What is Pressure?

The pressure of a substance is defined as the force per unit area applied to a given surface. Pressure is usually measured in bar, pascal, pounds per square inch (psi), or atmospheres (atm). It is one of the most important physical properties of a substance because it plays a major role in a variety of scientific and engineering applications. Pressure is also important in terms of thermodynamics, heat transfer, and fluid mechanics.

Pressure in Gas Form

When a gas is compressed, the number of molecules per volume decreases and the individual pressure created by the molecules increases. This is known as an increase in pressure. Conversely, when a gas is allowed to expand, the number of molecules per volume increases and the individual pressure created by each molecule decreases. This is known as a decrease in pressure. Generally, the greater the volume of the gas, the lower the pressure and the smaller the volume, the higher the pressure.

Pressure in Liquid Form

Pressure in liquids can be thought of as the sum of the forces exerted by molecules of the liquid on all surfaces in contact with the liquid, such as the walls of the container or pipe. The pressure in a liquid is usually high and uniform, since the molecules are very close together and interact strongly with each other. Pressure in liquids increases with depth because of the gravity-induced pressure gradient. This phenomenon is called hydrostatic pressure.

Pressure in Solid Form

Pressure in solids can be thought of as the total force exerted by the molecules in the solid on all of the surfaces that are in contact with the solid. Pressure in solids varies depending on the material and the circumstances, but it is usually much higher than in liquids and gases. For example, the pressure of a diamond is very high, since the individual molecules interact strongly with each other and are very close together, causing a great deal of force to be exerted on all of the surfaces in contact with the diamond.

In summary, pressure is the force per unit area applied to a given surface. It is an important physical property of a substance because it plays a role in many engineering, scientific, and thermodynamic applications. Pressure can vary depending on the form of the substance, and the pressure of a gas or liquid generally increases with depth. Pressure in solids can also vary depending on the material, but it is usually much higher than in liquids or gases.