The ratio of force to area. Atmospheric pressure at the surface of Earth is in the vicinity of 15 lbf/in.2 (1.0 × 105 Pa). Pressures in enclosed containers less than this value are spoken of as vacuum pressures; for example, the vacuum pressure inside a cathode-ray tube is 10-8 mmHg, meaning that the pressure is equal to the pressure that would be produced by a column of mercury, with no force acting above it, that is 10-8 mm high. This is absolute pressure measured above zero pressure as a reference level. Inside a steam boiler, the pressure may be 800 lbf/in.2 (5.5 × 106 Pa) or higher. Such pressure, measured above atmospheric pressure as a reference level, is gage pressure, designated psig. See Pressure measurement
a physical quantity characterizing the intensity of normal forces (perpendicular to the surface) with which one body acts on another’s surface (for example, the foundations of a building acting on the ground, a liquid acting on the walls of a vessel, and gas in the cylinder of a motor acting on the piston). If the forces are distributed uniformly over the surface, then the pressure ρ on any part of the surface i s p = F/S, where 5 is the area of the part and F is the sum of the forces applied perpendicular to it. If the distribution of forces is nonuniform, this equality gives the mean pressure on the given small area, whereas at the limit, with S tending toward zero, it gives the pressure at a given point. If the distribution of forces is uniform, the pressure at all points of the surface is the same; if the distribution is nonuniform, the pressure varies from point to point.
For a continuous medium, the concept of pressure at each point in the medium is similarly introduced; it plays an important part in the mechanics of liquids and gases. At any point in a quiescent liquid the pressure in all directions is the same;
| Table 1. Conversion of units of pressure | ||||||
|---|---|---|---|---|---|---|
| Nlm2 | bar | kgflcm2 | atm | mm Hg | mm H20 | |
| 1 N/m2(Pascal)................. | 1 | 10-5 | 1.01972 x 10-5; | 0.98692 x 10-5 | 750.06 x 10-5 | 0.101972 |
| 1 bar = 106dynes/cm2................. | 105 | 1 | 1.01972 | 0.98692 | 750.06 | 1.01972 x 104 |
| 1 kgf/cm2 = 1 at................. | 0.980665 x 105 | 0.980665 | 1 | 0.96784 | 735.56 | 104 |
| 1 atm................. | 1.01325 x 105 | 1.01325 | 1.0332 | 1 | 760 | 1.0332 x 104 |
| 1 mm Hg (torr)................. | 133.322 | 1.33322 x 10-3 | 1.35951 x 10-3 | 1.31579 x10--3 | 1 | 13.5951 |
| 1 mm H20................. | 9.80665 | 9.80665 x 10-5; | 10-4 | 9.67841 x 10-5 | 7.3556 x 10-4 | 1 |
this is true also of moving liquids or gases, if they may be considered ideal (frictionless). In a viscous liquid the value of the mean pressure for three mutually perpendicular directions is taken to be the pressure at a given point.
Pressure plays an important part in physical, chemical, mechanical, and biological phenomena.
S. M. TARG
In a gaseous medium pressure is associated with the transfer of momentum during collisions of thermally moving gas molecules with each other or with the surface of bodies adjacent to the gas. The pressure in gases, which may be called thermal, is proportional to the temperature (the kinetic energy of the particles). In condensed mediums (liquids and solids), unlike gases, in which the mean distances between randomly moving particles are much greater than the size of the particles themselves, interatomic distances are comparable to atomic dimensions and are determined by the equilibrium of interatomic (intermolecular) forces of repulsion and attraction. When atoms approach one another repulsion forces increase, bringing about so-called cold pressure. In condensed mediums the pressure also has a “thermal” component, which is associated with the thermal vibrations of the atoms (nuclei). Given a steady or diminishing volume of a condensed medium, the thermal pressure rises as the temperature increases. At temperatures of about 104 ° K or more, thermal excitation of electrons makes an appreciable contribution to the thermal pressure.
Pressure is measured with manometers, barometers, and vacuometers, as well as with various pressure sensors.
Units of pressure have the dimensions of force divided by area. In the International System of Units, the unit of pressure is the newton per sq m (N/m2); in the Mks system, it is the kilogram-force per sq cm (kgf/cm2). Subsidiary units of pressure also exist—for example, the physical atmosphere (atm), the technical atmosphere (at), the bar, and mm of water and mercury columns (torr), by means of which the pressure measured is compared with the pressure of a column of liquid (water or mercury). (See Table 1.)
In the USA and Great Britain pressure is expressed in pounds-force per square inch (lbf/in.2), poundals per square foot (pdl/ft2), inches of water (in. H20), feet of water (ft H20), and inches of mercury (in. Hg); 1 lbf/in.2 = 6,894.76 N/m2; 1 pdl/ft2 = 1.48816 N/m2; 1 in. H20 = 249.089 N/m2; 1 ft H20 =2,989.07 N/m2; 1 in. Hg = 3,386.39 N/m2.
L. D. LIVSHITS