User:Milton Beychok/Sandbox: Difference between revisions

Revision as of 01:29, 14 June 2008

Pressure (symbol: p) is the force over an area applied to an object in a direction perpendicular to the surface.

Pressure is a scalar quantity and it is a fundamental parameter in thermodynamics.

Formula

Mathematically:

p={\frac {F}{A}}

where:

p

is the pressure,

F

is the normal force,

A

is the area.

Units

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m^-2 or kg·m^-1·s^-2). This special name for the unit was added in 1971; before that, pressure in SI was expressed simply as N/m².

Template:Pressure Units

Absolute pressure versus gauge pressure

Absolute pressure is the pressure relative to zero pressure. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

An example of the difference is between gauge and absolute pressure is the air pressure in an automobile tire. A tire pressure gauge might read 220 kPaas the gauge pressure, but that means the pressure is 220 kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 101 kPa, the absolute pressure in the tire is therefore about 321 kPa.

In technical writing, this would be written as a gauge pressure of 220 kPa or as an absolute pressure of 321 kPa. Where space is limited, such as on pressure gauge dials, table headings or graph labels, the use of a modifier in parentheses, such as kPa (gauge) or kPa (absolute), is permitted and strongly encouraged. ^[1]

Gauge pressure is the relevant measure of pressure wherever one is interested in the stress on 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 101 kPa, a gas at 200 kPa (gauge), which is 301 kPa (absolute), is 50 percent more dense than the same gas at 100 kPa (gauge), which is 201 kPa (absolute). Focusing on the gauge values, one might erroneously conclude the gas at 200 kPa (gauge) had twice the density of the same gas at 100 kPa (gauge).

Negative pressures

While pressures are generally positive, there are certain situations in which negative pressures may be encountered. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of -21 kPa (i.e., 21 kPa below an atmospheric pressure of 101 kPa).

Examples effects of pressure

A finger can be pressed against a wall without making a lasting impression; however, the same finger pushing a thumbtack can easily make a small hole in 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.

Another example is that of a common knife. The flat side of the knife won't cut an apple. But if we use the thin side, it will easily cut the apple. When we use the thin side, more pressure is applied because the surface area is reduced and so it readily cuts the apple.

References

↑ NIST, Rules and Style Conventions for Expressing Values of Quantities, Sect. 7.4.

External links

[1] NIST, Rules and Style Conventions for Expressing Values of Quantities, Sect. 7.4.

[1]

