Two protons $A$ and $B$ are placed in space between plates of a parallel plate capacitor charged upto $V$ volts (See fig.) Forces on protons are ${F_A}$ and ${F_B}$, then

Two particle of equal mass $m$ and charge $q$ are placed at a distance of $16\, cm$. They do not experience any force. The value of $\frac{q}{m}$ is

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 .......... $N$

Point charges $ + 4q,\, - q$ and $ + 4q$ are kept on the $x - $axis at points $x = 0,\,x = a$ and $x = 2a$ respectively, then

Write expression of Coulombian force acting between two charges kept in medium.

The electrostatic force on a small sphere of charge $0.4 \;\mu\, C$ due to another small sphere of charge $-0.8 \;\mu \,C$ in air is $0.2\; N .$

$(a)$ What is the distance between the two spheres?

$(b)$ What is the force on the second sphere due to the first?