In science, pressure is a measurement of the force per unit area. The SI unit of pressure is the pascal (Pa), which is equivalent to N/m2 (newtons per meter squared).
If you had 1 newton (1 N) of force distributed over 1 square meter (1 m2), then the result is 1 N/1 m2 = 1 N/m2 = 1 Pa. This assumes that the force is directed perpendicularly toward the surface area.
If you increased the amount of force but applied it over the same area, then the pressure would increase proportionally. A 5 N force distributed over the same 1 square meter area would be 5 Pa. However, if you also expanded the force, then you would find that the pressure increases in an inverse proportion to the area increase.
If you had 5 N of force distributed over 2 square meters, you would get 5 N/2 m2 = 2.5 N/m2 = 2.5 Pa.
A bar is another metric unit of pressure, though it is not the SI unit. It is defined as 10,000 Pa. It was created in 1909 by British meteorologist William Napier Shaw.
Atmospheric pressure, often noted as pa, is the pressure of the Earth's atmosphere. When you are standing outside in the air, the atmospheric pressure is the average force of all of the air above and around you pushing in on your body.
The average value for the atmospheric pressure at sea level is defined as 1 atmosphere, or 1 atm. Given that this is an average of a physical quantity, the magnitude may change over time based on more precise measurement methods or possibly due to actual changes in the environment that could have a global impact on the average pressure of the atmosphere.
- 1 Pa = 1 N/m2
- 1 bar = 10,000 Pa
- 1 atm ≈ 1.013 × 105 Pa = 1.013 bar = 1013 millibar
How Pressure Works
The general concept of force is often treated as if it acts on an object in an idealized way. (This is actually common for most things in science, and particularly physics, as we create idealized models to highlight the phenomena we way to pay specific attention to and ignore as many other phenomena as we reasonably can.) In this idealized approach, if we say a force is acting on an object, we draw an arrow indicating the direction of the force, and act as if the force is all taking place at that point.
In reality, though, things are never quite that simple. If you push on a lever with your hand, the force is actually distributed across your hand and is pushing against the lever distributed across that area of the lever. To make things even more complicated in this situation, the force is almost certainly not distributed evenly.
This is where pressure comes into play. Physicists apply the concept of pressure to recognize that a force is distributed over a surface area.
Though we can talk about pressure in a variety of contexts, one of the earliest forms in which the concept came into discussion within science was in considering and analyzing gases. Well before the science of thermodynamics was formalized in the 1800s, it was recognized that gases, when heated, applied a force or pressure onto the object that contained them. Heated gas was used for levitation of hot air balloons starting in Europe in the 1700s, and the Chinese and other civilizations had made similar discoveries well before that. The 1800s also saw the advent of the steam engine (as depicted in the associated image), which uses the pressure built up within a boiler to generate mechanical motion, such as that needed to move a riverboat, train, or factory loom.
This pressure received its physical explanation with the kinetic theory of gases, in which scientists realized that if a gas contained a wide variety of particles (molecules), then the pressure detected could be represented physically by the average motion of those particles. This approach explains why pressure is closely related to the concepts of heat and temperature, which are also defined as motion of particles using the kinetic theory. One particular case of interest in thermodynamics is an isobaric process, which is a thermodynamic reaction where the pressure remains constant.
Edited by Anne Marie Helmenstine, Ph.D.