In this article, we’re going to concentrate solely on the unit Pascal (Pa), as it’s the standard unit accepted by the International System of Units (SI).
History of Pascals
Pa units are named after Blaise Pascal, a 17th-century mathematician and physicist. He was an important pioneer and contributor in both fields, including his many experiments with barometers. It was decided to use his name as the SI unit for pressure at the 1971 General Conference on Weights and Measures.
One Pascal unit is defined, in base units, as 1 kilogram per metre second squared (1kg/ms2) or 1 newton per metre squared (1N/m2).
In layman’s terms, it measures the pressure applied by one newton of force acting at a right angle on an area of one square metre.
The Pascal unit is often used by scientists, engineers and technicians to measure pressure, replacing PSI in countries that have adopted the metric system. Exceptions are countries where Imperial measurement is still in use, such as the US, or countries that commonly use both, such as the UK.
It can also be used to measure stress, for example, gigapascals are frequently used by Geophysicists to study tectonic stresses in the Earth’s plates.
Material scientists and engineers use Pascals in the measurement of stiffness and tensile and compressive strengths of materials.
The unit hectopascals (1hPa=100Pa) is used to measure meteorological air-pressure.
Pressure is measured in Pascals. As mentioned earlier, it is a measure of the pressure exerted by force on a specific area.
The formula F=A can be used to represent the proportionality of Force (F), and Area (A).
This can be rearranged to:
Let’s take a look at a couple of examples that neatly demonstrate the application of the pressure formula.
Imagine a man is pressing his thumb against a wall with a force of 200N. The surface area of his thumb that is contact with the wall is 0.0004m2. To calculate the pressure:
= F/A = 200/0.0004 = 500 000 Pa or 500 kPa (kilopascal)
Now let’s imagine using the same force to push a drawing pin or tack into the same wall. This time the surface area in contact with the wall is just the head of the pin, a tiny 0.0000004m2. If we perform the calculation again we get:
= F/A = 200/0.0000004 = 50 000 000 Pa or 50 MPa (megapascal)
This explains why pushing a drawing pin into a wall will often penetrate the wall when a moderate human pushing force is applied, whereas your thumb won’t.
Here are some common conversions for frequently used units of pressure:
- 1 pascal = 0.00001 bar
- 1 pascal = 0.00014503773801 pound force/square inch (psi)
- 1 pascal = 0.0000098692316931 atmosphere (atm, standard)
- 1 atm = 101325 Pa or 101.325 kPa. You’ll often find this value as a reference pressure. For example, it’s used in some national and international standards, including ISO 5024, 2533 and 2787.
Pascals are often used as the units of measure for Young’s Modulus, which is a numerical constant describing the elasticity of solid materials undergoing tension or compression. The higher the value, the less elastic the material is.
For instance, nylon has a Young’s modulus of around 2–4 GPa, whereas diamond has a Young’s modulus of 1220 GPa.
A notable contribution made by Pascal to the physical sciences is Pascal’s law. It is a fluid mechanics principle stating that a change in pressure anywhere in the fluid, creates the same change everywhere within that body of fluid. This helped in the development of hydraulic systems.