The earth is bathed in an atmosphere of gases that interact intimately
with the Earth's biosphere. The biogeochemical cycles that
replenish the Earth's supply of carbon, nitrogen and oxygen, gases that
are necessary for life on earth, all rely on the interactions between activities
on the surface of the earth and the envelope of gases the separates the
earth's atmosphere from vacant space. Then it is clear that a good
understanding of atmospheric science should include a basic understanding
of gas behavior.
The forces that attract gas molecules to one another are significantly
weaker than similar forces between liquid and solid phase materials.
Gas molecules move around more or less independantly of one another traveling
in straight lines until a collision occurs between molecules,container
walls or small airborne particles. The Kinetic
Molecular Theory very successfully describes the behavior
of gases and allows detailed predicitions of gas characteristics under
a full range of atmospheric conditions. The kinetic-molecular theory
is itself mathematical, But it can be summarized qualitatively. The theory
is is based on three general principals:
Size of Gas Particles
The volume of an individual gas molecule is vanishingly small when compared
with the space between molecules. As a result, the model considers
individual gas molecules to be points of mass and a sample of gas to be
nearly empty space.
Motion of Gas Molecule
Gas molecules are in constant, random, straight-line motion until they
collide with one another, airborne particles, or surface. Between
collisions the molecules do not influence each other. When they do
collide the collisions are elastic which means that the two particles in
the collision exchange energy but that the total kinetic energy of the
two molecules remains constant.
Particle Speed, Temperature and Energy
The molecules in a sample of gas have a range of speeds (u), with
the most probable speed near the average speed for the whole sample of
gas. As the temperature of a gas increases, so does the average particle speed. The increase
in speeds occurs
because the average kinetic energy of the molecules, that is the energy
associated with the motion of the gas particles, is proportional
to the absolute temperature. This means that at a given temperature all gases have the same average kinetic energy.
The figure at right shows this information graphically for gas samples at two different temperatures. The x-axis shows
the number of gas particles with a velocity (u). The average velocity at the higher temperature is
greater but the number of particles at the highest average velocity is smaller.
With these basic ideas in mind it is a small step to understand and
predict the relationship between pressure (P), volume (V), and the temperature
of a gas (T).
