Pressure
Pressure (symbol:
p) is a measure of
force per unit
area.
The
SI unit for pressure is the
pascal (Pa), equal to one
newton per
square metre (N·m
-2 or kg·s
-2·m
-1).\nNon--SI measures (still in use in some parts of the world) include the
pound-force per square
inch (
PSI) and the
bar.\nIn the
United States air pressure is still measured in inches of
mercury (as in the mercury
barometer). Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, because it gives the same numbers as the older millibar (mbar).
Again,\n:,
where
p equals pressure,
F equals
force, and
A equals
area. Often
F, is taken to be the of the magnitude of the mean vector force normal to the surface of area
A upon which it exerts; the "surface" not necessarily being a that of a body, but for example the cross sectional area of a conduit.
Pressure is sometimes measured not as an absolute pressure, but as the excess of that pressure above
atmospheric pressure, sometimes called
gauge pressure. \nAn example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure. \nIn technical work, this would be written as "30 PSIG" or, more commonly, "30 psig".
"Pressure is a scalar quantity, but teachers and authors do\nnot appear to believe this in their hearts."\n(McClelland, 1987)
The
standard atmosphere (atm) is a curious
unit of pressure, defined to six figures of precision to approximate a reality that varies constantly from place to place and moment to moment. It is approximately equal to typical air pressures at sea level and defined to be\n:1 atm = 101 325
Pa = 101.325 kPa = 1013.25 hPa. k = kilo and h = hecto.
Obsolete
manometric units of pressure such as inches or millimeters of mercury are based on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity. \nThey are effectively attempts to define a unit for expressing the readings of a
manometer.
Manometric pressure units should no longer be used for scientific or engineering purposes, due to the lack of repeatability inherent in their definitions.
Static pressure is the pressure due to the density and depth of a fluid.
Stagnation pressure is the pressure due to the velocity of a fluid, and is defined to include
static pressure. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid.
The pressure of a moving fluid can be measured using a Pitot probe, or one of its varations 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.
The
force density f (= ∂
F/∂
V) is equal to the
gradient of the pressure: ; if it concerns the gravitational force, the force density is the specific weight.
The
cgs unit of pressure is barye (ba). It is equal to 1 dyn·cm
-2.
\nPressure units and conversion factors
\n| | \nPascal | \nbar | \nN/mm2 | \nkp/m2 | \nkp/cm2 (=1 at) | \natm | \ntorr | \n
\n| 1 Pa (N/m2)= | \n1 | \n10-5 | \n10-6 | \n0.102 | \n0.102×10-4 | \n0.987×10-5 | \n0.0075 | \n
\n| 1 bar (daN/cm2) = | \n100,000 | \n1 | \n0.1 | \n10,200 | \n1.02 | \n0.987 | \n750 | \n
\n| 1 N/mm2 = | \n106 | \n10 | \n1 | \n1.02×105 | \n10.2 | \n9.87 | \n7,500 | \n
\n| 1 kp/m2 = | \n9.81 | \n9.81×10-5 | \n9.81×10-6 | \n1 | \n10-4 | \n0.968×10-4 | \n0.0736 | \n
\n| 1 kp/cm2 (1 at) = | \n98,100 | \n0.981 | \n0.0981 | \n10,000 | \n1 | \n0.968 | \n736 | \n
\n| 1 atm (760 torr) = | \n101,325 | \n1.013 | \n0.1013 | \n10,330 | \n1.033 | \n1 | \n760 | \n
\n| 1 torr (mmHg) = | \n133 | \n0.00133 | \n1.33×10-4 | \n13.6 | \n0.00132 | \n0.00132 | \n1 | \n
\n
Human body
In the human body, a baroreceptor is a pressure sensor. Arterial baroreceptors are located in each of the two internal carotid sinuses (carotid arteries, neck arteries) and in the aortic arch. They are part of a system, including the carotid sinus reflex, that regulates arterial blood pressure.
A microphone is a sensor too and our ear drums sense the pressure waves in air.
See also blood pressure.
See also
External links
Pressure can also be psychological, political, etc.; see also peer pressure.
Category:Physical quantity\nCategory:Diving\nCategory:Meteorology
\n \n \n \n \n \n \n \n \n
