Two unit negative charges are placed on a straight line. $A$ positive charge $q$ is placed exactly at the midpoint between these unit charges. If the system of these three charges is in equilibrium,the value of $q$ (in $C$) is

Write the expression for the Coulombian force acting between two point charges kept in a medium.

Two charges,each of $1\;C$,are at a distance of $1\;km$ apart. The force between them is:

Three charges of each magnitude $100 \mu C$ are placed at the corners $A, B$ and $C$ of an equilateral triangle of side $4 \text{ m}$. If the charges at points $A$ and $C$ are positive and the charge at point $B$ is negative,then the magnitude of the total force acting on the charge at $C$ and the angle made by it with $AC$ are:

Two spherical conductors $B$ and $C$ having equal radii and carrying equal charges $Q$ repel each other with a force $F$ when kept apart at a distance $r$. $A$ third spherical conductor having the same radius as that of $B$ but uncharged is brought in contact with $B$,then brought in contact with $C$,and finally removed away from both. The new force of repulsion between $B$ and $C$ is

An infinite number of charges,each of charge $1 \,\mu C$,are placed on the $x$-axis at coordinates $x = 1, 2, 4, 8, ... \infty$. If a charge of $1 \, C$ is kept at the origin,what is the net force acting on the $1 \, C$ charge in Newtons?