An alternative representation to
0x45's rendition of Coulomb's law accounts for the polarizability of the environment:
q1 q2
f = ----------
4 pi E r2
Where
E is actually epsilon.
E/
E0 is known as the
dielectric constant, or
relative permittivity.
E0 is the
permittivity of free space (of a vacuum).
The dielectric constant, subsumed in the constant
k in the previous writeup, accounts for screening of electrostatic interactions between charges by the environment. An environment that is highly polarizable will orient around a given charge, thereby effectively neutralizing the effects of electric field at long distances. Water has a high dielectric constant - around 80. As a result, ions in water have a much weaker attraction or repulsion than they do in the gas phase.