Definition of Unit Charge from Coulomb's Law

Substituting $\epsilon_0=8.85\times10^{-12}\,\text{C}^2\text{N}^{-1}\text{m}^{-2}$ into Coulomb's law in vacuum gives the numerical constant $\dfrac{1}{4\pi\epsilon_0}\approx9\times10^9\,\text{N m}^2\text{C}^{-2}$, so if $q_1=q_2=1$ C and $r=1.0$ m, the resulting force works out to $F=9.0\times10^9$ N. This calculation is used directly to give the formal SI definition of the coulomb itself: ONE COULOMB is the amount of charge which, when placed at a distance of one metre from another charge of the same magnitude in vacuum, experiences a force of $9.0\times10^9$ N.

This defining force, $9.0\times10^9$ N, is a tremendously, unrealistically large force -- roughly equivalent to about 10 million metric tonnes, or about one million truck-loads (taking a typical truck-load as roughly 10 metric tonnes) -- essentially never realisable in any practical laboratory situation with actual charges of a full coulomb each. Because of this, electrostatics problems almost always work with much smaller SUB-UNITS of the coulomb: the microcoulomb ($1\,\mu\text{C}=10^{-6}$ C), the nanocoulomb ($1$ nC $=10^{-9}$ C), and the picocoulomb ($1$ pC $=10^{-12}$ C) are the units that actual …