Check that the ratio $ke ^{2} / G m _{ e } m _{ p }$ is dimensionless. Look up a Table of Physical Constants and determine the value of this ratio. What does the ratio signify?

Two charges placed in air repel each other by a force of ${10^{ – 4}}\,N$. When oil is introduced between the charges, the force becomes $2.5 \times {10^{ – 5}}\,N$. The dielectric constant of oil is

Two electrons are separated by a distance of $1\,\mathop A\limits^o $. What is the coulomb force between them

Two identical charged spheres suspended from a common point by two massless strings of lengths $l,$ are initially at a distance $d\;(d < < l)$ apart because of their mutual repulsion. The charges begin to leak from both the spheres at a constant rate. As a result, the spheres approach each other with a velocity $v.$ Then $v$ varies as a function of the distance $x$ between the spheres, as 

Electric charges of $1\,\mu C,\, – 1\,\mu C$ and $2\,\mu C$ are placed in air at the corners $A$, $B$ and $C$ respectively of an equilateral triangle $ABC$ having length of each side $10 \,cm$. The resultant force on the charge at $C$ is……$N$