Two point charges $+3 \mu C$ and $+8 \mu C$ repel each other with a force of $40 \ N$. If a charge of $-5 \mu C$ is added to each of them,then the force between them will become: (in $N$)

If the distance between two equal point charges is doubled,what would happen to the force between them?

In one model of the electron,the electron of mass $m_e$ is thought to be a uniformly charged shell of radius $R$ and total charge $e$,whose electrostatic energy $E$ is equivalent to its mass $m_e$ via Einstein's mass-energy relation $E = m_e c^2$. In this model,$R$ is approximately ($m_e = 9.1 \times 10^{-31} \, kg$,$c = 3 \times 10^8 \, ms^{-1}$,$1 / 4 \pi \varepsilon_0 = 9 \times 10^9 \, Nm^2C^{-2}$,magnitude of the electron charge $e = 1.6 \times 10^{-19} \, C$).

Two identical charged conducting spheres $A$ and $B$ have their centres separated by a certain distance. Charge on each sphere is $q$ and the force of repulsion between them is $F$. $A$ third identical uncharged conducting sphere $C$ is brought in contact with sphere $A$ first and then with $B$ and finally removed from both. The new force of repulsion between spheres $A$ and $B$ (radii of $A$ and $B$ are negligible compared to the distance of separation so that for calculating force between them they can be considered as point charges) is best given as:

Two small spheres each having the charge $+Q$ are suspended by insulating threads of length $L$ from a hook. This arrangement is taken into space where there is no gravitational effect. Then the angle between the two suspensions and the tension in each will be:

Two small balls,each having mass $m$ and charge $q$,are suspended from a common point by insulating strings of length $L$ as shown in the figure. Assuming $\theta$ is very small such that $\tan \theta \approx \sin \theta$,find the value of $x$ at equilibrium.