Pressure - Unit, Definition, Measurement, & Types

Pressure

Pressure is defined as the force experienced by an object, acting in a perpendicular direction to the surface of the object per unit area of it. It is denoted by using the alphabets ‘P’ or ‘p’. Pressure is given by the formula.

P = F/ A

where

• P = the pressure experienced by the object
• F = the force acting perpendicular to the object
• A = the area over which the pressure is experienced

Pressure is a scalar quantity as it is measured only in terms of its magnitude, and there is no direction element to it. Even if the object's orientation is changed, only the direction in which the force acts on the object is changed accordingly, while the pressure remains the same. The dimension of pressure is [M L −1 T −2 ], and it is a physical quantity.

The area on which the force is acting greatly determines the pressure applied on the object. For example, if a knife is held by the blade and the handle is used to press on an object (like a vegetable) in a flat direction, the knife does not cut through the object. However, if the sharp edge of the knife’s blade is placed on the object and the same force is applied, the knife easily cuts through the object. This is because, even though the applied force is the same, the area of the knife in contact with the object is less when the sharp edge of the knife’s blade is used. This makes the applied force act concentrated on less area, bringing about more impact on the object and cutting it through.

Units of Pressure

Several different units are used to express pressure, and most of them are derived from a unit of force divided by a unit of area. The primary unit of pressure is its SI unit pascal (Pa), where 1 Pa is equal to 1 newton per square meter (n/m 2 ). The psi (pound force per square inch), which was the traditional unit of pressure, is still in use in the imperial and U.S. customary systems. Even though the SI units of the system advises against it, pressure is even at times expressed in gram force per square centimeter (g/cm2) or kilograms force per square centimeter (kg/ cm2).

When a system is under pressure, it tends to do a degree of work and owing to this reason; pressure can also be expressed in terms of potential energy stored per unit volume. Hence, it is related to energy density and can be denoted using units like joules per cubic meter (J/ m3). The pressure value at sea level on the Earth is given by the fixed value of standard atmosphere (atm), and its value is given as 1 atm = 101325 Pa.

Pascal: The SI unit of pressure, pascal (Pa) is used to measure the internal pressure in a container, stress experienced by an object, Young's modulus of an object or material, and the ultimate tensile strength of a material. The unit is named after Blaire Pascal, a French physicist.

1 Pa = 1 N/ m² = 1 kg/ ms² = 1 J/ m³

where

• Pa stands for pascal (pressure)
• N stands for newton (force)
• m stands for meter (distance)
• kg stands for kilogram (weight)
• s stands for seconds (time)
• J stands for joule (energy)

Pound force per square inch: It is defined as the pressure experienced when one pound of force is applied to an area of one square inch. It is stylized as psi. There are different multiples of psi that are used in certain fields.

1 ksi = 6.895 MPa (or alternatively) 1 MPa = 0.145 ksi

1 Mpsi = 6.895 GPa (or alternatively) 1 GPa = 0.145 Mpsi

where

• ksi stands for kilo pound per square inch (1000 lbf/ in 2 )
• MPa stands for mega pascal
• Mpsi stands for mega pound per square inch
• GPa stands for giga pascal

Standard atmosphere: Used as a reference pressure or standard pressure, it is defined as the Earth's atmospheric pressure value at sea level.

1 atm = 101325 Pa 

The pressure value of the standard atmosphere can also be expressed in terms of other units as shown below. 1 atm

• ≡ 1.01325 bar
• ≈ 1.033 kgf/ cm 2
• ≈ 1.033 technical atmosphere
• ≈ 10.33 m H 2 O at 4 °C
• ≡ 760 torr (Torr)
• ≈ 29.92 in Hg at 0 °C
• ≈ 406.782 in H 2 O at 4 °C
• ≈ 14.6959-pound force per square inch (lbf/in2)
• ≈ 2116.22-pound force per square foot (lbf/ft2)
• = 1 ata (atmosphere absolute)