If the electric flux entering and leaving an enclosed surface respectively is ${\mathrm{\varphi }}_1$ and ${\mathrm{\varphi }}_2$, the electric charge inside the surface will be

A charged particle $q$ is shot towards another charged particle $Q$ which is fixed, with a speed $v$. It approaches $Q$ up-to a closest distance $r$ and then returns. If $q$ was given a speed $2v$, the closest distance of approach would be

The diagram shows the arrangement of three small uniformly charged spheres $A\text{,} B$ and $C$. The arrows indicate the direction of the electrostatic forces acting between the spheres(for example, the left arrow on sphere $A$ indicates the electrostatic force on sphere $A$ due to sphere $B$). At least two of the spheres are positively charged. Which sphere, if any, could be negatively charged?

The figure, shown below, shows three situations involving a charged particle and a uniformly charged spherical shell. The charges and radii of the shells are indicated in the figure. If $F_1\text{,} F_2$ and $F_3$ are the magnitudes of the forces on the particle due to the presence of the shell in situations (I), (II) and (III) respectively then