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

$A$ force $F$ acts between sodium and chlorine ions of salt (sodium chloride) when placed $1 \ cm$ apart in air. The permittivity of air is $\varepsilon_0$ and the dielectric constant of water is $K$. When a piece of salt is placed in water,the electrical force acting between sodium and chlorine ions $1 \ cm$ apart is:

For any fixed distance,the electromagnetic force between two protons is $10^n$ times the gravitational force between them. Then $n=$

Two identical conducting spheres,having charges of opposite sign,attract each other with a force of $0.108 \ N$ when separated by $0.5 \ m$. The spheres are connected by a conducting wire,which is then removed,and thereafter,they repel each other with a force of $0.036 \ N$. The initial charges on the spheres are

Three equal charges $+q$ are placed at the three vertices of an equilateral triangle centered at the origin. They are held in equilibrium by a restoring force of magnitude $F(r) = k r$ directed towards the origin,where $k$ is a constant. What is the distance of the three charges from the origin?

Why is the Coulombian force called a two-body force?