Two identical charged spheres separated by a distance $r$ repel each other with a force $F$. If $10 \%$ of electrons are transferred from one sphere to the other,then the force between them becomes

Explain the superposition principle for static electric forces and write its general equation.

Two copper balls,each weighing $10\,g$,are kept in air $10\,cm$ apart. If one electron from every $10^6$ atoms is transferred from one ball to the other,the Coulomb force between them is (atomic weight of copper is $63.5$).

There are two charges $+1 \ \mu C$ and $+5 \ \mu C$. The ratio of the forces acting on them will be

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 a side length of $10 \ cm$. The resultant force on the charge at $C$ is: (in $N$)

Two identical spheres each of radius $R$ are kept at a center-to-center spacing of $4R$ as shown in the figure. They are charged,and the electrostatic force of interaction between them is first calculated assuming them to be point-like charges at their centers $(F_c)$,and the force is also measured experimentally $(F_m)$. ($F_c$ and $F_m$ denote the magnitude of the force.)