View source for Debye

{{Short description|CGS unit of electric dipole moment}}
{{other uses}}
The '''debye''' ({{IPAc-en|d|ɪ|ˈ|b|aɪ}} {{respell|dib|EYE}},<ref>{{cite web
|title = Debye
|url = http://www.dictionary.com/browse/debye
|work = Random House Dictionary
|date = 2013}}</ref> {{IPA|nl|dəˈbɛiə|lang}}; symbol: '''D''') is a [[CGS]] unit<ref>[http://www.unc.edu/~rowlett/units/cgsmks.html CGS units] {{Webarchive|url=https://web.archive.org/web/20110809085056/http://www.unc.edu/~rowlett/units/cgsmks.html |date=2011-08-09 }} R. Rowlett (University of North Carolina at Chapel Hill).</ref> (a non-[[SI]] [[metric system|metric]] unit) of [[electric dipole moment]]<ref group=note>Two equal and opposite charges separated by some distance constitute an electric dipole. This dipole possesses an [[electric dipole moment]] whose value is given as charge times length of separation. The dipole itself is a vector whose direction coincides with the position vector of the positive charge with respect to the negative charge:
: '''p''' = ''q'''''r'''.</ref> named in honour of the physicist [[Peter J. W. Debye]]. It is defined as {{val|e=-18}} [[statcoulomb]]-[[centimetre]]s.<ref group=note name=statC>The statcoulomb is also known as the franklin or electrostatic unit of charge.
: 1&nbsp;statC&nbsp;= 1&nbsp;Fr = 1&nbsp;esu = 1&nbsp;cm<sup>3/2</sup>⋅g<sup>1/2</sup>⋅s<sup>−1</sup>.</ref> Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10<sup>−10</sup>&nbsp;statcoulomb<ref group=note>10<sup>−10</sup>&nbsp;statcoulomb corresponds to approximately 0.2083&nbsp;units of elementary charge.</ref> (generally called e.s.u. (electrostatic unit) in older scientific literature), which were separated by 1&nbsp;[[ångström]].<ref group=note>The ångström is within an order of magnitude of the nuclear separation for a typical [[covalent bond]].
: 1&nbsp;Å = 100&nbsp;pm = 10<sup>−8</sup>&nbsp;cm = 10<sup>−10</sup>&nbsp;m.</ref> This gave a convenient unit for molecular dipole moments.
: {|
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|height=30|1&nbsp;D&nbsp;||= 10<sup>−18</sup>&nbsp;statC·cm
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|= 10<sup>−18</sup>&nbsp;cm<sup>5/2</sup>⋅g<sup>1/2</sup>⋅s<sup>−1</sup> <ref group=note>In any of the [[CGS system]]s, electromagnetic units are expressed in terms of the three base units, cm, g and s, albeit the systems are mutually inconsistent in this regard.</ref>
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|= 10<sup>−10</sup>&nbsp;statC·Å <ref group=note name=statC/>
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|≘ {{sfrac|{{val|299,792,458}}}}{{e|&minus;21}}&nbsp;[[coulomb|C]]·[[metre|m]]<ref group=note>One debye corresponds to {{val|e=-21|u=C·m<sup>2</sup>/s}} divided by the speed of light. Conversely, 1&nbsp;C·m ≘ {{val|2.9979|e=29|u=D}}.</ref>
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|≈ {{val|3.33564|e=-30|u=C·m}} <ref group=note>Also expressible as {{val|3.33564|u=qC·m}}.</ref>
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|≈ {{val|0.3934303|u=''e''·[[Bohr radius|''a''<sub>0</sub>]]}} <ref group=note>The product ''e''·''a''<sub>0</sub> is also called the ''[[atomic unit]] of electric dipole moment'' (see [http://physics.nist.gov/cgi-bin/cuu/Value?auedm Atomic unit of electric dipole moment], NIST.)</ref>
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|≈ {{val|0.2081943|u=''e''⋅Å}}
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|≈ {{val|0.02081943|u=[[elementary charge|''e'']]·[[nanometre|nm]]}}
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|}
Typical dipole moments for simple diatomic molecules are in the range of 0 to 11&nbsp;D. Molecules with symmetry point groups or containing inversion symmetry will not have a permanent dipole moment, while highly ionic molecular species have a very large dipole moment, e.g. gas-phase [[potassium bromide]], KBr, with a dipole moment of 10.41&nbsp;D.<ref>''Physical chemistry'' 2nd Edition (1966) G. M. Barrow. McGraw-Hill.</ref> A proton and an electron 1&nbsp;Å apart have  a dipole moment of 4.8&nbsp;D.
The debye is still used in [[atomic physics]] and [[chemistry]] because SI units have until recently been inconveniently large. The smallest SI unit of electric dipole moment is the quectocoulomb-metre,<ref group=note>With a value of 10<sup>−30</sup>, [[quecto-]] is the smallest [[SI prefix]], accepted as an SI prefix by the CGPM on 18 November 2022.</ref> which corresponds closely to 0.3&nbsp;D.
== See also ==
* [[Buckingham (unit)]] (CGS unit of electric quadrupole)
== Notes ==
<references group=note />
== References ==
{{reflist}}
[[Category:Non-SI metric units]]
[[Category:Peter Debye]]
[[Category:Centimetre–gram–second system of units]]