Two point charges of $1 \, \mu\text{C}$ and $5 \, \mu\text{C}$ are placed at a distance of $4 \, \text{cm}$ apart. The ratio of the forces exerted by them on each other will be:

Two identical charged spheres separated by a distance $r$ repel each other with a force $F$. If $10 \%$ of electrons are transferred from one sphere to the other,then the force between them becomes

Four charges of magnitude $-Q$ are placed at the four corners of a square,and a charge $q$ is placed at the center. If the system is in equilibrium,the value of $q$ is ......

Two point charges $+q_{1}$ and $+q_{2}$ are placed a finite distance '$d$' apart. It is desired to put a third charge $q_{3}$ in between these two charges so that $q_{3}$ is in equilibrium. This is

Two particles of equal mass $m$ and equal charge $q$ are separated by a distance of $16 \text{ cm}$. They do not experience any net force. The value of $\frac{q}{m}$ is . . . . . . (where $G$ is the universal gravitational constant and $\epsilon_0$ is the permittivity of free space).

Charges $3Q$,$q$,and $Q$ are placed along the $x$-axis at positions $x=0$,$x=l/3$,and $x=l$ respectively. When the net force on charge $Q$ is zero,the value of $q$ is: