Two identical charges repel each other with a force equal to $10 \text{ mg}$ wt when they are $0.6 \text{ m}$ apart in air $(g = 10 \text{ ms}^{-2})$. The value of each charge is:

Two charges are at a distance $d$ apart. If a copper plate (conducting medium) of thickness $d/2$ is placed between them,the effective force will be

Two identical conducting spheres with negligible volume have $2.1 \, nC$ and $-0.1 \, nC$ charges,respectively. They are brought into contact and then separated by a distance of $0.5 \, m$. The electrostatic force acting between the spheres is $.......... \times 10^{-9} \, N$. [Given: $\frac{1}{4 \pi \varepsilon_{0}} = 9 \times 10^{9} \, N \cdot m^{2}/C^{2}$]

Equal charges $q$ are placed at the four corners $A, B, C, D$ of a square of side length $a$. The magnitude of the net electrostatic force on the charge at $B$ is:

Three charges $4q$,$Q$,and $q$ are placed in a straight line at positions $0$,$l/2$,and $l$ respectively. The resultant force on $q$ will be zero if $Q = $

$A$ charge $Q$ is divided into two parts $Q_1$ and $Q_2$. These charges are placed at a distance $R$. What will be the values of $Q_1$ and $Q_2$ for the maximum repulsive force between them?