Difference between revisions of "Pressure" - New World Encyclopedia

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[[Image:Captain cook memorial fountain and national library.jpg|thumb|right|250px|The use of  
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{{Ebapproved}}{{Copyedited}}{{Images OK}}{{Submitted}}{{Approved}}{{Paid}}
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water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin, Canberra.]]
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[[Image:Captain cook memorial fountain and national library.jpg|thumb|right|300px|The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin, Canberra.]]
  
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'''Pressure ''' (symbol: ''p'') is the [[force (physics)|force ]] per unit [[area|area ]] applied on a surface in a direction [[surface normal|perpendicular]] to that surface. Mathematically:
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'''Pressure ''' (symbol "p") is the [[force (physics)|force ]] applied to a surface (in a direction [[surface normal|perpendicular]] to that surface) per unit [[area]] of the surface. If the force is concentrated on a small area, it will exert higher pressure than if the same force is distributed over a larger surface area. For example, a force applied with a [[finger]] may be able to push a [[thumbtack]] into a wall, but the same finger pressing directly against the wall (with the same force) may not make any lasting impression. This is because the thumbtack concentrates the force into a smaller area.
  
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:<math>p = \frac{F}{A}</math>
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Our understanding of the concept of pressure and our ability to manipulate the pressure in various systems have provided us with many practical benefits. For example, we have been able analyze [[weather]] patterns, move fluids using [[pump]]s, sterilize medical equipment at [[temperature]]s above the boiling point of [[water]], and conduct industrial processes at reduced temperatures.
  
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where:<br>
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Pressure arises from two fundamentally different kinds of sources: ambient and localized. Ambient sources of pressure are usually a gas or a liquid in which an entity is immersed, such as a human being on the surface of the [[earth]] or an [[octopus]] in the [[sea]]. Life forms are generally insensitive to ambient pressures and become aware of the source of that pressure when currents become strong enough that the fluid exerts a non-uniform localized pressure on the life form, such as when the [[wind]] blows. Humans also become sensitive to ambient pressure during the take off and landing of passenger [[airline]]s when they need to swallow to equalize the air pressure on both sides of the [[ear]] drum.
 +
 
 +
Localized pressure sources are usually discrete objects, such as the finger pressing on the wall, or the tires of a car pressed against the pavement. A liquid or gas can become the source of a localized
 +
pressure if either of them is forced through a narrow opening.
 +
{{toc}}
 +
[[Flight]] by [[bird]]s and [[airplane]]s occurs according to simple principles of pressure whereby the faster a fluid moves in one direction the lesser is the pressure it exerts in the direction perpendicular to its flow. The [[wing]]s of birds and airplanes are shaped in a way that their movement through the air requires a faster air flow over the top of the wing than over the bottom, which means that the upward force produced by the air flowing under the wing is greater than the downward force produced by the air flowing over the wing. The result is [[lift]] which pushes the bird or airplane upward.
 +
 
 +
== Mathematical expression ==
 +
 
 +
In mathematical terms, pressure can be expressed as:
 +
:<math>
 +
p = \frac{F}{A}\,
 +
</math>
 +
 
 +
where:<br/>
 
:<math>p</math> is the pressure
 
:<math>p</math> is the pressure
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:<math>F</math> is the [[normal force]]
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:<math>F</math> is the [[normal force]]<ref>The "normal force" is the component of a force perpendicular to the surface of contact.</ref>
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:<math>A</math> is the area.  
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:<math>A</math> is the area.
  
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Pressure is transmitted to solid boundaries or across arbitrary sections of fluid ''normal to'' these boundaries or sections at every point. It is a fundamental parameter in [[thermodynamics]] and it is [[conjugate variables (thermodynamics)|conjugate ]] to [[volume]].
+
The [[SI]] unit for force is the Newton, that for area is square meter, and that for pressure is called a [[Pascal (unit)|pascal]] (Pa). By definition, 1 Pa = 1 Newton per square meter (N/m<sup>2</sup>). [1 Pa is also equivalent to 1&nbsp;Joule per cubic meter (J•m<sup>&minus;3</sup>) or 1&nbsp;kilogram per meter per square second (kg•m<sup>&minus;1</sup>•s<sup>&minus;2</sup>).] The units for pressure are further discussed below.
  
