The key difference between pressure of solids and liquids is that pressure of solids occurs only due to the weight of the solid, whereas pressure of liquid occurs due to both weight and movement of the liquid molecules.
Pressure is a very important concept in physics. The concept of pressure plays a very important role in applications such as thermodynamics, aerodynamics, fluid mechanics, and deformations. Thus, it is vital to have a good understanding of pressure in order to excel in any field that uses pressure as a base concept.
CONTENTS
1. Overview and Key Difference
2. What is Pressure of Solids
3. What is Pressure of Liquids
4. Side by Side Comparison – Pressure of Solids vs Liquids in Tabular Form
6. Summary
What is the Pressure of Solids?
The pressure of a solid arises due to the weight of the solid. We can interpret this pressure using the argument based on liquid pressure. The atoms inside a solid are static. Therefore, there is no creation of pressure by the momentum change of a solid. But the weight of the solid column above a certain point is effective on the said point. This creates pressure inside the solid.
However, solids do not expand or contract by large amounts due to this pressure. The pressure on the side of the solid, which is perpendicular to the weight vector, is always zero. Therefore, the solid has its own shape, unlike liquids, which take the shape of the container.
What is the Pressure of Liquids?
To understand the concept of the pressure of liquids, we must first understand the concept of pressure in general. The pressure of a static fluid is equal to the weight of the fluid column above the point of the pressure we measure. Therefore, the pressure of a static (non-flowing) fluid is dependent only on the density of the fluid, the gravitational acceleration, the atmospheric pressure and the height of the liquid above the point the pressure is measured. Also, we can define pressure as the force exerted by the collisions of particles. In this sense, we can calculate pressure using the molecular kinetic theory of gasses and the gas equation. The term “hydro” means water and the term “static” means non-changing. This means hydrostatic pressure is the pressure of the non-flowing water. However, this is also applicable to any fluid, including gasses.
Since hydrostatic pressure is the weight of the fluid column above the measured point we can give it in an equation as P= hdg, where P is the hydrostatic pressure, h is the height of the surface of the fluid form the measured point, d is the density of the fluid and g is the gravitational acceleration.
Figure 01: Pressure of a Liquid
The total pressure on the measured point is the union of the hydrostatic pressure and the external pressure (i.e. atmospheric pressure) on the fluid surface. The pressure due to a moving fluid varies from that of static fluid. We can use the Bernoulli theorem to calculate the dynamic pressure of non-turbulent incompressible fluids.
What is the Difference Between Pressure of Solids and Liquids?
The key difference between pressure of solids and liquids is that pressure of solids arise only due to the weight of the solid, whereas pressure of liquid arises due to both weight and movement of the liquid molecules. When calculating these pressures, we can calculate the pressure of solids using the weight of solid and the pressure of liquids using both the weight of liquid and movement of liquid molecules. When considering the shapes of solids and liquids, solid has a definite shape because the pressure on the side of the solid, which is perpendicular to the weight vector, is always zero while liquid gets the shape of the container because liquid pressure acts on the sides of the liquid as well as the bottom.
Summary – Pressure of Solids vs Liquids
The key difference between pressure of solids and liquids is that pressure of solids arise only due to the weight of the solid, whereas pressure of liquid arises due to both weight and movement of the liquid molecules.
Reference:
1. “Liquid Pressure – Physics Video by Brightstorm.” Brightstorm, Available here.