Coulomb's law, F = k*(q₁q₂)/r² describes the force between two charges (q₁ and q₂) separated by a distance r in a vacuum (k is a constant). When q₁ and q₂ are both positive or both negative, the force is positive and repulsive. When one charge is positive and the other is negative, the force is negative and attractive. In biological systems, charges are separated by water, other molecules, or parts of molecules, not a vacuum. Thus, the cellular medium shields charges from each other.

To account for solvent shielding, a dimensionless number called the dielectric constant, , is inserted into the Coulomb equation, F = k*(q₁q₂)/(*r²). The dielectric constant is high for a polar solvent and low for nonpolar organic solvents.