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 sets nonnegative (resp. nonpositive) everywhere on "A". Of course, if μ is a nonnegative measure, every element of Σ is a positive set for μ. In the light of Radon–Nikodym theorem, if ν is a σ-finite positive measure such that |μ| « ν, a set "A" is a positive set for μ if and only if the Radon–Nikodym derivative dμ/dν is nonnegative ν-almost everywhere on "A". Similarly, a negative set is a set where dμ/dν ≤ 0 ν-almost everywhere. It follows from the definition that every measurable subset of a positive or negative set is also positive or negative. Also, the