Editing Pressure (section)

=== Negative pressures ===
[[File:13-07-23-kienbaum-unterdruckkammer-33.jpg|thumb|Low-pressure chamber in [[Bundesleistungszentrum Kienbaum]], Germany]]
While ''pressures'' are, in general, positive, there are several situations in which negative pressures may be encountered:
*When dealing in relative (gauge<!--Editors are asked to PLEASE check the discussion page for this article before making changes regarding "gauge" vs. "gage" spelling issues. Much debate has transpired on this issue.-->) pressures. For instance, an absolute pressure of 80&nbsp;kPa may be described as a gauge<!--Editors are asked to PLEASE check the discussion page for this article before making changes regarding "gauge" vs. "gage" spelling issues. Much debate has transpired on this issue.--> pressure of −21&nbsp;kPa (i.e., 21&nbsp;kPa below an atmospheric pressure of 101&nbsp;kPa). For example, [[abdominal decompression]] is an [[obstetric]] procedure during which negative gauge pressure is applied intermittently to a pregnant woman's abdomen.
*Negative absolute pressures are possible. They are effectively [[tension (physics)|tension]], and both bulk solids and bulk liquids can be put under negative absolute pressure by pulling on them.<ref name="Imre2007">{{cite book|last1=Imre|first1=A. R.|title=Soft Matter under Exogenic Impacts|chapter=How to generate and measure negative pressure in liquids?|series=NATO Science Series II: Mathematics, Physics and Chemistry | volume=242|year=2007|pages=379–388|issn=1568-2609|doi=10.1007/978-1-4020-5872-1_24|isbn=978-1-4020-5871-4}}</ref> Microscopically, the molecules in solids and liquids have attractive interactions that overpower the thermal kinetic energy, so some tension can be sustained. Thermodynamically, however, a bulk material under negative pressure is in a [[metastable]] state, and it is especially fragile in the case of liquids where the negative pressure state is similar to [[superheating]] and is easily susceptible to [[cavitation]].<ref name=liqneg>{{cite book |title=Liquids Under Negative Pressure (Nato Science Series II) |date=2002 |publisher=Springer | isbn=978-1-4020-0895-5 | doi=10.1007/978-94-010-0498-5|editor1-last=Imre |editor1-first=A. R |editor2-last=Maris |editor2-first=H. J |editor3-last=Williams |editor3-first=P. R }}</ref> In certain situations, the cavitation can be avoided and negative pressures sustained indefinitely,<ref name=liqneg/> for example, liquid mercury has been observed to sustain up to {{val|-425|u=atm}} in clean glass containers.<ref name="Briggs1953">{{cite journal|last1=Briggs|first1=Lyman J.|title=The Limiting Negative Pressure of Mercury in Pyrex Glass|journal=Journal of Applied Physics|volume=24|issue=4|year=1953|pages=488–490|issn=0021-8979|doi=10.1063/1.1721307|bibcode=1953JAP....24..488B}}</ref> Negative liquid pressures are thought to be involved in the [[ascent of sap]] in plants taller than 10&nbsp;m (the atmospheric [[pressure head]] of water).<ref>{{cite web | title=The Physics of Negative Pressure | url=http://discovermagazine.com/2003/mar/featscienceof | publisher=[[Discover (magazine)|Discover]] | author=Karen Wright | access-date=31 January 2015 | date=March 2003 | url-status=live | archive-url=https://web.archive.org/web/20150108182004/http://discovermagazine.com/2003/mar/featscienceof | archive-date=8 January 2015 }}</ref>
*The [[Casimir effect]] can create a small attractive force due to interactions with [[vacuum energy]]; this force is sometimes termed "vacuum pressure" (not to be confused with the negative ''gauge<!--Editors are asked to PLEASE check the discussion page for this article before making changes regarding "gauge" vs. "gage" spelling issues. Much debate has transpired on this issue.--> pressure'' of a vacuum).
*For non-isotropic stresses in rigid bodies, depending on how the orientation of a surface is chosen, the same distribution of forces may have a component of positive stress along one [[Normal (geometry)|surface normal]], with a component of negative stress acting along another surface normal. The pressure is then defined as the average of the three principal stresses.
**The stresses in an [[electromagnetic field]] are generally non-isotropic, with the stress normal to one surface element (the [[normal stress]]) being negative, and positive for surface elements perpendicular to this.
*In [[cosmology]], [[dark energy]] creates a very small yet cosmically significant amount of negative pressure, which accelerates the [[expansion of the universe]].