Two point charges $A$ and $B$ with magnitudes $+4q$ and $-4q$ are placed along a line separated by a distance $r$. The force acting between them is $F$. If $25\%$ of the charge from point $A$ is transferred to point $B$,the force between the charges now becomes:

Two small spheres each of mass $10 \, mg$ are suspended from a point by threads $0.5 \, m$ long. They are equally charged and repel each other to a distance of $0.20 \, m$. The charge on each of the sphere is $\frac{a}{21} \times 10^{-8} \, C$. The value of $a$ will be ...... . [Given $g = 10 \, ms^{-2}$]

In the given figure,if $O$ is the midpoint of $AB$,calculate the net force on charge $Q$.

Two identical spheres have charges $+q$ and $-q$ and are placed at a certain distance. The force between them is $F$. If a third identical sphere with charge $+q$ is placed exactly between the two spheres, what is the magnitude and direction of the force experienced by it?

Two small spheres of masses $M_1$ and $M_2$ are suspended by weightless insulating threads of lengths $L_1$ and $L_2$ respectively. The charges on the spheres are $Q_1$ and $Q_2$ respectively. The spheres are suspended such that they lie in a horizontal line and the threads make angles $\theta_1$ and $\theta_2$ with the vertical as shown in the figure. Which of the following conditions is necessary for $\theta_1 = \theta_2$?

What are the units of $K = 1/(4\pi \varepsilon_0)$?