See also: Pressure (homonymy)

A pressure is an intensive size report/ratio of a Force on a surface.

Definition

The pressure , noted p admits, according to the branches of the physics which one considers, several definitions which all coincide:

In Mechanical, the pressure is locally defined starting from the component of the normal force on the surface on which she is exerted. If one considers an elementary surface dS and its local normal in M $\backslash \; vec\; N\; (M)$ one has then the pressure p (M) by:

$\backslash \; vec\; F\; \backslash \; cdot\; \backslash \; vec\; N\; (M)\; =\; p\; (M)\; \backslash ,\; dS\; (M)$ In the case of a force perpendicular to a plane surface of surface S , one obtains the following scalar definition easily: $p\; =\; \backslash \; frac\; \{F\}\; \{S\}$

The term obtained by building the report/ratio of the component of the tangential force on the surface of exercise is called the forced tangential. It is homogeneous with a pressure and is concerned in the phenomena of Viscosité in particular.

In Mechanical of the continuous mediums, the pressure is defined like one the third of the trace of the Tenseur of the constraints i.e. the average of the diagonal terms of this tensor. In mechanics of the incompressible fluids, the pressure is the Multiplicateur of Lagrange making it possible to check the Incompressibilité material. One has then to make with a definition Implicite of the pressure.

In Thermodynamic, the pressure is defined starting from the energy interns $U\; (V,\; S,\; NR)$ by: $p=-\; \backslash \; biggl\; (\backslash \; frac\; \{\backslash \; partial\; U\}\; \{\backslash \; partial\; V\}\; \backslash \; biggr)\; \_\; \{S,\; NR\}\; \backslash ,\; \backslash !$
For a Newtonian fluid , the pressure is strictly positive because it is necessary to provide energy $\backslash \; Delta\; U>O$ to decrease volume

Pushed of Archimedes

The Pushed of Archimedes exerted by a fluid at rest on a body immersed perhaps definite as being the resultant of the compressive forces applying to the surface of this object.

Units and measurement

In the international system of units, the pressure is expressed in Pascal (Pa): a pressure of 1  Pascal corresponds to a force of 1  newton exerted on a Surface of 1  Mr.

The measuring device of the pressure is the Manomètre. For the Atmospheric pressure, one uses the Baromètre. One can also use a Vacuomètre to measure the pressure of a gas in a vacuum tube or a Hypsomètre, device based on the temperature of boiling of a liquid.

In a closed container, a liquid will exert an equal force on all surface.

Other usual units

• 1 bar = 10 Pa

• 1 MPa = 1 N/mm
• 1 kgf/cm = 0,981 bar (kgf = kilogram forces)
• 1 psi  = : 6894  N/m  = : 6894  Pa   = 0,068  94  bar (Psi = pound per public garden inch ( lb / in ), delivers per square inch , unit Anglo-Saxon)
• 1 atm =: 101325  Pa (atm  =  atmosphere)
• 1 mmHg = 1 Torr = 133,3  Pa (MmHg   = Millimetre of mercury)
• 1 bar = 401.463 inH2O or inch off toilets (39.2°C) English traditional unit
• 1 bar = 760 mmHg

Deep-sea diving

In Plunged underwater, the pressure which is exerted on the biological fabrics and on the Gaz inspired has a great importance. Its variation can be considerable according to the depth reached.

They then are differentiated:

• Atmospheric pressure: pressure of surface under usual conditions (normally in the neighborhoods of 1013 mb but usually regarded as equivalent with 1 bar)
• Hydrostatic pressure : variable according to the depth reached - this pressure increases by 1 bar by section of 10 Mètre S under water (0,98 bar in fresh water and 1,007 bar in sea water)
• absolute pressure: it is the nap atmospheric pressures and hydrostatic

Related articles

• partial Pressure

• Hydraulic
• Driving hydraulics
• Hydraulic pump
• Jack

External bonds

• Conversion of units of pressure
• Conversion of all units

Simple: Press

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