Pressure, a physical quantity characterising the intensity of the normal (perpendicular to the surface) forces with which one body acts on the surface of another (for example, the foundation of a building on the ground, a liquid on the walls of a vessel, the gas in the cylinder of an engine on the piston). If the forces are distributed uniformly over the surface, then the pressure p on any part of the surface is equal to: p=F/S, where S is the area of that part and F is the sum of the forces applied perpendicular to it. When the forces are distributed non-uniformly, this equality defines the mean pressure on the given area, and in the limit, as S tends to zero,— the pressure at the given point.
For a continuous medium the notion of “Pressure” at each point of the medium is introduced in the same way. At any point of a liquid or gas at rest the pressure is the same in all directions; this also holds for a moving liquid or gas if it can be regarded as ideal (free of friction). In a viscous liquid, the pressure at a given point is taken to mean the mean value of the pressure over three mutually perpendicular directions. According to the kinetic theory of gases, the pressure in a gaseous medium is connected with the transfer of momentum in the collisions of the gas molecules, which are in thermal motion, with one another or with the surface of the bodies bounding the gas. The pressure in gases (which may be called thermal) is proportional to the temperature (to the kinetic energy of the particles, (see Gas).
Pressure is measured with manometers, barometers and vacuum gauges, and also with various pressure sensors. The units of pressure have the dimension of force divided by area: in the International System of Units the unit of pressure is —1 Pa=N/m2, in the MKGFS system of units — 1 kgf/cm2. There are units of pressure outside the systems: the standard atmosphere (atm), the technical atmosphere (at), the bar, and also the millimetre of water and the millimetre of mercury (the torr). On the physical nature of the pressure of waves (sound, shock, electromagnetic) see the articles (see Acoustic radiation pressure, Shock wave, Light pressure).
For a continuous medium the notion of “Pressure” at each point of the medium is introduced in the same way. At any point of a liquid or gas at rest the pressure is the same in all directions; this also holds for a moving liquid or gas if it can be regarded as ideal (free of friction). In a viscous liquid, the pressure at a given point is taken to mean the mean value of the pressure over three mutually perpendicular directions. According to the kinetic theory of gases, the pressure in a gaseous medium is connected with the transfer of momentum in the collisions of the gas molecules, which are in thermal motion, with one another or with the surface of the bodies bounding the gas. The pressure in gases (which may be called thermal) is proportional to the temperature (to the kinetic energy of the particles, (see Gas).
Pressure is measured with manometers, barometers and vacuum gauges, and also with various pressure sensors. The units of pressure have the dimension of force divided by area: in the International System of Units the unit of pressure is —1 Pa=N/m2, in the MKGFS system of units — 1 kgf/cm2. There are units of pressure outside the systems: the standard atmosphere (atm), the technical atmosphere (at), the bar, and also the millimetre of water and the millimetre of mercury (the torr). On the physical nature of the pressure of waves (sound, shock, electromagnetic) see the articles (see Acoustic radiation pressure, Shock wave, Light pressure).