Pression

Pressure, termed pression in several languages, is the force that a surface experiences per unit area. In fluids and gases, pressure is typically transmitted equally in all directions and is described by Pascal's principle when the fluid is at rest. Pressure is a scalar quantity in such contexts, though the total stress in a solid body includes directional components as well.

Units and scales: The SI unit is the pascal (Pa), equal to one newton per square meter. Larger commonly used units include kilopascals (kPa), bar (1 bar = 100 kPa), and atmospheres (1 atm ≈ 101,325 Pa). In engineering, pounds per square inch (psi) is also used. The pressure measured relative to a vacuum is called absolute pressure; gauge pressure uses the surrounding ambient pressure as reference.

Types: Absolute, gauge, and differential pressure. Vacuum conditions correspond to negative gauge pressure. In fluids the hydrostatic pressure increases with depth: P = ρ g h. Dynamic pressure is given by q = 1/2 ρ v^2 in flowing fluids.

Measurement: Devices include manometers, bourdon tube gauges, piezoresistive sensors, and capacitive transducers. Techniques vary with range, temperature, and environment.

Applications and contexts: Atmospheric pressure governs weather and altitude effects. In medicine, blood pressure is reported as systolic/diastolic and measured in millimeters of mercury. In engineering, pressure is central to hydraulics, pneumatics, and the design of pressure vessels, piping, and aerodynamics. In materials science, pressure contributes to stress states, including hydrostatic pressure in liquids and compressive stresses in solids.