State an expression for the electrostatic potential at a distance r from a single point charge q$.$ What are the fundamental S$.$I. units of electrostatic potential? $[2$ marks$]$ Consider, next, an arrangement of four point charges, two of charge q and two of charge $-$q$.$ These are equally spaced around a circular ring of radius a$.$ The pattern is alternating, so that the charges q are at $12$ o$$clock and $6$ o$$clock$,$ while the charges $-$q are at $3$ o$$clock and $9$ o$$clock$.$ Write an expression for the total electrostatic potential due to this arrangement, at an arbitrary $($i$.$e$.,$ general$)$ point of measurement. Be sure to define any variables that you introduce. Continued from Question $3.($b$)[3$ marks$]$ By taking a Taylor expansion, or otherwise, calculate the leading $($first non vanishing$)$ term in the electrostatic potential in the $$far$-$field regime$,$ that is at large distances from the centre point of the circular arrangement of charges in $3.($b$).$ Without needing to calculate it explicitly, with what exponent does the electric field magnitude fall off with $($large$)$ distance from the centre point? $[10$ mark