Editing Triboelectric effect (section)

=== Work function differences ===
[[File:Work function mismatch gold aluminum.svg|thumb|When the two metals depicted here are in thermodynamic equilibrium with each other as shown (equal [[Fermi level]]s), the vacuum [[electrostatic potential]] ''ϕ'' is not flat due to a difference in [[work function]].]]
In all materials there is a positive electrostatic potential from the positive atomic nuclei, partially balanced by a negative electrostatic potential of what can be described as a sea of electrons.<ref name=":16">{{Cite book |last1=Ashcroft |first1=Neil W. |last2=Mermin |first2=N. David |title=Solid State Physics |date=1976 |publisher=Cengage Learning |isbn=978-0-03-083993-1}}</ref> The average potential is positive, what is called the ''mean inner potential'' (MIP). Different materials have different MIPs, depending upon the types of atoms and how close they are. At a surface the electrons also spill out a little into the vacuum, as analyzed in detail by [[Walter Kohn|Kohn]] and Liang.<ref name=":16" /><ref name=":20">{{Cite journal |last1=Lang |first1=N. D. |last2=Kohn |first2=W. |date=1971 |title=Theory of Metal Surfaces: Work Function |url= https://link.aps.org/doi/10.1103/PhysRevB.3.1215 |journal=Physical Review B |volume=3 |issue=4 |pages=1215–1223 |doi=10.1103/PhysRevB.3.1215 |bibcode=1971PhRvB...3.1215L |issn=0556-2805}}</ref> This leads to a [[dipole]] at the surface. Combined, the dipole and the MIP lead to a potential barrier for electrons to leave a material which is called the [[work function]].<ref name=":16" />

A rationalization of the triboelectric series is that different members have different work functions, so electrons can go from the material with a small work function to one with a large.<ref name=":1" /> The potential difference between the two materials is called the [[Volta potential]], also called the ''contact potential''. Experiments have validated the importance of this for metals and other materials.<ref name=":4" />  However, because the surface dipoles vary for different surfaces of any solid<ref name=":16" /><ref name=":20" /> the contact potential is not a universal parameter. By itself it cannot explain many of the results which were established in the early 20th century.<ref name=":6" /><ref name=":7" /><ref name=":8" />