Whose result is the whole of electrostatics?

Three identical charged balls,each of charge $2 \, C$,are suspended from a common point $P$ by silk threads of $2 \, m$ each (as shown in the figure). They form an equilateral triangle of side $1 \, m$. The ratio of the net electrostatic force on one charged ball to the force between any two charged balls is ...........

Four charges,each of magnitude $+Q$,are placed at the corners of a square of side $a$. What is the net force on one of the charges?

The force between two point charges $A$ and $B$ is $F$. If $75\%$ of the charge on $A$ is transferred to $B$,then the new force between $A$ and $B$ is:

Four charges are placed at the circumference of a dial clock as shown in the figure. If the clock has only an hour hand,then the resultant force on a charge $q_0$ placed at the center points in the direction which shows the time as (in $:30$)

If $\varepsilon_0$ and $\varepsilon$ represent the permittivity of free space and absolute permittivity of a medium,then the relative permittivity of the medium is: