View source for Pascal (unit)

{{redirect|Pascals|other uses|pascal (disambiguation)}}
{{Short description|SI unit of pressure}}
{{Use dmy dates|date=October 2011}}
{{Infobox Unit
| bgcolour =
| name = Pascal
| image = [[File:Psidial.jpg|250px]]
| caption = A [[pressure gauge]] reading in [[Pounds per square inch|psi]] (red scale) and kPa (black scale)
| standard = [[SI unit]]
| quantity = [[Pressure]] or [[Stress (physics)|stress]]
| symbol = Pa
| namedafter = [[Blaise Pascal]]
| units1 = [[SI base unit]]s:
| inunits1 = [[Kilogram|kg]]&sdot;[[metre|m]]<sup>−1</sup>&sdot;[[second|s]]<sup>−2</sup>
| units2 = [[US customary units]]:
| inunits2 = 1.450 × 10<sup>−4</sup> [[Pounds per square inch|psi]]
| units3 = [[Atmosphere (unit)|atmosphere]]:
| inunits3 = 9.869 × 10<sup>−6</sup> atm
| units4 = [[Bar (unit)|bar]]:
| inunits4 = 10<sup>−5</sup> bar
}}
The '''pascal''' (symbol: '''Pa''') is the [[SI derived unit]] of [[pressure]] used to quantify [[internal pressure]], [[stress (physics)|stress]], [[Young's modulus]] and [[ultimate tensile strength]]. The unit, named after [[Blaise Pascal]], is defined as one [[Newton (unit)|newton]] per [[square metre]].<ref>{{SIbrochure8th|page=118}}</ref> The unit of measurement called [[Atmosphere (unit)|standard atmosphere (atm)]] is defined as {{gaps|101|325|u=Pa}}.<ref name="BIPM">{{cite web|url=http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4 |publisher=[[BIPM]] |title=Definition of the standard atmosphere|accessdate=2015-02-16}}</ref> 

Common multiple units of the pascal are the hectopascal (1&nbsp;hPa = 100&nbsp;Pa) which is equal to one [[Bar (unit)#Usage|millibar]], and the kilopascal (1&nbsp;kPa = 1000&nbsp;Pa) which is equal to one centibar. [[Meteorology|Meteorological]] forecasts typically report [[atmospheric pressure]] in hectopascals per the recommendation of the [[World Meteorological Organization]]. Forecasts in the United States typically use [[Bar (unit)#Usage|millibars]],<ref>{{Cite web|url=https://www.wpc.ncep.noaa.gov/sfc/90fwbg.gif|title=US government atmospheric pressure map}}</ref><ref>{{Cite web|url=https://s.w-x.co/util/image/map/WEB_Current_Weather_Map_1280x720.jpg?v=ap&w=1280&h=720&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0|title=The Weather Channel}}</ref> in Canada these reports are given in kilopascals.<!-- I live in Canada --><ref>{{Cite web|url=https://weather.gc.ca/canada_e.html|title=Canadian Weather - Environment Canada|first=Environment|last=Canada|date=16 April 2013|website=weather.gc.ca}}</ref>

== Etymology ==
The unit is named after [[Blaise Pascal]], noted for his contributions to hydrodynamics and hydrostatics, and experiments with a [[barometer]]. The name pascal was adopted for the SI unit newton per square metre (N/m<sup>2</sup>) by the 14th [[General Conference on Weights and Measures]] in 1971.<ref>[http://www.bipm.fr/en/convention/cgpm/14/pascal-siemens.html bipm.fr] {{webarchive|url=https://web.archive.org/web/20070630020548/http://www.bipm.fr/en/convention/cgpm/14/pascal-siemens.html |date=30 June 2007 }}</ref><ref name="CGPM14">[http://www.bipm.org/utils/common/pdf/CGPM/CGPM14.pdf#page=78 Minutes of the 14. General Conference on Weights and Measures], 1971, page 78</ref>

== Definition ==
The pascal can be expressed using [[SI derived unit]]s, or alternatively solely [[SI base unit]]s, as:
:<math>{\rm 1~Pa = 1~\frac{N}{m^2} =  1~\frac{kg}{m \cdot s^2}  = 1~\frac{J}{m^3} }</math>
where N is the [[Newton (unit)|newton]], m is the [[metre]], kg is the [[kilogram]], s is the [[second]], and J is the [[joule]].<ref>[http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html Table 3 (Section 2.2.2)] {{webarchive|url=https://web.archive.org/web/20070618123613/http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html |date=18 June 2007 }}, ''SI Brochure'', [[International Bureau of Weights and Measures]]</ref>

One pascal is the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square metre.

== Standard units ==
The unit of measurement called an [[Atmosphere (unit)|atmosphere or a standard atmosphere (atm)]] is {{convert|101325|Pa|kPa|abbr=on|comma=gaps}}.<ref>{{cite web
|url = http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4
|title = Resolution 4 of the 10th meeting of the CGPM
|publisher=[[General Conference on Weights and Measures|Conférence Générale des Poids et Mesures (CGPM)]]
|year = 1954
|accessdate = 2010-04-05}}</ref> This value is often used as a reference pressure and specified as such in some national and international standards, such as the [[International Organization for Standardization]]'s ISO&nbsp;2787 (pneumatic tools and compressors), ISO&nbsp;2533 (aerospace) and ISO&nbsp;5024 (petroleum). In contrast, [[International Union of Pure and Applied Chemistry]] (IUPAC) recommends the use of 100&nbsp;kPa as a standard pressure when reporting the properties of substances.<ref>IUPAC.org, Gold Book, [http://goldbook.iupac.org/S05921.html ''Standard Pressure'']</ref>

[[Unicode]] has dedicated code-points {{Unichar|33a9|Square Pa}} and {{Unichar|33AA|Square kPa}} in the [[CJK Compatibility]] block, but these exist only for backward-compatibility with some older ideographic character-sets and are therefore [[deprecate]]d.<ref>{{cite web|url=https://www.unicode.org/charts/PDF/U3300.pdf|title=CJK Compatibility|accessdate=2016-02-21|date=2015}}</ref><ref>{{cite web|publisher=The Unicode Consortium|title=The Unicode Standard, Version 8.0.0|location=Mountain View, CA|date=2015|isbn=978-1-936213-10-8|url=https://www.unicode.org/versions/Unicode8.0.0|accessdate=2016-02-21}}</ref>

== Uses ==
{{refimprove section|date=January 2016}}
The pascal (Pa) or kilopascal (kPa) as a unit of pressure measurement is widely used throughout the world and has largely replaced the [[pounds per square inch]] (psi) unit, except in some countries that still use the [[imperial measurement system]] or the [[US customary system]], including the United States.

[[Geophysics|Geophysicists]] use the gigapascal (GPa) in measuring or calculating tectonic stresses and pressures within the [[Earth]].

Medical [[elastography]] measures tissue stiffness non-invasively with [[medical ultrasonography|ultrasound]] or [[magnetic resonance imaging]], and often displays the [[Young's modulus]] or [[shear modulus]] of tissue in kilopascals.

In [[materials science]] and [[engineering]], the pascal measures the [[Young's modulus|stiffness]], [[tensile strength]] and [[compressive strength]] of materials. In engineering use, because the pascal represents a very small quantity, the megapascal (MPa) is the preferred unit for these uses.

{| class="wikitable"
|+ Approximate Young's modulus for common substances <ref>{{cite web|url=http://www.engineeringtoolbox.com/young-modulus-d_417.html|title=Tensile Modulus - Modulus of Elasticity or Young's Modulus - for some common Materials|accessdate=2015-02-16}}</ref>
! Material !! Young's modulus
|-
| [[nylon|nylon 6]] || 2–4&nbsp;GPa
|-
| [[hemp]] fibre || 35&nbsp;GPa
|-
| [[aluminium]] || 69&nbsp;GPa
|-
| [[tooth enamel]] || 83&nbsp;GPa
|-
| [[copper]] || 117&nbsp;GPa
|-
| [[steel|structural steel]] || 200&nbsp;GPa
|-
| [[diamond]] || 1220&nbsp;GPa
|}

The pascal is also equivalent to the SI unit of [[energy density]], the joule per cubic metre. This applies not only to the thermodynamics of pressurised gases, but also to the energy density of [[electric potential energy|electric]], [[magnetic field#Energy stored in magnetic fields|magnetic]], and [[gravitational energy|gravitational]] fields.

