Pressure

Pressure is a fundamental physical concept used to describe how force is distributed. Specifically, pressure is the force applied perpendicular to a surface per unit area. Scientists use the symbol p or P to represent this value. The IUPAC recommends using a lower-case p for pressure. However, many researchers use an upper-case P depending on their specific field of study. Pressure is a scalar quantity, which means it has no specific direction. While the force acting on a surface has a direction, the pressure itself does not.

The relationship between force, area, and pressure is a matter of concentration. Mathematically, pressure is calculated by dividing the magnitude of the normal force by the area of the contact surface. If you apply the same force to a larger area, the pressure decreases. If you apply that same force to a much smaller area, the pressure increases significantly. Consider a thumbtack versus a finger. Pushing a finger against a wall applies force, but the area is large, so the pressure is low. A thumbtack concentrates that same force onto a tiny point. This creates enough pressure to damage the surface.

Pressure can be measured in many different ways depending on the scientific discipline. The International System of Units (SI) uses the pascal (Pa) as the standard unit. One pascal is defined as one newton of force per square metre. In the United States, the imperial system often uses pounds per square inch (psi). Other common units include the bar and the decibar. Meteorologists often use the hectopascal (hPa) to measure atmospheric air pressure. This is the same as the older unit called the millibar (mbar).

Another way to express pressure is through manometric units. These units measure the height of a column of fluid in a device called a manometer. Common choices for these fluids are water or mercury. Mercury is often preferred because it is very dense. A high density allows for a shorter column to measure a given pressure. This makes the measuring tools smaller and easier to use. One common manometric unit is the torr, which is approximately equal to one millimetre of mercury.

In specific environments, scientists use specialized units to ensure accuracy. Oceanographers often measure underwater pressure in decibars (dbar). This is useful because pressure in the ocean increases by roughly one decibar for every metre of depth. Divers use units like metres of sea water (msw) or feet of sea water (fsw). These units help them manage pressure exposure and safety in diving chambers. It is important to note that 10 msw is not exactly the same as a linear metre of depth. Instead, 1 msw is defined as 0.1 bar, or 10,000 Pa.

There are different ways to report pressure values, such as gauge and absolute pressure. Gauge pressure is the pressure measured relative to the ambient atmospheric pressure. For example, a car tire might be listed at 32 psi gauge. This means the pressure inside is 32 psi higher than the air outside. Absolute pressure, however, is the total pressure including the atmosphere. When calculating properties like gas density, scientists must use absolute pressure. Using gauge pressure alone can lead to incorrect calculations of how dense a gas is.

At a microscopic level, pressure in a gas comes from constant motion. Even in a gas that looks still, individual molecules are moving randomly in all directions. When these molecules are confined in a container, they collide with the walls. These constant collisions create the force that we measure as pressure. On a much larger scale, planets have atmospheres because of gravity. The gravitational pull of a planet creates a pressure gradient. This force balances the outward push of the gas, preventing the atmosphere from escaping into space.