In a medium, the force of attraction between two point charges, distance $d$ apart, is $F$. What distance apart should these point charges be kept in the same medium, so that the force between them becomes $16\, F$ ?

Two positive ions, each carrying a charge $q,$ are separated by a distance $d.$ If $F$ is the force of repulsion between the ions, the number of electrons missing from each ion will be  ($e$ being the charge on an electron)

Two point charges $3 \times 10^{-6} \,C$ and  $8 \times 10^{-6} \, C$ repel each other by a force of  $6 \times 10^{-3} \, N$. If each of them is given an additional charge $-6 \times 10^{-6} \, C$, the force between them will be

If two charges of $1$ coulomb each are placed $1 \,km$ apart, then the force between them will be ……….. $N$

Two small spheres each of mass $10 \,mg$ are suspended from a point by threads $0.5 \,m$ long. They are equally charged and repel each other to a distance of $0.20 \,m$. The charge on each of the sphere is $\frac{ a }{21} \times 10^{-8} \, C$. The value of $a$ will be …… .

$\left[\right.$ Given $\left.g=10 \,ms ^{-2}\right]$