What is the Difference Between Air Pressure and Liquid Pressure

The key difference between air pressure and liquid pressure is that air pressure allows the gaseous state of matter to be compressible, whereas liquid pressure makes a liquid incompressible.

Liquid pressure is the pressure that we can observe in a liquid. Air pressure is also known as atmospheric pressure, and it is the pressure as the force exerted by the collisions of particles in the air.

1. Overview and Key Difference
2. What is Air Pressure
3. What is Liquid Pressure
4. Air Pressure vs Liquid Pressure in Tabular Form
5. Summary

What is Air Pressure?

Air pressure is also known as atmospheric pressure, and it is the pressure as a force exerted by the collisions of particles in the air. It is important to understand the concept of pressure in order to understand atmospheric pressure. We can define pressure as the force per unit area applied perpendicularly on a surface. The pressure of a static fluid is equal to the weight of the fluid column above the point we measure the pressure. Therefore, the pressure of a static (non-flowing) fluid depends only on the density of the fluid, gravitational acceleration, atmospheric pressure, and the height of the liquid above the point the pressure is measured.

Moreover, we can define pressure as the force exerted by the collisions of particles. In this sense, we can calculate pressure using the kinetic molecular theory of gasses and the gas equation. Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth’s atmosphere.

What is Liquid Pressure?

Liquid pressure is the pressure that we can observe in a liquid. This type of pressure can act equally in all directions. Further, the liquid pressure is not affected by the shape, size, and surface area of the liquid. When considering two points at the same depth of the same liquid, we can say that the liquid pressure is equal at those two points. However, the liquid pressure depends on the depth of the point we are going to measure the pressure from the liquid’s surface. For example, the deeper the point of measurement goes, the higher the liquid pressure becomes.

For example, we can observe that the bubble coming from deep down a liquid get bigger upon rising to the surface of the sea. This is mainly because the pressure at the deep down of the liquid is high, and upon rising upwards towards the liquid surface, the pressure gradually decreases, allowing the bubble to get bigger than it was at a depth.

We can determine the liquid pressure using the simple equation: liquid pressure = pressure of liquid + atmospheric pressure, given mathematically as follows:

P = P_atm + pgh

Where P is liquid pressure, P_atm is atmospheric pressure, p is the density of the liquid, g is the gravitation, and h is the depth to the point of measurement from the surface of the liquid.

There are different applications of liquid pressure, including public water supply systems where a reservoir is placed at a location that is elevated compared to lower ground, which allows it to have sufficient pressure to flow to the consumers at the ground level. Similarly, dams are constructed in a way that the broad and thicker base of the dam can withstand high water pressure. Another important application is the infusion of medicine to a patient where the medicine bottle is placed at an elevated position so that the liquid inside that bottle has sufficient pressure to flow down towards the patient.

What is the Difference Between Air Pressure and Liquid Pressure?

Liquid pressure is the pressure we can observe in a liquid. Air pressure or atmospheric pressure is the pressure exerted by the collisions of particles in the air. The key difference between air pressure and liquid pressure is that air pressure allows the gaseous state of matter to be compressible, whereas liquid pressure makes a liquid incompressible.

The following table summarizes the difference between air pressure and liquid pressure.

Summary – Air Pressure vs Liquid Pressure

Liquid pressure is the pressure that we can observe in a liquid. Air pressure or atmospheric pressure is the pressure as the force exerted by the collisions of particles in the air. The key difference between air pressure and liquid pressure is that air pressure allows the gaseous state of matter to be compressible, whereas liquid pressure makes a liquid incompressible.

Reference:

1. Vyvian Leow. “Pressure in Liquid.” SlideShare.

Image Courtesy:

1. “Aircraft altimeter” By User:CambridgeBayWeather – Own work (Public Domain) via Commons Wikimedia
2. “Plastic bottle at 14000 feet, 9000 feet and 1000 feet, sealed at 14000 feet” By Quantockgoblin – Own work (Public Domain) via Commons Wikimedia