$A$ charge $q_1$ exerts some force on a second charge $q_2$. If a third charge $q_3$ is brought near,the force of $q_1$ exerted on $q_2$:

Two identical conducting spheres $P$ and $S$ with charge $Q$ on each, repel each other with a force $16 \,N$. $A$ third identical uncharged conducting sphere $R$ is successively brought in contact with the two spheres. The new force of repulsion between $P$ and $S$ is: (in $\,N$)

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

Write the general equation for the Coulombian force on a charge ${q_1}$ due to a system of charges ${q_1}, {q_2}, \dots, {q_n}$.

An oil drop of $12$ excess electrons is held stationary under a constant electric field of $2.55 \times 10^{4} \; N \, C^{-1}$ (Millikan's oil drop experiment). The density of the oil is $1.26 \; g \, cm^{-3}$. Estimate the radius of the drop. $(g = 9.81 \; m \, s^{-2}; e = 1.60 \times 10^{-19} \; C)$

Assume that you have $10^{23}$ carbon atoms. All the nuclei are placed at the North Pole of the Earth and all the electrons are placed at the South Pole of the Earth. (Radius of Earth = $6400 \ km$). The force between the charges is approximately: