In this screen cast we are going to introduce you to fluid mechanics by describing some
of the characteristics of a fluid.
We are going to start with what is a fluid.
So most people, if asked what a fluid is, think of liquids, such as water.
Other people think of both liquids and gases, however, the real definition is that a fluid
is something that deforms continuously and that is one of the important words, when acted
upon by another important word, a shear stress.
So let's start by defining that.
What is a shear stress.
So this is a tangential force which acts parallel to a surface.
So let's look at an example of that.
So let's say, here we have some fluid, the tangential force, or shear stress will act
in this direction on the fluid.
And we call this shear stress tao.
So let's see why it is important that we call it continuous.
Pretty much everything deforms, however a solid deforms to a certain point and then
it stops.
In contrast a fluid deforms continuously.
And what we call the deformation, is just flow.
So why is this?
Why do fluids deform continuously while solids don't?
And the reason for that is what is known as molecular spacing.
In other words the distance between molecules.
And if you look at the difference between the spacing in solids versus liquids and gases
you can see that the molecular spacing is quite different.
Let's start with a solid.
So here you can see the closely packed structure of the molecules in a solid.
However, now let's look at a liquid or a gas.
So as we saw in the closely packed structure of the solid the molecules are really not
free to move.
However, here, we have a liquid and you can see these intermolecular spaces which allows
the fluid molecules to move more easily.
And you can see this even more so in a gas where there are large spaces between the molecules
making the ability for these molecules to move even greater.
In addition because of this spacing or in terms of the solid, lack of spacing, the intermolecular
forces between the molecules and fluids are smaller.
Again leading to more ease of movement.
So let's sum up the differences.
We will start with deform continuously.
Again, this is how we define a fluid.
A solid does not deform continuously, however, a liquid or a gas does.
Next we will look at large molecular spacing.
So again if we look at the pictures what we see in these three diagrams is that solids
do not have large molecular spacing, however, liquids do and gases do even more.
We talked about small intermolecular forces, again caused by this spacing.
And what we find is that solids have larger intermolecular forces, whereas liquids, and
in particular, gases, have smaller intermolecular forces.
And finally, something we haven't talked abut is compressibility.
Are these systems compressible?
Solids are not compressible but here is where you have the difference even though liquid
and gases are both fluids, liquids are not compressible, while gases are.
And this leads to the differences between gases and liquids even though they are both
considered fluid.
Another characteristic of fluids is that we have to look at fluids as a continuum.
In other words properties in a fluid vary continuously.
And what this allows us therefore to do is we can determine an average property.
This is because the volume of a fluid, or the volume of the fluid molecules is large
compared to the average distance between the molecules.
For example, if we look at the spacing between molecules for a liquid versus a gas.
So the spacing between molecules is about 10^-6 mm for a liquid.
And for a gas it's 10^-7 mm.
So if we look in one cubic mm of fluids, then the number of molecules range from 10^18 all
the way to 10^21st.
So their volume is going to be very large compared to the spacing between the molecules.
Therefore we can treat the fluid as a continuum and say that properties such as pressure or
velocity, vary continuously throughout the fluid.