Today we continue with Physics, and more specifically the branch of "Classical Mechanics", in order to start with a new chapter : "Fluid Mechanics". This article will be a small introduction to Fluids, and Density and Pressure.
So, without further ado, let's get straight into it!
Fluids and Fluid Mechanics
In Physics, any substance that "flows" is considered a Fluid. Thus, both the liquid and the gas state of matter is considered a fluid.
The study of the motion of such fluids is known as Fluid Mechanics. This branch of Physics can further be divided into Fluid Statics, which studies fluids at rest, and Fluid Dynamics, which studies fluids in motion.
The first sub-branch covers simpler concepts such as density, pressure, and Pascal's and Archimedes' principles, whilst the second one is one of the most complicated branches of Mechanics! Therefore, we will only get into concepts that are based on idealized models (such as Newton's Laws and Conservation of Energy), which are easier to comprehend.
Density
Before we can get into the more interesting topics, we first have to understand the basics. Let's start off with density...
Density is a characteristic of substances, which is defined as the mass per unit volume. It's denoted by the Greek letter ρ. The S.I. unit of density is the Kg / m3.
So, mathematically, density can be expressed as:
The density isn't always constant throughout the volume of a substance. Only a homogeneous substance has the same density across it's volume.
Local density can be obtained using a limit, where the volume approaches zero:
The Effects of Temperature and Pressure
The density of gases is much less than the density of liquids and solids. Additionally, density is also affected by temperature. In the case of gases it's also affected by pressure.
The densities of gases are commonly displayed at standard temperature (0° C) and standard pressure (101.3 kPa), whilst solids and liquids are displayed at standard temperature (0° C). The density usually increases with the decrease of temperature.
Specific Gravity
In order to make it easier to compare the density of various substances, which is a dimensional property (units must be taken into consideration), we define a quantity which is dimensionless. This quantity is known as specific gravity, and is defined as the ratio of the density of the substance by the density of water at 4° C and 1 atm of pressure. The density of water at these conditions is exactly 1000 Kg / m3 or 1 g / cm3.
For example, in the case of aluminum, which has a density of 2.7 g / cm3, its specific gravity is 2.7 which is much easier to work with.
Pressure
Next up is the topic of pressure...
Pressure is the normal force per the unit area over which the force is applied. So, it's the force perpendicular to the area divided by the area itself:
The pressure at a specific point is defined as:
Pressure is a scalar and not a vector unit. The S.I. unit is the pascal (Pa), which is a shorter way of saying N / m2. Other common units are the bar, which equals 105 Pa and the millibar, which equals 100 Pa.
Atmospheric Pressure
Atmospheric pressure, pa, is the pressure exerted by the atmosphere of the Earth. Its value depends on the weather, height etc.
A standard atmosphere, denoted by atm, is a unit of pressure, which equals about 1.013 x 105 Pa = 1.013 bar.
