Two electric charges of $9 \mu C$ and $-3 \mu C$ are placed $0.16 \ m$ apart in air. There will be a point $P$ on the line joining the two charges and between them where the electric potential is zero. The distance of $P$ from the $9 \mu C$ charge is: (in $m$)

Two particles with charges $+q$ and $-q$ are separated by a distance of $2d$. What is the electric potential at the midpoint of the line joining them?

Twenty-seven drops of water of the same size are equally and similarly charged. They are then united to form a bigger drop. By what factor will the electrical potential change?

If $27$ charged water droplets, each of radius $10^{-3} \,m$ and charge $10^{-12} \,C$ coalesce to form a single big spherical drop, then the potential of the big drop is (in $\,V$)

The figure shows a solid hemisphere with a charge of $5 \ nC$ distributed uniformly throughout its volume. The hemisphere lies on a plane and point $P$ is located on this plane,along a radial line from the center of curvature at a distance of $15 \ cm$. The electric potential at point $P$ due to the hemisphere is ..... $V$.

Two points $P$ and $Q$ are at potentials of $10 \ V$ and $-4 \ V$ respectively. The work done in moving $100$ electrons from $P$ to $Q$ is .....