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{{Conjugate variables (thermodynamics)}}
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== Kinetic nature of pressure in a gas ==
  
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== Example ==
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In 1738, [[Swizerland|Swiss]] [[physics|physician]] and [[mathematics|mathematician]] [[Daniel Bernoulli]] (1700-1782) published ''Hydrodynamica'', which laid the basis for the kinetic theory of gases. In this work, Bernoulli made the argument, still used to this day, that [[gas]]es consist of great numbers of [[molecule]]s moving in all directions, and that their impact on a surface causes the gas pressure that we feel. In addition, he proposed that what we experience as [[heat]] is simply the kinetic energy of their motion.
−
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 [[gradient]] of pressure is called the [[force density]].
+
==Relative pressure or gauge pressure==
  
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==Relative or gauge pressure==
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For gases, pressure is sometimes measured not as an ''absolute pressure'', but relative to [[atmospheric pressure]]. Such a measurement is sometimes called ''gauge pressure''. For example, if the air pressure in an [[automobile]] [[tire]] is given as "220 [[kPa]]," it is actually 220 kPa above atmospheric pressure. Given that atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written as "a gauge pressure of 220 kPa." Where space is limited, such as on [[pressure gauge]]s, [[name plate]]s, 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, gauge pressure is sometimes written as "32 psig (pounds per square inch gauge)."
−
For gases, pressure is sometimes measured not as an '''absolute pressure''', but relative to [[atmospheric pressure]]; such measurements are sometimes called '''gauge pressure'''. An example of this is the air pressure in an [[automobile]] [[tire]], which might be said to be "220 [[kPa]]", but is actually 220 kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written "a gauge pressure of 220 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 is sometimes written as "32 psig", though the other methods explained above that avoid attaching characters to the unit of pressure are preferred [http://physics.nist.gov/Pubs/SP811/sec07.html#7.4].
 
  
 
==Scalar nature of pressure==
 
==Scalar nature of pressure==
−
In a static gas, the gas as a whole does not appear to move.  The individual molecules of the gas, however, are in constant 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 a magnitude but no direction 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 to the surface.
 
  
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A closely related quantity is the [[stress (physics)|stress]] tensor '''σ''' which relates the vector force '''F''' to the vector area '''A''' via
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The pressure within a fluid (gas or liquid) is a [[scalar (physics)|scalar]] quantity—that is, it has magnitude but no particular direction associated with it in space. When the fluid meets a solid surface, this pressure is transmitted to the solid in a direction perpendicular to that surface. For instance, a swimmer at the bottom of a swimming pool may find that no matter how he turns his head (while maintaining the same depth), the water pressure against his eardrums remains the same, and it seems to be perpendicular to the eardrums.
  
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:<math>\mathbf{F}=\mathbf{\sigma A}</math>
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A gas is said to be ''static'' if it as a whole does not appear to move. The individual molecules of the gas, however, are in constant random motion. Given that there is an extremely large number of molecules and 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, the molecules collide with the walls of the container and exert pressure against the walls. The pressure at any point within the gas is independent of direction.
  
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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.
+
==Negative pressure==
  
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== Kinetic nature of pressure ==
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Although pressure generally has a positive value, it may have a negative value in some situations. Some examples follow.
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{{main|kinetic theory}}
 
  
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In 1738, Swiss physician and mathematician [[Daniel Bernoulli]] published ''Hydrodynamica'' which laid the basis for the kinetic theory of gases. In this work, Bernoulli positioned the argument, still used to this day, that gases consist of great numbers of molecules moving in all directions, that their impact on a surface causes the gas '''pressure''' that we feel, and that what we experience as [[heat]] is simply the kinetic energy of their motion.
+
* A relative (gauge) pressure can be negative. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of -21 kPa (that is, 21 kPa below atmospheric pressure).
 +
* At times, attractive forces (such as [[Van der Waals forces]]) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable, because the particles will move closer together until repulsive forces balance attractive forces.
 +
* Negative pressure exists in the [[transpiration pull]] of plants.
  
