Pressure

Pressure is the force exerted per unit area on a surface. In physics it is a scalar quantity describing how a force is distributed over an area. Its SI unit is the pascal (Pa), equal to one newton per square meter. Larger pressures are measured in kilopascals (kPa) or megapascals (MPa), and in meteorology and engineering other units such as bar or atmosphere are commonly used.

In fluids at rest, pressure increases with depth due to the weight of the overlying fluid. The hydrostatic pressure at depth h is P = P0 + ρ g h, where P0 is the pressure at the surface, ρ is density, g is gravitational acceleration.

There are different types of pressure: absolute pressure P_abs is measured relative to a vacuum; gauge pressure P_g = P_abs − P_atm measures relative to atmospheric pressure; atmospheric pressure is about 101.3 kPa at sea level.

Measurement devices: barometer measures atmospheric pressure; manometer and U-tube measure gauge or differential pressures; Bourdon tube and diaphragm sensors convert pressure to a mechanical or electrical signal.

Applications and relations: force equals pressure times area (F = P A). In fluids, Pascal's principle states that pressure applied to an enclosed incompressible fluid is transmitted undiminished. For gases, the ideal gas law relates pressure to volume and temperature: P V = n R T.

Special contexts: acoustic pressure p represents fluctuations in pressure due to sound; sound pressure level quantifies it. In physiology, blood pressure measures arterial pressure. Vacuum and negative gauge pressures occur when P_abs < P_atm.

Units and conversions: 1 bar = 100 kPa; 1 atm = 101,325 Pa; common gauge pressures are given in kPa or bar.