The usefulness of cgs units is clear when considering the coulomb force between two charged particles with z1 and z2 units of electronic charge separated by a distance r: where the coulomb law constant kc = (1/4πε。)-8.988 x 109 m/farad (f) in the si system (where 1 f = 1 amp-s/v) and kc 1 in the cgs system. in cgs units, the value of e equals 4.803 x 10-10 stat c, which leads to a quick conversion factor for coulomb nteractions of e2 = 1.44 x 10-13 mev-cm-1.44 ev-nm. in this book we use kc = 1 and rely on eq. 1.5 for e2. the masses of particles, given in kg in si units, are generally expressed in unified mass units (u), a measure that replaces the older atomic mass units, or amu. the neutral carbon atom with 6 protons, 6 neutrons, and 6 electrons is the reference for the unified mass unit (u), which is defined as 1/12th the mass of the neutral 12c carbon atom (where the superscript indicates the