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
Negative number the rules of signs for expanding the multiplication formula_3, and al-Karaji wrote in his "al-Fakhrī" that "negative quantities must be counted as terms". In the 10th century, Abū al-Wafā' al-Būzjānī considered debts as negative numbers in "A Book on What Is Necessary from the Science of Arithmetic for Scribes and Businessmen". By the 12th century, al-Karaji's successors were to state the general rules of signs and use them to solve polynomial divisions. As al-Samaw'al writes: the product of a negative number — "al-nāqiṣ" — by a positive number — "al-zāʾid" — is negative, and by a negative number is positive.