−
==Negative pressures==
+
==Stagnation pressure==
−
While pressures are generally positive, there are several situations in which a negative pressure may be encountered:
 
  
−
* When dealing in relative (gauge) pressures. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of -21 kPa (i.e. 21 kPa below atmospheric pressure).
+
[[Stagnation pressure]] is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower ''static pressure'', it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the [[Mach number]] of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid.<ref>Note that Bernoulli's equation applies only to incompressible flow.</ref>
−
* When attractive forces (e.g. [[Van der Waals forces]]) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces. Negative pressure exists in the [[transpiration pull]] of plants.
 
−
* 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 pressure'' of a vacuum).
 
−
* Depending on how the [[Orientability|orientation]] of a surface is chosen, the same distribution of forces may be described either as a positive pressure along one [[surface normal]], or as a negative pressure acting along the opposite surface normal.
 
  
−
==Hydrostatic pressure (Head pressure)==
+
The pressure of a moving fluid can be measured using a [[Pitot probe]] (or one of its variations) connected to a [[manometer]]. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.
−
[[Hydrostatic pressure]] is the pressure due to the [[weight]] of a fluid.
 
  
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::<math>p = \rho g h\,</math>
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==The pressure/volume conjugate pair ==
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::where:
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{{Conjugate variables (thermodynamics)}}
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::*''[[rho|ρ]]'' (rho) is the [[density]] of the fluid (i.e. the practical density of fresh water is 1000 kg/m<sup>3</sup>);
+
 
−
::*''[[acceleration due to gravity|g]]'' is the acceleration due to gravity (approx. 9.81 m/s<sup>2</sup> on Earth's surface);
+
In thermodynamics, pressure and volume are said to form a pair of conjugate variables (for nonviscous fluids). The pressure acts as a generalized force—pressure differences force a change in [[volume]], and their product is the energy lost by the system due to [[mechanical work]]. Pressure is the driving force, and volume is the associated displacement.
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::*''h'' is the height of the fluid column (i.e. meters or feet).
 
  
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See also [[Pascal's law]].
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== Calculation of hydrostatic pressure ==
  
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==Stagnation pressure==
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At a given spot within a fluid, the [[hydrostatic pressure]] (or "head pressure") is the pressure resulting from the [[weight]] of the fluid column above that spot. It may be calculated by the following formula:
−
[[Stagnation pressure]] is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower '''static pressure''', it may have a higher stagnation pressure when forced to a standstill.  Static pressure and stagnation pressure are related by the [[Mach number]] of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See [[Bernoulli's equation]] (note: Bernoulli's equation only applies for incompressible flow).
 
  
−
The pressure of a moving fluid can be measured using a [[Pitot probe]], or one of its variations such as a [[Kiel probe]] or [[Cobra probe]], connected to a [[manometer]]. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.
+
::<math>p = \rho g h\,</math>
 +
::where:
 +
::*''[[rho|ρ]]'' (rho) is the [[density]] of the fluid (for example, the density of freshwater is about 1000 kg/m<sup>3</sup>);
 +
::*''[[acceleration due to gravity|g]]'' is the acceleration due to gravity (approximately 9.81 m/s<sup>2</sup> on Earth's surface);
 +
::*''h'' is the height of the fluid column (in meters).
  
 
==Units==
 
==Units==
−
{{Pressure Units}}
 
  
−
 
+
The name for the unit of pressure, the ([[pascal|Pascal]]), was added in 1971. Before that, pressure in SI was expressed by using its units (N•m<sup>-2</sup>).
−
The [[SI]] unit for pressure is the [[pascal]] ([[pascal|Pa]]), equal to one [[newton]] per [[square metre]] (N·m<sup>-2</sup> or kg·m<sup>-1</sup>·s<sup>-2</sup>). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N/m².
 
