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To study the dynamic behavior of gases some basic knowledge is required about the molecular structure of this form of matter, about the difference between thermal motion and drift motion, on the definition of pressure, et al.
Properties of a gases
1.1.1. Microscopic view of a gas.
Given is a long, gas-filled pipe, of which only a small section is shown
Thermal velocity of gas atoms or molecules
Such a gas consists of isolated atomic or molecular particles moving at relatively high speeds and constantly colliding with each other or with the walls.
Thermal velocity of gas atoms or molecules
1.1.2. Data of air at room temperature
Air is a mixture of about 20% Oxygen molecules O2 and 80% Nitrogen molecules N2. At room temperature and normal atmospheric pressure the average velocity of the Oxygen and Nitrogen molecules is about 480 m/s. The average number of collisions per second of a molecule with its neigbourgh is of the order of 8 / s. The mean free path between two collisions is about 6 x10-8 m.
1.1.3. Pressure and density
In physics the term pressure describes a state where a force is acting on a certain area. If the seize of the force F and the seize of the area a is known, the amount of pressure can be determined as F/a.
Pressure on a wall, caused by colliding particles
If the area a on the right, which is constantly hit by the particle, is movable and can be kept in balance by a force F, then the pressure on this area is equal to P=F/a.
The pressure of a gas is depending on the number of atoms or molecules which strike the chosen area per unit time. This number is proportional to the density of a gas, i.e. the number of particles per unit volume. Pressure is furthermore depending on the mean velocity of the microscopic particles.
Pressure on a wall, caused by colliding particles
1.1.4. Thermal velocity and homogenous distribution
The temperature of a gas is explained at the microscopic level by the random movement of the atoms or molecules.
The higher the mean velocity of the molecules, the higher the temperature of the gas. This thermal motion and the high number of collisions causes the particles to spread evenly through the available space.
This homogenous distribution can only be disturbed by external influences. Without any external disturbance this homogeneous distribution will always remain. After an external temporary disturbance the gas will automatically revert to the original even distribution.
1.1.5. Drifting velocity
The thermal velocity has to be distinguished from another type of motion, the flow of a gas, where all the particles in a gas receive the same drift velocity in one direction. Since all molecules are involved in the same sense, such a flow is observable on a macroscopic scale.
Thermal velocity and drift velocity are superimposed in a gas.
Drifting velocity (without indication of thermal velocity)
As a rule the magnitude of the drifting velocity is much smaller than the magnitude of the thermal velocity. A typical value ranges between several mm to m per second while the average thermal velocity of oxygen or nitrogen molecules in a gas at normal conditions is about 450 m/s.
Drifting velocity (without indication of thermal velocity)
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