Editing Pressure (section)

===Examples===
[[File:Aluminium cylinder.jpg|120px|thumbnail|right|The effects of an external pressure of 700&nbsp;bar on an aluminum cylinder with {{convert|5|mm|in|3|abbr=on}} wall thickness]]
As an example of varying pressures, a finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a [[thumbtack]] can easily damage the wall. Although the force applied to the surface is the same, the thumbtack applies more pressure because the point concentrates that force into a smaller area. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid ''normal to'' these boundaries or sections at every point. Unlike [[stress (physics)|stress]], pressure is defined as a [[Scalar (physics)|scalar quantity]]. The negative [[gradient]] of pressure is called the [[force density]].<ref>{{cite book |last1=Lautrup |first1=Benny |title=Physics of continuous matter : exotic and everyday phenomena in the macroscopic world |date=2005 |publisher=Institute of Physics |location=Bristol |isbn=9780750307529 |page=50}}</ref>

Another example is a knife. If the flat edge is used, force is distributed over a larger surface area resulting in less pressure, and it will not cut. Whereas using the sharp edge, which has less surface area, results in greater pressure, and so the knife cuts smoothly. This is one example of a practical application of pressure.<ref>{{cite book |last1=Breithaupt |first1=Jim |title=Physics |date=2015 |location=Basingstoke |isbn=9781137443243 |page=106 |edition=Fourth |publisher=Palgrave Macmillan }}</ref>

For gases, pressure is sometimes measured not as an ''absolute pressure'', but relative to [[atmospheric pressure]]; such measurements are called ''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''. An example of this is the air pressure in an [[automobile]] [[tire]], which might be said to be "{{convert|220 |kPa |psi|abbr=on|lk=in}}", but is actually 220&nbsp;kPa (32&nbsp;psi) above atmospheric pressure. Since atmospheric pressure at sea level is about 100&nbsp;kPa (14.7&nbsp;psi), the absolute pressure in the tire is therefore about {{convert|320|kPa|psi|abbr=on}}. In technical work, this is written "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 {{convert|220|kPa|psi|abbr=on}}".

Where space is limited, such as on [[pressure 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.-->]]s, [[name plate]]s, graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (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.-->)" or "kPa (absolute)", is permitted.<ref> {{Cite book|author=Institute of Electrical and Electronics Engineers |title=268-1992 |author-link=Institute of Electrical and Electronics Engineers |year=1992}}</ref> In non-[[SI]] technical work, 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 {{convert|32 |psi|kPa|abbr=on}} is sometimes written as "32&nbsp;psig", and an absolute pressure as "32&nbsp;psia", though the other methods explained above that avoid attaching characters to the unit of pressure are preferred.<ref name = pubs/>

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 is the relevant measure of pressure wherever one is interested in the stress on [[Pressure vessel|storage vessels]] and the plumbing components of fluidics systems. However, whenever equation-of-state properties, such as densities or changes in densities, must be calculated, pressures must be expressed in terms of their absolute values. For instance, if the atmospheric pressure is {{convert|100|kPa|psi|abbr=on}}, a gas (such as helium) at {{convert|200|kPa|psi|abbr=on}} (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.-->) ({{convert|300|kPa|psi|abbr=on|disp=or}} [absolute]) is 50% denser than the same gas at {{convert|100|kPa|psi|abbr=on}} (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.-->) ({{convert|200|kPa|psi|abbr=on|disp=or}} [absolute]). Focusing on 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.--> values, one might erroneously conclude the first sample had twice the density of the second one.{{citation needed|date=October 2021}}