  
 
Non-SI measures (still in use in some parts of the world) include the [[pound-force]] per square [[inch]] ([[pound-force per square inch|psi]]) and the [[bar (unit)|bar]].
 
Non-SI measures (still in use in some parts of the world) include the [[pound-force]] per square [[inch]] ([[pound-force per square inch|psi]]) and the [[bar (unit)|bar]].
  
−
The [[cgs]] unit of pressure is the [[barye]] (ba). It is equal to 1 dyn·cm<sup>-2</sup>.
+
The [[cgs]] unit of pressure is the [[barye]] (ba), which is equal to one dyne per square centimeter (dyn•cm<sup>-2</sup>).
  
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Pressure is still sometimes expressed in [[kgf]]/cm² or grams-force/cm² (sometimes as [[Kilogram per centimeter squared|kg/cm²]] and g/cm² without properly identifying the force units). But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is expressly forbidden in SI; the unit of force in SI is the newton (N). The [[technical atmosphere]] (symbol: at) is 1 kgf/cm².
+
The [[standard atmosphere]] (atm) of pressure is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows:
 +
:standard atmosphere = 101,325 [[pascal|Pa]] = 101.325 kPa = 1013.25 hPa.
  
−
Some [[meteorologist]]s prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit [[millibar]] (mbar). Similar pressures are given in kilopascals (kPa) in practically all other fields, where the hecto prefix is hardly ever used. In Canadian weather reports, the normal unit is kPa. The obsolete unit ''inch of mercury'' ([[inHg]], see below) is still sometimes used in the United States.
+
Some [[meteorologist]]s prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit [[millibar]] (mbar). Similar pressures are given in kilopascals (kPa) in practically all other fields, where the hecto prefix is hardly ever used. In Canadian weather reports, the normal unit is kPa.
  
−
The [[standard atmosphere]] (atm) is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows.
+
Because pressure may be measured by the displacement of a column of liquid in a [[manometer]], it is sometimes expressed as the depth (in inches) of a particular fluid. The most common choices of fluid are [[Mercury (element)|mercury]] (Hg) and [[water]], and pressure was once expressed in terms of ''inches of mercury'' ([[in Hg]]). Water is nontoxic and readily available, but mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The pressure exerted by a column of liquid of height ''h'' and density ''ρ'' is given by the hydrostatic pressure equation noted above: ''p = hgρ''.
  
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:standard atmosphere = 101325 [[pascal|Pa]] = 101.325 kPa = 1013.25 hPa.
+
Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When '[[torr|millimeters of mercury]]' or '[[inHg|inches of mercury]]' are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. (The water-based units still depend on the density of water, a measured, rather than defined, quantity.)
  
−
Because pressure is commonly measured by its ability to displace a column of liquid in a [[manometer]], pressures are often expressed as a depth of a particular fluid (e.g. inches of water). The most common choices are [[Mercury (element)|mercury]] (Hg) and [[water]]; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The press exerted by a column of liquid of height ''h'' and density ''ρ'' is given by the hydrostatic pressure equation as above: ''p = hgρ''.
+
Although no longer favored in physics, these ''manometric'' units are still encountered in certain fields. [[Blood pressure]] is measured in millimeters of mercury in most parts of the world, and lung pressure in centimeters of water is also common. [[Natural gas]] pipeline pressures are measured in inches of water, expressed as '''WC''' ('Water Column'). [[Scuba diving|Scuba divers]] often use a manometric [[rule of thumb]]: the pressure exerted by ten meters depth of water is approximately equal to one atmosphere.
  
