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## Pressure
## Additional recommended knowledge
## Definition## Formulaic
Mathematically: where: *p*is the pressure,*F*is the normal force,*A*is the area.
Pressure is a scalar quantity, and has SI units of pascals; 1 Pa = 1 N/m Pressure is transmitted to solid boundaries or across arbitrary sections of fluid ## Units
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m Non-SI measures such as 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 most other fields, where the hecto prefix is rarely used. The unit 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: - standard atmosphere = 101325 Pa = 101.325 kPa = 1013.25 hPa.
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 (Hg) and water; water is nontoxic and readily available, while mercury's high 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 Presently or formerly popular pressure units 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), ton-force (long), pound-force, ounce-force, and poundal per square inch
- pound-force, ton-force (short), and ton-force (long)
- non-SI metric units:
- bar, decibar, millibar
- kilogram-force, or kilopond, per square centimetre (technical atmosphere)
- gram-force and tonne-force (metric ton-force) per square centimetre
- barye (dyne per square centimetre)
- kilogram-force and tonne-force per square metre
- sthene per square metre (pieze)
## ExamplesAs 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 The gradient of pressure is called the force density.
For gases, pressure is sometimes measured not as an 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 kPa, a gas (such as helium) at 200 kPa (gauge) (300 kPa [absolute]) is 50 % more dense than the same gas at 100 kPa (gauge) (200 kPa [absolute]). Focusing on gauge values, one might erroneously conclude the first sample had twice the density of the second. ## Scalar natureIn 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. A closely related quantity is the stress tensor 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 ## Types## Explosion or deflagration pressuresExplosion or deflagration pressures are the result of the ignition of explosible gases, mists, dust/air suspensions, in unconfined and confined spaces. ## Negative pressuresWhile 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 an atmospheric pressure of 101 kPa).
- 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 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.
- In the cosmological constant.
## Hydrostatic pressure (head pressure)Hydrostatic pressure is the pressure due to the weight of a fluid. where: *ρ*(rho) is the density of the fluid (i.e., the practical density of fresh water is 1000 kg/m^{3});*g*is the acceleration due to gravity (approximately 9.81 m/s^{2}on earth's surface);*h*is the height of the fluid column (in metres). Other units can be used if the rest of the units used in the equation are defined in a consistent way.
See also Pascal's law. ## Stagnation pressureStagnation 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 The pressure of a moving fluid can be measured using a Pitot tube, 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. ## Surface pressureThere is a two-dimensional analog of pressure -- the lateral force per unit length applied on a line perpendicular to the force. Surface pressure is denoted by π and shares many similar properties with three-dimensional pressure. Properties of surface chemicals can be investigated by measuring pressure/area isotherms, as the two-dimensional analog of Boyle's law, ## See also- Atmospheric pressure
- Blood pressure
- Boyle's Law
- Combined gas law
- Conversion of units
- Units conversion by factor-label
- Ideal gas law
- Kinetic theory
- Partial pressure
- Sound pressure
- Microphone
- Timeline of temperature and pressure measurement technology
- Vacuum
- Vacuum pump
- Pressure pulse
## Notes**^**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*FAQ page and Fluke Corporation’s product search page).**^**NIST,*Rules and Style Conventions for Expressing Values of Quantities*, Sect. 7.4.
Categories: Atmospheric thermodynamics | Thermodynamics |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pressure". A list of authors is available in Wikipedia. |