$A$ glass rod rubbed with silk is used to charge a gold leaf electroscope and the leaves are observed to diverge. The electroscope thus charged is exposed to $X$-rays for a short period. Then

Two particles with charges $+Q$ and $-Q$ are placed at a certain distance. The force between them is $F$. If a particle with charge $+Q$ is placed exactly in the middle of these two particles, what is the net force acting on it?

Three point charges are placed at the corners of an equilateral triangle of side $L$ as shown in the figure.

In free space,a particle $A$ of charge $1\,\mu C$ is held fixed at a point $P.$ Another particle $B$ of the same charge and mass $4\,\mu g$ is kept at a distance of $1\,mm$ from $P$. If $B$ is released,then its velocity at a distance of $9\,mm$ from $P$ is [ Take $\frac{1}{4\pi \varepsilon_0} = 9 \times 10^9\,N m^2 C^{-2}$ ]

Charges are placed on the vertices of a square as shown. Let $\vec E$ be the electric field and $V$ be the potential at the centre. If the charges on $A$ and $B$ are interchanged with those on $D$ and $C$ respectively,then

Two small spheres of each charge $q$,mass $m$ and material density $d$ are suspended from a fixed point with the help of inextensible light threads. When the spheres are in air,the angle between the threads is $90^{\circ}$. When the spheres are suspended in a liquid of density $\frac{2}{3} d$,the angle between the threads is $60^{\circ}$. The value of the dielectric constant of the liquid is