−
Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When '[[torr|millimetres of mercury]]' or '[[inHg|inches of mercury]]' are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. The water-based units still depend on the density of water, a measured, rather than defined, quantity.
+
Pressure is still sometimes expressed in [[kgf]]/cm² or grams-force/cm² (sometimes as [[Kilogram per centimeter squared|kg/cm²]] and g/cm² without properly identifying the force units). But using the terms kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is expressly forbidden in SI. (The unit of force in SI is the newton, N).
−
 
 
−
Although no longer favoured in physics, these '''manometric units''' are still encountered in many fields. [[Blood pressure]] is measured in millimetres of mercury in most of the world, and lung pressures in centimeters of water are still common. [[Natural gas]] pipeline pressures are measured in inches of water, expressed as '"WC' ('Water Column'). [[Scuba diving|Scuba divers]] often use a manometric [[rule of thumb]]: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere.
 
  
 
Non-SI units presently or formerly in use include the following:
 
Non-SI units presently or formerly in use include the following:
 
*[[atmosphere (unit)|atmosphere]].
 
*[[atmosphere (unit)|atmosphere]].
 
*manometric units:
 
*manometric units:
−
**centimetre, inch, and millimetre of mercury ([[Torr]]).
+
**centimeter, inch, and millimeter of mercury ([[Torr]]).
−
**millimetre, [[centimetre of water|centimetre]], metre, inch, and foot of water.
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**millimeter, [[centimetre of water|centimeter]], meter, inch, and foot of water.
 
*imperial units:
 
*imperial units:
−
**[[kip (unit)|kip]], [[ton]]-force (short), ton-force (long), pound-force, ounce-force, and [[poundal]] per square inch.
+
**[[kip (unit)|kip]], [[ton]]-force (short)<ref>One short ton equals 2,000 pounds, or 90.72 percent of a metric ton.</ref>, ton-force (long)<ref>One long ton equals 2,240 pounds, or 101.605 percent of a metric ton.</ref>, pound-force, ounce-force, and [[poundal]] per square inch.
 
**pound-force, ton-force (short), and ton-force (long) per square foot.
 
**pound-force, ton-force (short), and ton-force (long) per square foot.
 
*non-SI metric units:
 
*non-SI metric units:
 
**[[bar (unit)|bar]], [[millibar]].
 
**[[bar (unit)|bar]], [[millibar]].
−
**kilogram-force, or kilopond, per square centimetre ([[technical atmosphere]]).
+
**kilogram-force, or kilopond, per square centimeter ([[technical atmosphere]]) (symbol: at) is 1 kgf/cm².
−
** gram-force and tonne-force (metric ton-force) per square centimetre.
+
** gram-force and tonne-force (metric ton-force) per square centimeter.
−
**[[barye]] ([[dyne]] per square centimetre).
+
**[[barye]] ([[dyne]] per square centimeter).
−
**kilogram-force and tonne-force per square metre.
+
**kilogram-force and tonne-force (metric ton-force) per square meter.
−
**[[sthene]] per square metre ([[pieze]]).
+
**[[sthene]] per square meter ([[pieze]]).
  
−
== Biology ==
+
==See also==
−
In the human body there are several types of pressure receptor. [[Baroreceptor]]s monitor [[blood pressure]] in the [[carotid artery|carotid arteries]], [[aortic arch]] and [[right atrium]] of the heart. [[Mechanoreceptor]]s are part of the [[somatosensory system]] and are present in the [[dermis]] of the [[skin]] and in deeper tissues. They respond to different forms of touch and pressure; the main types of mechanoreceptor are [[Pacinian corpuscle]]s, [[Meissner's corpuscle]]s, [[Merkel cell]]s and [[Ruffini ending]]s.
 
−
 
 
−
== Moments ==
 
−
Moments is the scientific name given to someting that turns on a pivot (fulcrum). The pressure of the moment can also be measured using a similar calculation to one given earlier in this article. The thing to consider or the relationship is that the further away the load (effort force) is from the pivot the larger the force. For example, a door, it has its handle (effort force) right at the end furthest away from the hinges (pivot). This allows the door to move with ease.
 
