Од Википедија — слободната енциклопедија
Прејди на: содржини, барај
p, P
SI-единица Pascal (Pa)
Во Основни-единици N/m2 or 1 kg/(m·s2)
Изведенки од
други величини
p = F / A
A figure showing pressure exerted by particle collisions inside a closed container. The collisions that exert the pressure are highlighted in red.
Pressure as exerted by particle collisions inside a closed container.


Притисок (symbol: p or P) е сила применета нормално на површината на едно тело за единица површина на која е распределена таа сила. Баждарниот притисок е притисокот релативен на амбиенталниот притисок.

Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre; similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and US customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure and the torr is defined as 1760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.

Definition[уреди | уреди извор]

Pressure is the amount of force acting per unit area. The symbol for it is p or P.[1] The IUPAC recommendation for pressure is a lower-case p.[2] However, upper-case P is widely used. The usage of P vs p depends on the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum, and on writing style.

Formula[уреди | уреди извор]

Шаблон:Conjugate variables (thermodynamics)

Pressure force area.svg



is the pressure,
is the normal force,
is the area of the surface on contact.

Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors:

The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outward. The equation has meaning in that, for any surface S in contact with the fluid, the total force exerted by the fluid on that surface is the surface integral over S of the right-hand side of the above equation.

It is incorrect (although rather usual) to say "the pressure is directed in such or such direction". The pressure, as a scalar, has no direction. The force given by the previous relationship to the quantity has a direction, but the pressure does not. If we change the orientation of the surface element, the direction of the normal force changes accordingly, but the pressure remains the same.

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 to volume.

Units[уреди | уреди извор]

Mercury column

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m2 or kg·m−1·s−2). This name for the unit was added in 1971;[3] before that, pressure in SI was expressed simply in newtons per square metre.

Other units of pressure, such as pounds per square inch and bar, are also in common use. The CGS unit of pressure is the barye (Ba), equal to 1 dyn·cm−2 or 0.1 Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre (g/cm2 or kg/cm2) and the like without properly identifying the force units. But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as units of force is expressly forbidden in SI. The technical atmosphere (symbol: at) is 1 kgf/cm2 (98.0665 kPa or 14.223 psi).

Since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume. It is therefore related to energy density and may be expressed in units such as joules per cubic metre (J/m3, which is equal to Pa). Mathematically:

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 inch of mercury is still used in the United States. Oceanographers usually measure underwater pressure in decibars (dbar) because pressure in the ocean increases by approximately one decibar per metre depth.

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 .

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., centimetres of water, millimetres of mercury or inches of mercury). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's high density allows a shorter column (and so a smaller manometer) to be used 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 p = ρgh, where g is the gravitational acceleration. 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 millimetres 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.[се бара извор] One millimetre of mercury is approximately equal to one torr. The water-based units still depend on the density of water, a measured, rather than defined, quantity. 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 centimetres of water are still common.

Underwater divers use the metre sea water (msw or MSW) and foot sea water (fsw or FSW) units of pressure, and these are the standard units for pressure gauges used to measure pressure exposure in diving chambers and personal decompression computers. A msw is defined as 0.1 bar, and is not the same as a linear metre of depth, and 33.066 fsw = 1 atm.[4] Note that the pressure conversion from msw to fsw is different from the length conversion: 10 msw = 32.6336 fsw, while 10 m = 32.8083 ft

Gauge pressure is often given in units with 'g' appended, e.g. 'kPag', 'barg' or 'psig', and units for measurements of absolute pressure are sometimes given a suffix of 'a', to avoid confusion, for example 'kPaa', 'psia'. However, the US National Institute of Standards and Technology recommends that, to avoid confusion, any modifiers be instead applied to the quantity being measured rather than the unit of measure[5] For example, "pg = 100 psi" rather than "p = 100 psig".

Differential pressure is expressed in units with 'd' appended; this type of measurement is useful when considering sealing performance or whether a valve will open or close.

Presently or formerly popular pressure units include the following:

  • atmosphere (atm)
  • manometric units:
    • centimetre, inch, millimetre (torr) and micrometre (mTorr, micron) of mercury
    • Height of equivalent column of water, including millimetre (mm H2O), centimetre (cm H2O), metre, inch, and foot of water
  • imperial and customary units:
  • non-SI metric units:
    • bar, decibar, millibar
      • msw (metres sea water), used in underwater diving, particularly in connection with diving pressure exposure and decompression
    • 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)
Единици за притисок
паскал бар техничка атмосфера стандардна
Милиметар на
живиниот столб
фунта на
квадратен инч
Pa bar at atm torr psi
1 Pa ≡ 1 N/m2 10−5 1,0197×10−5 9,8692×10−6 7,5006×10−3 145,04×10−6
1 bar 105 ≡ 106 dyn/cm2 1,0197 0,98692 750,06 14,5037744
1 at 0,980665 ×105 0,980665 ≡ 1 kp/cm2 0,96784 735,56 14,223
1 atm 1,01325 ×105 1,01325 1,0332 p0 760 14,696
1 torr 133,322 1,3332×10−3 1,3595×10−3 1,3158×10−3 = 1 mmHg 19,337×10−3
1 psi 6,895×103 68,948×10−3 70,307×10−3 68,046×10−3 51,715 ≡ 1 lbF/in2

  1. Giancoli, Douglas G. (2004). Physics: principles with applications. Upper Saddle River, N.J.: Pearson Education. ISBN 0-13-060620-0. 
  2. McNaught, A. D.; Wilkinson, A.; Nic, M.; Jirat, J.; Kosata, B.; Jenkins, A. (2014). IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book").. Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.P04819. ISBN 0-9678550-9-8. http://goldbook.iupac.org/E02281.html. 
  3. „14th Conference of the International Bureau of Weights and Measures“. Bipm.fr. http://www.bipm.fr/en/convention/cgpm/14/pascal-siemens.html. конс. 27 март 2012 г. 
  4. US Navy (2006). US Navy Diving Manual, 6th revision. United States: US Naval Sea Systems Command. стр. 2–32. http://www.supsalv.org/00c3_publications.asp?destPage=00c3&pageID=3.9. конс. 15 јуни 2008 г. 
  5. „Rules and Style Conventions for Expressing Values of Quantities“. NIST. http://physics.nist.gov/Pubs/SP811/sec07.html#7.4. конс. 7 јули 2009 г.