Charges $4Q$, $q$ and $Q$ and placed along $x$-axis at positions $x = 0,x = l/2$ and $x = l$, respectively. Find the value of $q$ so that force on charge $Q$ is zero

A negatively charged particle $p$ is placed, initially at rest, in $a$ constant, uniform gravitational field and $a$ constant, uniform electric field as shown in the diagram. What qualitatively, is the shape of the trajectory of the electron?

A charge $+q$ is situated at a distance $d$ away from both the sides of a grounded conducting $L$ shaped sheet as shown in the figure.The force acting on the charge $+q$ is 

$A$ and $B$ are two identical blocks made of a conducting material. These are placed on a horizontal frictionless table and connected by a light conducting spring of force constant $‘K’$. Unstretched length of the spring is $L_0$. Charge $Q/2$ is given  to each block. Consequently, the spring stretches to an equilibrium length $L$. Value of $Q$ is

A point charge $q_1=4 q_0$ is placed at origin. Another point charge $q_2=-q_0$ is placed at $x =12\,cm$. Charge of proton is $q_0$. The proton is placed on $x$-axis so that the electrostatic force on the proton in zero. In this situation, the position of the proton from the origin is $..........cm$.

Write limitation of Coulomb’s law.