The force of repulsion between two identical positive charges when kept with a separation $r$ in air is $F$. Half the gap between the two charges is filled by a dielectric slab of dielectric constant $=4$. Then, the new force of repulsion between those two charges becomes

The work done to move a charge on an equipotential surface is

Two point charges $A=+3 \mathrm{nC}$ and $B=+1 \mathrm{nC}$ are placed 5 cm apart in air. The work done to move charge $B$ towards $A$ by 1 cm is

$4 \times 10^{10}$ electrons are removed from a neutral metal sphere of diameter 20 cm placed in air. The magnitude of the electric field (in $\mathrm{NC}^{-1}$ ) at a distance of 20 cm from its centre is