This screencast is going to discuss a little bit about pressure. The types of pressure, and
how to convert from one to another, or how to use them in calculations, and so forth.
This will just be a quick overview. So the first kind of pressure we are going to talk
about is a big tall glass of water, and talk about fluid pressure. If we put a hole in
the glass, then the pressure that fluid exerts would be equal to the pressure you would have
to exert on a cork to fill that hole. The forces counter balance one another. We know
that pressure is a force over an area. There are many different units, but let's go with
SI. We know we can write force in Newtons and we will just choose meters squared. So
very typically you will see pressure in Newtons per meters squared, which is also written
as a Pascal (Pa). Other units might see atmosphere, psi, mm-Hg, bar, and of course Pascals. The next
type I want to mention is what is considered hydro-static pressure, and that is the idea
that this fluid has a weight to it, and we so we are going to take whatever the pressure
at the top of that water is and the weight of the water, add those pressures together,
and that gives us our hydro-static pressure at the bottom. Typically if this is open atmosphere
we will refer to this as atmospheric pressure Po (the initial pressure) and then the weight
of the fluid and so we will write that as the total pressure here, the hydro-static pressure
is Po, the fluid is rho*g*h, and you are probably wondering
how is rho*g*h a pressure or does it have the same units? If we write out rho*g*h, we
know that it’s mass over volume and we're multiplying that by an acceleration and height, and so
we can see that the height over the volume, we get a mass times acceleration over an area.
Mass times an acceleration is a force, force over an area is pressure. So those units
do work out, and so that is how we calculate the hydrostatic pressure. Another type of
pressure you are going to hear about is head pressure. And what that is is if we create
a hypothetical column of this water and we just refer to that column. Then we could call
this water having a head pressure of whatever its height is, and therefore, we are going
to refer to the head pressure as the height of the fluid and that is regardless of the
diameter of that fluid it can be very large or very small. It’s whatever height that
column is, and so in this case maybe it’s 2 meters. So we are going to say this is 2
meters of water, and it is important to designate what type of fluid that is. So we can say we have
a hypothetical column 2 meters of water. Therefore that is the head pressure of the water, and
we might want to convert that head pressure to a real pressure like atmospheres or psi.
I don't mean "real" necessarily but something that you could translate and compare to other systems. In that
case again it’s going to be very similar to what we saw over here. So the density of the fluid,
times gravity, times the head of that fluid. That gives us whatever the pressure of that
fluid is at the bottom. Let's do an example. So here we are given a situation where you
are diving underwater and we want to know the pressure on those ear drums of yours is, in bar specifically,
that's acting there, and we are given that atmospheric pressure is 1 bar, and so all
we know is that we are 15 feet under water when our ears start hurting. And there are a couple
conversion factors that I pulled out of a book for you that you can use.
Let's go back to the idea of using head pressure, simplifying this whole situation. Instead of trying to
figure out the exact weight all the fluid acting on your ears, we can say the depth is 15 feet. So the
head pressure in this case is 15 feet of water, and we know that atmospheric pressure is 1
bar. We can convert that over to a pressure that is related to feet of water. In this
case the total pressure is going to be the initial pressure plus that head pressure.
We are looking at 33.5 plus 15 and this is going to give us 48.5 feet of water acting
on our ears. Now if we want it in bar we just convert it over. So this comes out to about
1.45 bar and we can use that head pressure to simplify this entire calculation and convert
it over to a pressure that is acting at that point.