Editing Elementary charge (section)

{{short description|Charge carried by one proton or electron}}
{{Infobox physical quantity
| name = Elementary charge
| unit = coulomb
| otherunits = 
| symbols = <math>e</math>
| dimension = <math>\mathsf{T I}</math>
| derivations = 
| value = {{physical constants|e}}
}}

The '''elementary charge''', usually denoted by {{Var|e}}, is a fundamental [[physical constant]], defined as the [[electric charge]] [[Charge carrier|carried]] by a single [[proton]]  (+1 ''e'') or, equivalently, the magnitude of the negative electric charge carried by a single [[electron]], which has charge −1&nbsp;{{Var|e}}.<ref name="SIBrochure9thEd">{{citation |title=The International System of Units (SI) |author=International Bureau of Weights and Measures |author-link=New SI |date=20 May 2019  |edition=9th |isbn=978-92-822-2272-0 |url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf| archive-url = https://web.archive.org/web/20211018184555/https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232 |archive-date=18 October 2021 |url-status=live}}</ref>{{efn|The symbol ''e'' has another useful mathematical meaning due to which its use as label for elementary charge is avoided in [[theoretical physics]]. For example, in [[quantum mechanics]] one wants to be able to write compactly [[plane waves]] <math> e^{i \mathbf k \cdot \mathbf r}</math> with the use of [[E (mathematical constant)|Euler's number]] <math> e = e^{1} = \exp(1)</math>. In the US, [[E (mathematical constant)|Euler's number]] is often denoted ''e'' (italicized), while it is usually denoted e (roman type) in the UK and Continental Europe. Somewhat confusingly, in [[atomic physics]], ''e'' sometimes denotes the electron charge, i.e. the ''negative'' of the elementary charge. The symbol ''q''<sub>e</sub> is also used for the charge of an electron.}}

In [[International System of Units|SI units]], the value of the elementary charge is exactly defined as {{nowrap|<math>e</math> {{=}} {{val|1.602176634|e=−19|u=}} [[coulomb]]s,}} or 160.2176634 [[Orders of magnitude (numbers)#10−21|zepto]]<nowiki/>coulombs (zC).<ref name="SI2019">
{{cite book
 | last1 = Newell
 | first1 = David B.
 | last2 = Tiesinga
 | first2 = Eite
 | year = 2019
 | title = The International System of Units (SI)
 | series = NIST Special Publication 330
 | publisher = National Institute of Standards and Technology
 | location = Gaithersburg, Maryland
 | url = https://www.nist.gov/si-redefinition/meet-constants
 | doi = 10.6028/nist.sp.330-2019
 | s2cid = 242934226
}}</ref>  Since the [[2019 revision of the SI]], the seven [[SI base unit]]s are defined in terms of seven fundamental physical constants, of which the elementary charge is one.

In the [[centimetre–gram–second system of units]] (CGS), the corresponding quantity is {{val|4.8032047|end=...|e=-10|u=[[statcoulomb]]s}}.{{efn|This is derived from the [[CODATA]] 2018 value, since one coulomb corresponds to exactly {{val|2997924580}} statcoulombs. The conversion factor is ten times the numerical value of [[speed of light]] in [[metres per second]].}}

[[Robert Andrews Millikan|Robert A. Millikan]] and [[Harvey Fletcher]]'s [[oil drop experiment]] first directly measured the magnitude of the elementary charge in 1909, differing from the modern accepted value by just 0.6%.<ref name="Millikan1910">
{{cite journal
 | author = Millikan, R. A.
 | title = The isolation of an ion, a precision measurement of its charge, and the correction of Stokes's law
 | journal = Science
 | volume = 32
 | number = 822
 | pages = 436-448
 | doi = 10.1126/science.32.822.436
 | year = 1910
}}</ref><ref name="Fletcher1982">
{{cite journal
 | author = Fletcher, Harvey
 | title = My work with Millikan on the oil-drop experiment
 | journal = Physics Today
 | volume = 35
 | number = 6
 | pages = 43-47
 | doi = 10.1063/1.2915126
 | year = 1982
}}</ref>  Under assumptions of the then-disputed [[atomic theory]], the elementary charge had also been indirectly inferred to ~3% accuracy from [[Planck%27s_law_of_black-body_radiation|blackbody spectra]] by [[Max Planck]] in 1901<ref name="Klein">{{cite journal|first1=Martin J.|last1=Klein|title=Max Planck and the beginnings of the quantum theory|url=https://doi.org/10.1007/BF00327765|journal=Archive for History of Exact Sciences|date=1 October 1961|issn=1432-0657|volume=1|issue=5|pages=459–479 |doi=10.1007/BF00327765|s2cid=121189755 |url-access=subscription}}</ref> and (through the [[Faraday constant]]) at order-of-magnitude accuracy by [[Johann_Josef_Loschmidt| Johann Loschmidt]]'s measurement of the [[Avogadro constant]] in 1865.