Plasma (physics) potential in the space between charged particles, independent of how it can be measured, is called the "plasma potential", or the "space potential". If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a Debye sheath. The good electrical conductivity of plasmas makes their electric fields very small. This 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 (formula_6), but on the scale of the Debye length
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