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

Two spheres of electric charges +2 nC and -8 nC are placed at a distance $d$ apart. If they are allowed to touch each other, what is the new distance between them to get a repulsive force of same magnitude as before?

Three point charges of $+2 q,+2 q$ and $-4 q$ are placed at the corners $A, B$ and $C$ of an equilateral triangle $A B C$ of side $x$. The magnitude of the electric dipole moment of this system is