JANET: In these two screen cast we're going to
discuss some of the properties that fluids have.
So in part one what we're gonna look at are
density and what's known as the bulk modulus.
So lets start with density.
So we have density and we write this as row.
And it has units of kilograms per a meter cubed
and therefor its definition is mass per unit volume.
We can also describe it in English units.
We can have the units of slugs per feet cubed.
Even though that's not used as often as the SI units.
So density is a strong function of temperature and gasses
think the ideal gas law but not as much for liquids.
So why is this a function of position.
Well as heat is added to the gas.
The density decreases.
The more heat, the less dense.
So as the gas goes down the tube more heat is added
the density decreases therefor that density is going
to be a function of position.
So often the value of the density is not given,
but can be calculated from other values.
So for example, you can use the ideal gas law.
Such that density is equal to the pressure times
the molar muscular molecular weight
of the fluid, divided by R,
which is the universal gas constant, times T.
You can also sometimes see what
known as the specific volume.
Which is just one over row.
So you can see its units are meters cubed per a kilogram.
And we don't really use that very much
in fluids but you will
see it in thermodynamics.
We also have what's known as the specific weight
of the fluid.
And that uses the Greek letter gamma,
and it is similar
to density except that it is the weight per unit volume.
And the way we find our specific weight, that it is
equal to the density of the fluid times gravity.
And its units are gonna be in newtons per a meter cubed.
Probably the most common value given in the tables
is what's known as the specific gravity.
And that's the density of your fluid, divided by
the reference density.
And the reference density for liquids and solids is
water at four degrees c.
So in SI units the reference density is a
thousand kilograms per a meter cubed.
So in order to get the fluid density
that you're interested in you take the specific
gravity which is unit less, that you find from
a table, and multiply it by one thousand kilograms
per meter cubed.
So usually in a basic fluids course
we assume incompressible fluids,
which we defined as a fluid with no change in density.
However many important systems use compressible fluids.
So we need a way to determine how compressible a fluid is.
And what we use is EV which is known as the
bulk modulus of the fluid.
And so this bulk modulus,
is equal to minus dp
divided by dv over v.
And what that means is are dp is this differential
change in pressure in the bulk modulus
therefor is this differential change in pressure
that's needed to create a differential change in volume.
Note that this expression is negative.
Why?
Because in increase in pressure will
cause a decrease in volume, and vice versa.
We can also write this in terms of density.
So here's this differential pressure, change in pressure
that's needed to cause the differential change in density.
So here the value is positive.
So sense you take a mass which is divide
as the density times the volume.
If the volume decreases the density must increase
the conservation in mass and therefor this is going
to be positive.
So again the bulk modulus is defined
as the differential change in pressure
needed to create a differential change in density.
Since liquids are considerably more dense than gasses,
that number is going to have to be higher for liquids.
In fact it's in the range with about
ten to the ninth pascals, compared to around
ten to the fifth for gasses.
So in our next screen cast we're gonna talk about
two other properties, one of them is vapor pressure
and the other is service tension.