Gases
The first step to understanding gases is to spell out what exactly a gas is.
Gases have two properties that set them apart from solids and liquids. First,
gases spontaneously expand to fill the container they occupy, no matter its
size. In other words, a gas has no fixed volume or shape. Secondly, gases are
easily compressible.
You can imagine a gas as a busy swarm of molecules. Each molecule moves
randomly and travels great distances before bouncing off another molecule. This
occurs because the individual molecules comprising a gas are generally far
apart. In fact, for a gas at low pressure, we can approximate that aside
from a few random collisions, individual gas molecules do not interact. This
approximation is what separates gases from solids and liquids, whose molecules
always interact. The series of SparkNotes on Gases SparkNote seek to use this
approximation about gases to establish the ideal gas law
and the kinetic molecular theory. The ideal gas law
macroscopically describes how gases behave under nearly all conditions. The
kinetic molecular theory describes how sub-microscopic gas molecules interact
with each other.
Pressure
Of the three general terms used to describe gases (volume, temperature,
pressure), pressure is the least familiar. Before we can delve into the gas
theories, we need a firm understanding of it. Pressure is defined as force
divided by the area over which the force acts:
pressure
P =  |
|
Ice skates are familiar examples of the effects of pressure. The area of the
blades of a skate are much smaller than, say, the soles of your feet. So if you
strap on ice skates, your weight will act on an area much smaller than it would
if you were wearing normal shoes. Since
A decreases while
F stays the same,
by @@Equation@@, the pressure you exert on the ice will be much
greater if you're wearing skates. This pressure is often enough to melt a layer
of ice, which allows your skate to glide smoothly across an ice rink. If you
try the same maneuver with normal shoes, you will not generate enough pressure
to melt the ice and won't get anywhere fast.
So how does pressure relate to gases? If you will remember, a gas will fill any
container that holds it. It is easy to see why with our swarm analogy. If a
compact swarm of molecules is placed into a large container, the individual
molecules will move about randomly and eventually stray from their original
dimensions. Eventually, some intrepid molecules will reach the walls of the
container. When they do, they will impact the walls of the container. These
impacts generate a force, and hence a pressure on the walls of the container.