−
The calculation used to measure force of moments is:
 
−
 
 
−
Distance from Pivot (mm/cm/m/km/M)  X  Force (N)  =  
 
−
 
 
−
Example:
 
−
A door has its handle 70cm away from its pivot and the force exerted on the door is 8 (N)
 
−
 
 
−
Distance from Pivot (70)    X  Force (8) = 560 Ncm2
 
  
−
==See also==
 
 
* [[Atmospheric pressure]]
 
* [[Atmospheric pressure]]
 
* [[Blood pressure]]
 
* [[Blood pressure]]
 +
* [[Kinetic theory Pressure]]
 
* [[Combined gas law]]
 
* [[Combined gas law]]
 
* [[Conversion of units]]
 
* [[Conversion of units]]
−
* [[Units conversion by factor-label]]
 
 
* [[Ideal gas law]]
 
* [[Ideal gas law]]
−
* [[Kinetic theory#Pressure]]
 
 
* [[Partial pressure]]
 
* [[Partial pressure]]
−
* [[Sound pressure]] (audio)
 
−
* [[Microphone]]
 
−
* [[Timeline of temperature and pressure measurement technology]]
 
 
* [[Vacuum]]
 
* [[Vacuum]]
 
* [[Boyle's Law]]
 
* [[Boyle's Law]]
 +
 +
== Notes ==
 +
<references/>
 +
 +
== References ==
 +
 +
* Crowell, Benjamin. 2006. [http://www.lightandmatter.com/html_books/0sn/ch05/ch05.html Simple Nature: Thermodynamics] ''lightandmatter.com''. Retrieved December 3, 2007.
 +
* Nave, Carl R. 2007. [http://hyperphysics.phy-astr.gsu.edu/hbase/press.html#pre Pressure] ''Hyperphysics''. Retrieved December 3, 2007.
 +
* Young, Hugh D., and Roger A. Freedman. 2003. ''Physics for Scientists and Engineers'', 11th ed. San Francisco, CA: Pearson. ISBN 080538684X
  
 
== External links ==
 
== External links ==
−
* [http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?An_Exercise_In_Air_Pressure An exercise in air pressure]
+
All links retrieved November 30, 2022.
−
* [http://www.grc.nasa.gov/WWW/K-12/airplane/pressure.html Pressure being a scalar quantity]
+
* [http://hyperphysics.phy-astr.gsu.edu/hbase/press.html HyperPhysics: Pressure] ''Department of Physics and Astronomy, Georgia State University''.
 +
* [http://www.grc.nasa.gov/WWW/K-12/airplane/pressure.html Gas Pressure] ''Glenn Research Center, NASA''.
 +
* [http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/prs/def.rxml Atmospheric Pressure] ''Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign''.
 +
 
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]
 
[[Category:Physics]]
 
[[Category:Physics]]
 +
[[Category:Chemistry]]
  
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{{credit|75569314}}
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{{credits|Pressure|75569314|Conjugate_variables_(thermodynamics)|131529396}}

Latest revision as of 22:39, 30 November 2022

The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin, Canberra.

Pressure (symbol "p") is the force applied to a surface (in a direction perpendicular to that surface) per unit area of the surface. If the force is concentrated on a small area, it will exert higher pressure than if the same force is distributed over a larger surface area. For example, a force applied with a finger may be able to push a thumbtack into a wall, but the same finger pressing directly against the wall (with the same force) may not make any lasting impression. This is because the thumbtack concentrates the force into a smaller area.

Our understanding of the concept of pressure and our ability to manipulate the pressure in various systems have provided us with many practical benefits. For example, we have been able analyze weather patterns, move fluids using pumps, sterilize medical equipment at temperatures above the boiling point of water, and conduct industrial processes at reduced temperatures.

Pressure arises from two fundamentally different kinds of sources: ambient and localized. Ambient sources of pressure are usually a gas or a liquid in which an entity is immersed, such as a human being on the surface of the earth or an octopus in the sea. Life forms are generally insensitive to ambient pressures and become aware of the source of that pressure when currents become strong enough that the fluid exerts a non-uniform localized pressure on the life form, such as when the wind blows. Humans also become sensitive to ambient pressure during the take off and landing of passenger airlines when they need to swallow to equalize the air pressure on both sides of the ear drum.

