During lightning,a current pulse,as shown in the figure,flows from a cloud at a height of $1.5 \ km$ to the ground. If the breakdown electric field of humid air is about $400 \ kVm^{-1}$,the energy released during lightning would be (in units of $10^9 \ J$):

Four charges of $1\ \mu C, 2\ \mu C, 3\ \mu C,$ and $-6\ \mu C$ are placed at the corners of a square of side $1\ m$. The square lies in the $x-y$ plane with its center at the origin.

Three identical charges are placed at the three corners of an equilateral triangle as shown in the figure. Which of the following statements is true for the electric field $E$ and electric potential $V$ at the center $O$?

Two identical charged spheres are suspended by strings of equal lengths. The strings make an angle of $30^o$ with each other. When suspended in a liquid of density $1 \, g \, cm^{-3}$,the angle remains the same. If the density of the material of the sphere is $4/3 \, g \, cm^{-3}$,the dielectric constant of the liquid is:

Two fixed charges $+4\,Q$ and $-Q$ are located at $A$ and $B$ respectively,with the distance $AB = 3\,m$.

An empty thick conducting shell of inner radius $a$ and outer radius $b$ is shown in the figure. It is observed that the inner face of the shell carries a uniform charge density $-\sigma$ and the outer surface carries a uniform charge density $+\sigma'$. If a point charge $q_A$ is slowly moved inside the cavity of the shell,then choose the correct statement$(s)$.