Pressure
Pressure is force per unit area, a scalar quantity.
Pressure is the force applied perpendicular to the surface of an object per unit area. It is a fundamental parameter in thermodynamics and is expressed in various units, including the pascal (Pa), which is the SI unit equal to one newton per square metre.
- symbol
- p or P
- SI unit
- pascal (Pa), equal to one newton per square metre (N/m²)
- common units
- pound-force per square inch (psi), atmosphere (atm), torr, millimetre of mercury, centimetre of water
- gauge pressure
- pressure relative to ambient pressure
- CGS unit
- barye (Ba), equal to 0.1 Pa
Lore & Background
Pressure is mathematically defined as p = F/A, where F is the magnitude of the normal force and A is the area of the surface in contact. It is a scalar quantity, meaning it has no direction; the force resulting from pressure has direction, but pressure itself does not. The IUPAC recommends a lower-case p, but upper-case P is also widely used depending on the field and context.
Reader's Guide
Pressure is a key concept in physics and engineering, measured in many units. The SI unit is the pascal (Pa), but other units like pounds per square inch (psi), atmosphere (atm), and manometric units such as millimetres of mercury remain common. Gauge pressure is measured relative to ambient pressure, often indicated by suffixes like 'g' (e.g., psig). Pressure is also related to energy density, expressed in joules per cubic metre. Its definition as a derivative of internal energy with respect to volume at fixed entropy and particle number underscores its role in thermodynamics. Manometric units, such as centimetres of water or millimetres of mercury, are based on the height of a fluid column in a manometer, though modern definitions may be expressed in SI terms.
Did You Know?
- Pressure is a scalar quantity and has no direction.
- Gauge pressure is often indicated by appending 'g' to the unit, such as 'psig'.
More in Classical Mechanics 1-21
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