Symbol Definition Units p volume charge density coul/A 3 D displacement or flux vector coul/lk 2 relative permittivity of a dielectric dimensionless~ o permittivity of free space 8.85 x 10-12 coul2/nm 2 E electric field vector V/m surface potential mV K Debye reciprocal length constant A-1 kM affinity constant for a cation M(1/mole) D membrane thickness A d average spacing per monovalent surface charge A o surface charge density A-2 (esu/A2)~ d induced charge density adjacent to the membrane surface A-2 (esu/A 2) q the charge of one site on a membrane coul or esu esu electrostatic unit 1.6 X 10-19 coul/charge G arbitrary constant 270 A 2 (mole/l)* F