Three charges $+4q, Q$ and $q$ are placed in a straight line of length $l$ at points at distance $0, \raisebox{1ex}{$l$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ and $l$, respectively, from one end of line. What should be the value of $Q$ in order to make the net force on $q$ to be zero?

Two-point charges of the same magnitude and opposite sign are fixed at points $A$ and $B$. A third small point charge is to be balanced at point $P$ by the electrostatic force due to these two charges. The point $P$

Two point charges of same magnitude and opposite sign are fixed at points $A$ and $B.$ A third small point charge is to be balanced at point $P$ by the electrostatic force due to these two charges. The point $P:$

Two pith balls each with mass $m$ are suspended from insulating threads. When the pith balls are given equal positive charge $Q$, they hang in equilibrium as shown. We now increase the charge on the left pith ball from $Q$ to $2Q$ while leaving its mass essentially unchanged. Which of the following diagrams best represents the new equilibrium configuration?

As shown in the figure, two particles, each of mass $\text{'}m\text{'}$ tied at the ends of a light string of length $2a$ are kept on a frictionless horizontal surface. When the mid point $\left(P\right)$ of the string is pulled vertically upwards with a small but constant force $F,$ the particles move towards each other on the surface. Magnitude of acceleration of each particle, when the separation between them becomes $2x$ is