Three point charges $q,-2 q$ and $2 q$ are placed on $x$-axis at a distance $x=0, x=\frac{3}{4} R$ and $x=R$ respectively from origin as shown. If $q =2 \times 10^{-6}\,C$ and $R =2\,cm$, the magnitude of net force experienced by the charge $-2 q$ is .......... $N$

Two pith balls carrying equal charges are suspended from a common point by strings of equal length, the equilibrium separation between them is $r.$ Now the strings are rigidly clamped at half the height. The equilibrium separation between the balls now become

Four point charges, each of $+ q$, are rigidly fixed at the four corners of a square planar soap film of side ' $a$ ' The surface tension of the soap film is $\gamma$. The system of charges and planar film are in equilibrium, and $a=k\left[\frac{q^2}{\gamma}\right]^{1 / N}$, where ' $k$ ' is a constant. Then $N$ is

Force of attraction between two point charges $Q$ and $-Q$ separated by $d\,$ metre is ${F_e}$. When these charges are placed on two identical spheres of radius $R = 0.3\,d$ whose centres are $d\,$ metre apart, the force of attraction between them is

Equal charges $q$ are placed at the four corners $A,\,B,\,C,\,D$ of a square of length $a$. The magnitude of the force on the charge at $B$ will be

A cube of side $b$ has a charge $q$ at each of its vertices. The electric field due to this charge distribution at the centre of this cube will be