Two identical conducting spheres $A$  and $B,$  carry equal charge. They are separated by a distance much larger than their diameter, and the force between them is $F$ . A third identical conducting sphere, $C,$  is uncharged. Sphere $C$  is first touched to $A,$ then to $B,$  and then removed. As a result, the force between $A$  and $B$  would be equal to

${F_g}$ and ${F_e}$ represents gravitational and electrostatic force respectively between electrons situated at a distance $10\, cm$. The ratio of ${F_g}/{F_e}$ is of the order of

Three charge $q$, $Q$ and $4q$ are placed in a straight line of length $l$ at points distant $0,\,\frac {l}{2}$ and $l$ respectively from one end. In order to make the net froce on $q$ zero, the charge $Q$ must be equal to

Two equal charges of magnitude $Q$ each are placed at a distance $d$ apart. Their electrostatic energy is $E$. A third charge $-Q / 2$ is brought midway between these two charges. The electrostatic energy of the system is now

By using Coulomb’s law, define unit charge.