Nine point charges are placed on a cube as shown in the figure. The charge $q$ is placed at the body centre whereas all other charges are at the vertices. The electrostatic potential energy of the system will be

If an $\alpha$-particle and a proton are accelerated from rest by a potential difference of 1 megavolt then the ratio of their kinetic energy will be

Two charges $-q$ each are separated by distance $2d$. A third charge $+ q$ is kept at mid point $O$. Find potential energy of $+ q$ as a function of small distance $x$ from $O$ due to $-q$ charges. Sketch $P.E.$ $v/s$ $x$ and convince yourself that the charge at $O$ is in an unstable equilibrium.

State which of the following is correct

The mean free path of electrons in a metal is $4 \times 10^{-8} \;m$. The electric field which can give on an average $2 \;eV$ energy to an electron in the metal will be in units of $V / m$

There are two equipotential surface as shown in figure. The distance between them is $r$. The charge of $-q\,$ coulomb is taken from the surface $A$ to $B$, the resultant work done will be