Basic concepts

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Chapter 1

Summary Basic concepts

After a brief historical review, we discuss in ‘basic concepts’ the fundamental physical laws which govern the behaviour of vacuum and play a role in the technique of evacuation. To this end, it is necessary to have some insight in the characteristics and structure of gases, but also of liquids and solids. The properties of rarefied gases at different pressures and temperatures are quite well explained by the kinetic theory of gases. The basic principles of this theory will be discussed, including topics such as the thermal motion of gas particles, the mean free path, rate of incidence, vapour pressure, evaporation rate etc. Also the empirical gas laws of Boyle/Gay-Lussac, Avogadro and Dalton have their formal basis in the kinetic gas hypothesis. Finally, we will focus on some non-equilibrium properties of gases at high and low pressures (thermal conductivity, viscosity, diffusion). The reasonably good agreement between the kinetic transport laws and experimen­tal practice evidences the validity and useful­ness of the kinetic gas theory. We conclude, that ‘basic concepts’ will give you a clear view on vacuum-related physical laws and phenomena.

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Contents Basic concepts

1.1        Introduction 1
1.2        Historical overview 2
1.3        Molecules and atoms 5
1.4        Physical states of matter 8
1.5        Kinetic theory of gases 9
1.6        Molecular velocities and energies 10
1.7        Pressure of a gas 16
1.8        Ideal gas law 20
1.9        Dalton’s law 21
1.10      Avogadro’s law; equation of state for an ideal gas 21
11      Van der Waals equation of state 23
1.12      Mean free path 25
1.13      Rate of incidence of gas particles on a surface 30
1.14      Energy flow to a wall 32
1.15      Vapour pressure; evaporation rate of 34
1:16      Transport phenomena in gases 36
17      Transportation of a physical quantity in a viscous gas G 37
18      Viscosity 40
1.18.1   Viscosity in a dense gas (Kn «1) 40
1.18.2   Viscosity in a rarefied gas (Kn »1) 42
1.19      Thermal transpiration (thermo-molecular flow) 45
20      Thermal conductivity 46
1.20.1   Thermal conductivity at high pressures (Kn «1) 46
1.20.2   Thermal conductivity at low pressures (Kn »1) 50
1.21      Diffusion of gases 54
1.21.1   Fick’s diffusion laws 54
1.21.2   Self-diffusion 55
1.21.3   Diffusion in gas mixtures 57

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