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Blood Pressure
Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface. Mathematically: more...
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where:
- p is the pressure
- F is the normal force
- A the area.
Pressure is a scalar, and has SI units of pascals, 1 Pa = 1 N/m2.
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.
Example
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, pressure is defined as a scalar quantity.
The gradient of pressure is called the force density.
For gases, pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are called gauge pressure (also often spelled gage 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 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, 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.
Gauge pressure is a critical 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, at an altitude of 112 m, the mean atmospheric pressure is 100 kPa. At this altitude, a pressure vessel containing any gas (such as helium) at 200 kPa-gauge (300 kPa-absolute) is 50% more dense than at 100 kPa-gauge (200 kPa-absolute); not double the density as one might assume by focusing on gauge values.
Read more at Wikipedia.org
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