@@ Line 1: / Line 1: @@
-'''Pressure ''' (symbol: 'p') is the [[force (physics)|force]] over an [[area]] applied to an object in a direction [[surface normal|perpendicular]] to the surface.  '''Gauge pressure''' is the pressure relative to the local atmospheric or ambient pressure.
+'''Pressure''' (symbol: '''''p''''') is the [[force (physics)|force]] over an [[area]] applied to an object in a direction [[surface normal|perpendicular]] to the surface.
+Pressure is a scalar quantity and it is a fundamental parameter in [[thermodynamics]].
 === Formula ===
@@ Line 11: / Line 12: @@
 :<math>F</math> is the [[normal force]],
 :<math>A</math> is the area.
-Pressure is a scalar quantity, and has [[SI]] units of [[pascal (unit)|pascals]]; 1 Pa = 1 N/m<sup>2</sup>, and has [[United States customary units|EES]] units of [[Pounds per square inch|psi]]; 1 psi = 1 lb/in<sup>2</sup>.
-It is a fundamental parameter in [[thermodynamics]] and it is [[conjugate variables (thermodynamics)|conjugate ]] to [[volume]].
 === Units ===
@@ Line 22: / Line 19: @@
 {{Pressure Units}}
-=== Examples ===
+== Absolute pressure versus gauge pressure ==
-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]].
+'''Absolute pressure''' is the pressure relative to zero pressure. '''Gauge pressure''' is the pressure relative to the local atmospheric or ambient pressure.
+An example of the difference is between gauge and absolute pressure is the air pressure in an automobile tire. A tire [[pressure gauge]] might read 220 [[pascal (unit)|kPa]]as the gauge pressure, but that  means the pressure is 220 kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 101 kPa, the absolute pressure in the tire is therefore about 321 kPa.
-Another example is of a common ''knife.'' If we try and cut a fruit with the flat side it obviously won't cut. But if we take the thin side, it will cut smoothly. The reason is, the flat side has a greater surface area and so it does not cut  the fruit. When we take the thin side, the surface area is reduced and so it cuts the fruit easily and quickly. This shows one of the good effects of ''Pressure''.
+In technical writing, this would be written as ''a gauge pressure of 220 kPa'' or as ''an absolute pressure of 321 kPa. Where space is limited, such as on pressure gauge dials, table headings or graph labels, the use of a modifier in parentheses, such as ''kPa (gauge)'' or ''kPa (absolute)'', is permitted and strongly encouraged. <ref>NIST, [http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 ''Rules and Style Conventions for Expressing Values of Quantities''], Sect. 7.4.</ref>
-The [[gradient]] of pressure is called the [[force density]].
+Gauge pressure is the relevant measure of pressure wherever one is interested in the stress on 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 101 kPa, a gas  at 200 kPa (gauge), which is 301 kPa (absolute), is 50 percent more dense than the same gas at 100 kPa (gauge), which is 201 kPa (absolute). Focusing on the gauge values, one might erroneously conclude the gas at 200 kPa (gauge) had twice the density of the same gas at 100 kPa (gauge).
-For gases, pressure is sometimes measured not as an '''absolute pressure''', but relative to [[atmospheric pressure]]; such measurements are called '''gauge pressure''' (also sometimes spelled ''gage pressure'').<!--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.--><ref>The preferred spelling varies by country and even by industry. Further, both spellings are often used ''within'' a particular industry or country. Industries in British English-speaking countries typically use the "gauge" spelling. Many of the largest American manufacturers of pressure transducers and instrumentation use the spelling "gage pressure" in their most formal documentation (''Honeywell-Sensotec’s'' [http://www.sensotec.com/pressurefaq.shtml FAQ page] and Fluke Corporation’s [http://us.fluke.com/usen/Home/Search.asp?txtSearchBox=%22gage+pressure%22&x=0&y=0 product search page]).</ref> An example of this is the air pressure in an [[automobile]] [[tire]], which might be said to be "220&nbsp;[[pascal (unit)|kPa]]", but is actually 220&nbsp;kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 100&nbsp;kPa, the absolute pressure in the tire is therefore about 320&nbsp;kPa. In technical work, this is written "a gauge pressure of 220&nbsp;kPa". Where space is limited, such as on [[pressure gauge]]s, [[name plates]], graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)", is permitted. In non-[[SI]] technical work, a gauge pressure of 32 psi is sometimes written as "32&nbsp;psig", though the other methods explained above that avoid attaching characters to the unit of pressure are preferred.<ref>NIST, [http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 ''Rules and Style Conventions for Expressing Values of Quantities''], Sect. 7.4.</ref>
-Gauge pressure is the relevant measure of pressure wherever one is interested in the stress on 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 100&nbsp;kPa, a gas (such as helium) at 200&nbsp;kPa (gauge) (300&nbsp;kPa [absolute]) is 50&nbsp;% more dense than the same gas at 100&nbsp;kPa (gauge) (200&nbsp;kPa [absolute]). Focusing on gauge values, one might erroneously conclude the first sample had twice the density of the second one.
+==Negative pressures==
-===Scalar nature===
+While pressures are generally positive, there are certain situations in which negative pressures may be encountered. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of -21 kPa (i.e., 21 kPa below an atmospheric pressure of 101 kPa).
-In a static [[gas]], the gas as a whole does not appear to move.  The individual molecules of the gas, however, are in constant [[Brownian motion|random motion]]. Because we are dealing with an extremely large number of molecules and because the motion of the individual molecules is random in every direction, we do not detect any motion. If we enclose the gas within a container, we detect a pressure in the gas from the molecules colliding with the walls of our container. We can put the walls of our container anywhere inside the gas, and the force per unit area (the pressure) is the same. We can shrink the size of our "container" down to an infinitely small point, and the pressure has a single value at that point.  Therefore, pressure is a scalar quantity, not a vector quantity. It has magnitude but no direction sense associated with it. Pressure acts in all directions at a point inside a gas. At the surface of a gas, the pressure force acts perpendicular (at right angle) to the surface.
-A closely related quantity is the [[stress (physics)|stress]] tensor '''σ''', which relates the vector force '''F''' to the [[vector area]] '''A''' via
+== Examples effects of pressure ==
-:<math>
-\mathbf{F}=\mathbf{\sigma A}\,
-</math>
-This [[tensor]] may be divided up into a scalar part (pressure) and a traceless tensor part [[shear]]. The shear tensor gives the force in directions ''parallel'' to the surface, usually due to viscous or frictional forces. The stress tensor is sometimes called the pressure tensor, but in the following, the term "pressure" will refer only to the scalar pressure.
+A finger can be pressed against a wall without making a lasting impression; however, the same finger pushing a thumbtack can easily make a small hole in 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.
-==Negative pressures==
+Another example is that of a common knife. The flat side of the knife won't cut an apple. But if we use the thin side, it will easily cut the apple. When we use the thin side, more pressure is applied because the surface area is reduced and so it readily cuts the apple.
-While pressures are generally positive, there are several situations in which negative pressures may be encountered:
-*When dealing in relative (gauge) pressures. For instance, an absolute pressure of 80&nbsp;kPa may be described as a gauge pressure of -21&nbsp;kPa (i.e., 21&nbsp;kPa below an atmospheric pressure of 101&nbsp;kPa).
 ==References==