Localized pressure sources are usually discrete objects, such as the finger pressing on the wall, or the tires of a car pressed against the pavement. A liquid or gas can become the source of a localized pressure if either of them is forced through a narrow opening.

Flight by birds and airplanes occurs according to simple principles of pressure whereby the faster a fluid moves in one direction the lesser is the pressure it exerts in the direction perpendicular to its flow. The wings of birds and airplanes are shaped in a way that their movement through the air requires a faster air flow over the top of the wing than over the bottom, which means that the upward force produced by the air flowing under the wing is greater than the downward force produced by the air flowing over the wing. The result is lift which pushes the bird or airplane upward.

Mathematical expression

In mathematical terms, pressure can be expressed as:

where:

is the pressure
is the normal force[1]
is the area.

The SI unit for force is the Newton, that for area is square meter, and that for pressure is called a pascal (Pa). By definition, 1 Pa = 1 Newton per square meter (N/m2). [1 Pa is also equivalent to 1 Joule per cubic meter (J•m−3) or 1 kilogram per meter per square second (kg•m−1•s−2).] The units for pressure are further discussed below.

Kinetic nature of pressure in a gas

In 1738, Swiss physician and mathematician Daniel Bernoulli (1700-1782) published Hydrodynamica, which laid the basis for the kinetic theory of gases. In this work, Bernoulli made the argument, still used to this day, that gases consist of great numbers of molecules moving in all directions, and that their impact on a surface causes the gas pressure that we feel. In addition, he proposed that what we experience as heat is simply the kinetic energy of their motion.

Relative pressure or gauge pressure

For gases, pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure. Such a measurement is sometimes called gauge pressure. For example, if the air pressure in an automobile tire is given as "220 kPa," it is actually 220 kPa above atmospheric pressure. Given that atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written as "a gauge pressure of 220 kPa." Where space is limited, such as on pressure gauges, 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, gauge pressure is sometimes written as "32 psig (pounds per square inch gauge)."

Scalar nature of pressure

The pressure within a fluid (gas or liquid) is a scalar quantity—that is, it has magnitude but no particular direction associated with it in space. When the fluid meets a solid surface, this pressure is transmitted to the solid in a direction perpendicular to that surface. For instance, a swimmer at the bottom of a swimming pool may find that no matter how he turns his head (while maintaining the same depth), the water pressure against his eardrums remains the same, and it seems to be perpendicular to the eardrums.

A gas is said to be static if it as a whole does not appear to move. The individual molecules of the gas, however, are in constant random motion. Given that there is an extremely large number of molecules and 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, the molecules collide with the walls of the container and exert pressure against the walls. The pressure at any point within the gas is independent of direction.

Negative pressure

Although pressure generally has a positive value, it may have a negative value in some situations. Some examples follow.

  • A relative (gauge) pressure can be negative. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of -21 kPa (that is, 21 kPa below atmospheric pressure).
  • At times, attractive forces (such as Van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable, because the particles will move closer together until repulsive forces balance attractive forces.
  • Negative pressure exists in the transpiration pull of plants.

Stagnation pressure

Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid.[2]

The pressure of a moving fluid can be measured using a Pitot probe (or one of its variations) connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.

The pressure/volume conjugate pair

Conjugate variables
of thermodynamics
Pressure Volume
Temperature Entropy
Chem. potential Particle no.

In thermodynamics, pressure and volume are said to form a pair of conjugate variables (for nonviscous fluids). The pressure acts as a generalized force—pressure differences force a change in volume, and their product is the energy lost by the system due to mechanical work. Pressure is the driving force, and volume is the associated displacement.

