Chapter 5 - States of Matter
 Fluids & Pressure
Fluids are any substance that can flow. So when we talk of states of matter
there are two of them that are fluids, liquids and gases.
Any fluid will produce pressure because the particles that they are made of are in
motion and hitting the edges of the container.  In the graphic below you can see
one particle bouncing around, when it hits the edge it pushes on it. Now if there
are billions of particles hitting it will produce more pressure.
Pressure is measured by amount of force divided by the area (force / area), in
standard terms we might use pounds per square inch, such as in the pressure of a
bicycle tire, but in metrics (SI) we use Newtons per square meters. (N / M  ). One
Newton over one square meter is called a
Pascal (Pa) and is a very small amount,
so usually we measure pressure in a Kilopascal (KPa).  This doesn't mean we
want to go around killing Pascal, it means 1000 Pascals.
2
Blaise Pascal was a great
scientist/mathematician/philosopher who discovered one
of the basic principles of fluid pressure so it is called
Pascal's principle.  It says that when there is an
increase in pressure at any point in a confined fluid,
there is an equal increase at every other point in the
container.  Unlike a solid, when you push on it the force
goes straight through to the other side, in a fluid the
force goes equally in all directions.

Think of a balloon, if you push down on the top it will
start pushing out on the sides too.  This also explains
why a bubble is round, the air inside is pushing equally in
all direcftions.
Remember we said that a gas does not have a definite shape or
volume
because the molecules are spread apart and in motion.  If
you squeeze the gas into a smaller volume, the pressure will
increase.  Notice in the graphic to the left, when the space gets
smaller the particles hit the walls more often.  This is a basic rule
of gases, if you decrease the volume you will increase the pressure.