The constant of proportionality k in Coulomb’s law is a matter of
choice if the unit of charge is to be defined using Coulomb’s law In SI
units, however, what is defined is the unit of current (A) via its magnetic
effect (Ampere’s law) and the unit of charge (coulomb) is simply defined
by (1C = 1 A s) In this case, the value of k is no longer arbitrary; it is
approximately 9 × 109 N m2 C–2 4