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 there is a partition of "X" into a positive and a negative set; such a partition ("P","N") is unique up to μ-null sets, and is called a "Hahn decomposition" of the signed measure μ. Given a Hahn decomposition ("P","N") of "X", it is easy to show that "A" ⊆ "X" is a positive set if and only if "A" differs from a subset of "P" by a μ-null set; equivalently, if "A"−"P" is μ-null. The same is true for negative sets, if "N" is used instead of "P". Positive and negative sets In measure theory, given a measurable space ("X",Σ)