Calculation of hydrostatic pressure

At a given spot within a fluid, the hydrostatic pressure (or "head pressure") is the pressure resulting from the weight of the fluid column above that spot. It may be calculated by the following formula:

where:
  • ρ (rho) is the density of the fluid (for example, the density of freshwater is about 1000 kg/m3);
  • g is the acceleration due to gravity (approximately 9.81 m/s2 on Earth's surface);
  • h is the height of the fluid column (in meters).

Units

The name for the unit of pressure, the (Pascal), was added in 1971. Before that, pressure in SI was expressed by using its units (N•m-2).

Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (psi) and the bar.

The cgs unit of pressure is the barye (ba), which is equal to one dyne per square centimeter (dyn•cm-2).

The standard atmosphere (atm) of pressure is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows:

standard atmosphere = 101,325 Pa = 101.325 kPa = 1013.25 hPa.

Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar). Similar pressures are given in kilopascals (kPa) in practically all other fields, where the hecto prefix is hardly ever used. In Canadian weather reports, the normal unit is kPa.

Because pressure may be measured by the displacement of a column of liquid in a manometer, it is sometimes expressed as the depth (in inches) of a particular fluid. The most common choices of fluid are mercury (Hg) and water, and pressure was once expressed in terms of inches of mercury (in Hg). Water is nontoxic and readily available, but mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The pressure exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation noted above: p = hgρ.

Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When 'millimeters of mercury' or 'inches of mercury' are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. (The water-based units still depend on the density of water, a measured, rather than defined, quantity.)

Although no longer favored in physics, these manometric units are still encountered in certain fields. Blood pressure is measured in millimeters of mercury in most parts of the world, and lung pressure in centimeters of water is also common. Natural gas pipeline pressures are measured in inches of water, expressed as WC ('Water Column'). Scuba divers often use a manometric rule of thumb: the pressure exerted by ten meters depth of water is approximately equal to one atmosphere.

Pressure is still sometimes expressed in kgf/cm² or grams-force/cm² (sometimes as kg/cm² and g/cm² without properly identifying the force units). But using the terms kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is expressly forbidden in SI. (The unit of force in SI is the newton, N).

Non-SI units presently or formerly in use include the following:

  • atmosphere.
  • manometric units:
    • centimeter, inch, and millimeter of mercury (Torr).
    • millimeter, centimeter, meter, inch, and foot of water.
  • imperial units:
    • kip, ton-force (short)[3], ton-force (long)[4], pound-force, ounce-force, and poundal per square inch.
    • pound-force, ton-force (short), and ton-force (long) per square foot.
  • non-SI metric units:
    • bar, millibar.
    • kilogram-force, or kilopond, per square centimeter (technical atmosphere) (symbol: at) is 1 kgf/cm².
    • gram-force and tonne-force (metric ton-force) per square centimeter.
    • barye (dyne per square centimeter).
    • kilogram-force and tonne-force (metric ton-force) per square meter.
    • sthene per square meter (pieze).

See also

  • Atmospheric pressure
  • Blood pressure
  • Kinetic theory Pressure
  • Combined gas law
  • Conversion of units
  • Ideal gas law
  • Partial pressure
  • Vacuum
  • Boyle's Law

Notes

  1. ↑ The "normal force" is the component of a force perpendicular to the surface of contact.
  2. ↑ Note that Bernoulli's equation applies only to incompressible flow.
  3. ↑ One short ton equals 2,000 pounds, or 90.72 percent of a metric ton.
  4. ↑ One long ton equals 2,240 pounds, or 101.605 percent of a metric ton.

References
ISBN links support NWE through referral fees

  • Crowell, Benjamin. 2006. Simple Nature: Thermodynamics lightandmatter.com. Retrieved December 3, 2007.
  • Nave, Carl R. 2007. Pressure Hyperphysics. Retrieved December 3, 2007.
  • Young, Hugh D., and Roger A. Freedman. 2003. Physics for Scientists and Engineers, 11th ed. San Francisco, CA: Pearson. ISBN 080538684X

External links

All links retrieved November 30, 2022.

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