Write the value of the Coulombian constant $k$ in $SI$ units.

The force between two identical spheres charged with the same charge is $F$. If $75\%$ of the charge from one sphere is transferred to the other sphere,then the new force will be:

Two identical conducting spheres $P$ and $S$ with charge $Q$ on each, repel each other with a force $16 \,N$. $A$ third identical uncharged conducting sphere $R$ is successively brought in contact with the two spheres. The new force of repulsion between $P$ and $S$ is: (in $\,N$)

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 $q_2 = 3 \times 10^{-6} \ C$ and $q_1 = 5 \times 10^{-6} \ C$ are located at $B(3, 5, 1) \ m$ and $A(1, 3, 2) \ m$ respectively. Find the magnitude of the force on $q_1$ due to $q_2$.

Four charges equal to $-Q$ are placed at the four corners of a square of side length $a$,and a charge $q$ is at its centre. If the system is in equilibrium,the value of $q$ is: