the charges are halved and the distance between them is doubled,then the new force acting on each charge is

Two point charges of equal magnitude but opposite sign are placed at a certain distance. The force between them is $F$. If $75\%$ of the charge from one is transferred to the other,what will be the new force between them?