Electrical resistivity and conductivity results in the important concept of "quasineutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma ("n" = <Z>"n"), but on the scale of the Debye length there can be charge imbalance. In the special case that "double layers" are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation: Differentiating this
Positive and negative parts be expressed in terms of "f" and "f" as Also note that Using these two equations one may express the positive and negative parts as Another representation, using the Iverson bracket is One may define the positive and negative part of any function with values in a linearly ordered group. Given a measurable space ("X",Σ), an extended real-valued function "f" is measurable if and only if its positive and negative parts are. Therefore, if such a function "f" is measurable, so is its absolute value |"f"|, being the sum of two measurable functions. The converse, though, does not necessarily hold: