Q.Find the electric potential at a point $30\ \text{cm}$ from a point charge of $+5\ \mu\text{C}$.

Derived by integrating the point-charge field from infinity to $r$: $V(r) = kq/r$, with $k=1/(4\pi\epsilon_0)=9\times10^9\ \text{N}\,\text{m}^2/\text{C}^2$. Sign follows the charge's own sign (positive charge, positive $V$ everywhere; negative charge, negative $V$ everywhere), unlike field magnitude which is always positive. Falls off as $1/r$, one power gentler than the field's $1/r^2$. Undefined ($\to\infty$) exactly at the point charge itself, reflecting the point-charge idealisation.