What is the force (in $N$) between two small charged spheres having charges of $2 \times 10^{-7} \;C$ and $3 \times 10^{-7} \;C$ placed $30\; cm$ apart in air?

Two small spherical balls each carrying a charge $Q = 10\,\mu C$ ($10$ micro-coulomb) are suspended by two insulating threads of equal lengths $1\,m$ each, from a point fixed in the ceiling. It is found that in equilibrium threads are separated by an angle ${60^o}$ between them, as shown in the figure. What is the tension in the threads......$N$ (Given: $\frac{1}{{(4\pi {\varepsilon _0})}} = 9 \times {10^9}\,Nm/{C^2}$)

Two identical charged spheres are suspended by strings of equal lengths. The strings make an angle of $30^{\circ}$ with each other. When suspended in a liquid of density $0.8 \;g\, cm ^{-3}$, the angle remains the same. If density of the material of the sphere is $1.6\; g \,cm ^{-3}$, the dielectric constant of the liquid is

If $g_E$ and $g_M$ are the accelerations due to gravity on the surfaces of the earth and the moon respectively and if Millikan's oil drop experiment could be performed on the two surfaces, one will find the ratio (electronic charge on the moon/electronic charge on the earth) to be

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

Charges $4Q$, $q$ and $Q$ and placed along $x$-axis at positions $x = 0,x = l/2$ and $x = l$, respectively. Find the value of $q$ so that force on charge $Q$ is zero