On moving a charge of $20 \mathrm{C}$  by $2 \mathrm{cm}$, between two point $A \& B$,  $2 \mathrm{J}$ of work is done by external agent on this charge, then the potential difference between the points $A \& B$ is

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?

Two semi-circular rings lying in the same plane, of uniform linear charge density $\mathrm{\lambda }$ have radius $r$ and $2r$. They are joined using two straight uniformly charged wires of linear charge density $\mathrm{\lambda }$ and length $r$ as shown in the figure. The magnitude of the electric field at the common center of semi-circular rings is