Gases can be
studied by considering the small scale action of individual molecules
or by considering the large scale action of the gas as a whole. We
can directly measure, or sense, the large scale action
of the gas. But to study the action of the molecules, we must use a
theoretical model. The model, called the kinetic theory of
gases, assumes that the molecules are very small relative to the
distance between molecules. The molecules are in constant, random
motion and frequently collide with each other and with the walls of
any container.
The individual molecules possess the standard physical properties
of mass, momentum, and energy. The density
of a gas is simply the sum of the mass of the molecules divided by
the volume which the gas occupies. The pressure of a
gas is a measure of the linear momentum of the molecules. As the gas
molecules collide with the walls of a container, the molecules impart
momentum to the walls, producing a force that can be measured. The
force divided by the area is defined to be the pressure. The
temperature of a gas is a measure of the
mean kinetic energy of the gas. The molecules are in constant random
motion, and there is an energy (mass x square of the velocity)
associated with that motion. The higher the temperature, the greater
the motion.
In a solid, the location of the molecules relative to each other
remains almost constant. But in a gas, the molecules can move around
and interact with each other and with their surroundings in different
ways. As mentioned above, there is always a random component
of molecular motion. The entire fluid can be made to move as well in
an ordered motion (flow). The ordered motion is superimposed, or
added to, the normal random motion of the molecules. At the molecular
level, there is no distinction between the random component and the
ordered component. We measure the
pressure produced by the random component as the static
pressure.
The pressure produced by the
ordered motion is called
dynamic pressure.
And
Bernoulli's equation
tells us that the sum of the static and dynamic pressure
is the total pressure which we can also measure.
Guided Tours

Gas Statics:
Activities:
Temperature and Kinetic Energy Activity: Grade 89