pressures

Pressure is the force exerted per unit area on a surface. It is a scalar quantity that describes how concentrated external forces are over a given area. In fluids, pressure acts isotropically, pushing equally in all directions; in solids the related concept is stress, of which pressure is the uniform normal component.

The SI unit of pressure is the pascal (Pa), equal to one newton per square meter. Commonly used units include the bar (1 bar = 100,000 Pa), the atmosphere (1 atm ≈ 101,325 Pa), and the torr or millimeter of mercury (1 atm ≈ 760 Torr). The defining relation is P = F/A, where F is the normal force and A is the area over which it acts.

Pressure can be categorized in several ways. Atmospheric pressure is the pressure exerted by the Earth's atmosphere on surfaces. Gauge pressure is the pressure relative to atmospheric pressure, while absolute pressure is the total pressure, including atmospheric. Thus p_absolute = p_gauge + p_atm. In fluids, hydrostatic pressure increases with depth according to p = ρ g h, where ρ is fluid density, g is gravitational acceleration, and h is depth.

Measurement techniques include barometers for atmospheric pressure, and manometers or gauge instruments for pressure differences. Devices such as bourdon tubes, diaphragms, and capacitive sensors convert pressure changes into mechanical displacement or electrical signals.

Pressure is central to many principles and applications. Pascal's law states that in a closed incompressible fluid, pressure is transmitted undiminished in all directions, underpinning hydraulic systems. In weather, pressure patterns influence storms and wind. In physiology, blood pressure indicates arterial force. In engineering, pressure ratings determine the design and safety of vessels, pipes, and filtration systems.