In measurements of [[sound pressure]] or loudness of sound, one pascal is equal to 94 decibels [[Sound pressure#Sound pressure level|sound pressure level]] (SPL).  The quietest sound a human can hear, known as the [[threshold of hearing]], is 0&nbsp;dB&nbsp;SPL, or 20&nbsp;µPa.

[[Blower door|The airtightness of buildings]] is measured at 50&nbsp;Pa.<ref>{{cite web
 |url         = http://www.resnet.us/hotnews/revised_chapter_seven_final_9-16-10.pdf
 |title       = Chapter 7 ResNet Standards: ResNet National Standard for Home Energy Audits
 |publisher   = ResNet
 |year        = 2010
 |accessdate  = 2011-03-03
 |url-status     = dead
 |archiveurl  = https://web.archive.org/web/20110726045004/http://www.resnet.us/hotnews/revised_chapter_seven_final_9-16-10.pdf
 |archivedate = 26 July 2011
 |df          = dmy-all
}}</ref>

===Hectopascal and millibar units===
{{main|Bar (unit)}}
The units of atmospheric pressure commonly used in [[meteorology]] were formerly the [[Bar (unit)|bar]], which was close to the average air pressure on Earth, and the millibar. Since the introduction of [[International System of Units|SI units]], meteorologists generally measure pressures in hectopascals (hPa) unit, equal to 100 pascals or 1 millibar.<ref>{{cite web|url=http://www.knmi.nl/actueel/|title=KNMI - Weer - Waarnemingen|publisher=|accessdate=4 December 2016}}</ref><ref>{{cite web|url=http://www.meteo.be/meteo/view/fr/68771-FAQ%20sur%20le%20temps.html?view=195695|title=Comment convertir la pression? - IRM|publisher=|accessdate=4 December 2016}}</ref><ref>{{Cite web|url=http://www.dwd.de/de/WundK/W_aktuell/Beobachtungen.htm|title=DWD}}</ref><ref>{{cite web|url=http://www.jma.go.jp/en/g3/index.html|title=Japan Meteorological Agency - Weather Maps|publisher=|accessdate=4 December 2016}}</ref><ref>{{Cite web|url=http://www.meteoam.it/modules.php?name=analisiPrevisioniSuolo|archiveurl=https://web.archive.org/web/20060506192242/http://www.meteoam.it/modules.php?name=analisiPrevisioniSuolo|url-status=dead|title=MDD|archivedate=6 May 2006}}</ref><ref>[http://weather.noaa.gov/weather/current/KDCA.html NOAA]</ref><ref>{{cite web|url=http://www.metoffice.gov.uk/guide/weather/symbols#units|title=Key to symbols and terms|first=Met Office|last=United Kingdom|publisher=|accessdate=4 December 2016}}</ref> Exceptions include Canada, which use kilopascals (kPa). In many other fields of science, prefixes that are a power of 1000 are preferred, which excludes the hectopascal from use.<ref>{{Cite web|url=http://www.ctv.ca/servlet/HTMLTemplate/!ctvDynNews/Weather/Weather?City=Montreal|archiveurl=https://web.archive.org/web/20110604131321/http://www.ctv.ca/servlet/HTMLTemplate/!ctvDynNews/Weather/Weather?City=Montreal|url-status=dead|title=CTV News, weather; current conditions in Montreal|archivedate=4 June 2011}}</ref><ref>{{cite web|url=http://www.weatheroffice.gc.ca/city/pages/qc-147_metric_e.html|title=Montréal, QC - 7 Day Forecast - Environment Canada|first=Environment|last=Canada|publisher=|accessdate=4 December 2016}}</ref>

Many countries also use millibars. In practically all other fields, the kilopascal (1000 pascals) is used instead.<ref>Ambler Thompson (Editor) {{google books|pTw-SCI7EkoC|Guide for the Use of the International System of Units (SI) (rev. ): The ...|page=66}}</ref>

== See also ==
* [[Atmospheric pressure]] which gives the usage of the hbar and the mbar
* [[Centimetre of water]]
* [[Meteorology]]
* [[Metric prefix]]
* [[Orders of magnitude (pressure)]]
* [[Pascal's law]]
* [[Pressure measurement]]

== References ==
{{Reflist|30em}}

==External links==

{{SI units}}

[[Category:SI derived units]]
[[Category:Units of pressure]]
[[Category:Blaise Pascal]]