If the Coulombian force acting between two protons separated by a distance $r$ is $F$, what would be the force acting between two alpha particles separated by a distance $2r$?

Three charges $q$,$Q$,and $4q$ are placed in a straight line of length $l$ at points distant $0$,$\frac{l}{2}$,and $l$ respectively from one end. In order to make the net force on $q$ zero,the charge $Q$ must be equal to

The magnitude of the electric force on a $2\, \mu C$ charge placed at the centre $O$ of two equilateral triangles,each of side $10\, cm$,as shown in the figure,is $P$. If the charges at $A, B, C, D, E,$ and $F$ are $2\, \mu C, 2\, \mu C, 2\, \mu C, -2\, \mu C, -2\, \mu C,$ and $-2\, \mu C$ respectively,then $P$ is: ..... $N$

Two point charges $+9e$ and $+e$ are placed at a distance of $16\, cm$ from each other. Where should a third charge $q$ be placed between them so that the system remains in equilibrium?

$A$ point charge $q$ is placed at a distance $d$ from one end of a thin non-conducting rod of length $L$ having a charge $Q$ uniformly distributed along its length,as shown in the figure. Find the magnitude of the electric force between them.

Three equal charges are placed on the three corners of a square as shown below. If the magnitude of force between $q_1$ and $q_2$ is $F_{12}$ and that between $q_1$ and $q_3$ is $F_{13}$,then the ratio of $F_{13}$ to $F_{12}$ is