Two charges are at a distance $d$ apart. If a copper plate (conducting medium) of thickness $d/2$ is placed between them,the effective force will be

$A$ charge $Q$ is placed at the midpoint of the line joining two identical point charges $q$. For what value of $q$ will the system be in equilibrium?

Two charges placed in air repel each other with a force of $10^{-4} \ N$. When oil is introduced between the charges, the force becomes $2.5 \times 10^{-5} \ N$. The dielectric constant of the oil is:

Three charges of equal magnitude are placed at three corners of a square of side $a$. If $F_{12}$ is the force between the charges at corners $1$ and $2$,and $F_{13}$ is the force between the charges at corners $1$ and $3$,what is the ratio $F_{12}/F_{13}$?

In a right-angled triangle $ABC$,$AB = 3 \ cm$,$BC = 4 \ cm$,and $\angle ABC = \frac{\pi}{2}$. Charges of $+15 \ e.s.u.$,$+12 \ e.s.u.$,and $-20 \ e.s.u.$ are placed at $A$,$B$,and $C$ respectively. What is the net force on the charge at $B$ in $dynes$?

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