By A. M. Howatson (Auth.)
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If a sufficiently fast electron collides with an already excited atom, further excitation or ionization can take place. In this case the cross-section for, say, ionization is not only greater for a given value of si9 but has, of course, a lower threshold value. Data for electron ionization and excitation cross-sections, or efficiencies, are available(3) for a wide range of gases. FUNDAMENTAL PROCESSES 27 The reverse of excitation by a colliding electron is a superelastic collision, in which the excitation energy is given up to the electron as kinetic energy; similarly the reverse of ionization is a three-body collision of a positive ion with two electrons, one of which recombines with the ion, releasing to the other kinetic energy equivalent to the ionization potential.
ENGEL, A. VON. Ionized Gases. Clarendon Press, Oxford, 1965, p. 125. JEANS, J. H. Kinetic Theory of Gases. Cambridge, 1940, p. 198. HASTED, J. B. Physics of Atomic Collisions. Butterworths, London, 1964, p. 19. GENERAL BIBLIOGRAPHY BORN, M. Atomic Physics. Blackie, London, 1962. BROWN, S. C. Basic Data of Plasma Physics, 1966. T. Press, London, 1967. LOEB, L. B. Basic Processes of Gaseous Electronics. California, 1955. ENGEL, A. VON. Ionized Gases. Clarendon Press, Oxford, 1965. FLÜGGE, S. ). Handbuch der Physik, 21.
The average kinetic energy is then constant and so also is the average directed velocity, the value of which is then known as the drift velocity of the electron. It is comparable with the terminal velocity attained by a body passing through a viscous fluid under the action of a constant force. The collisions experienced by the electron, which serve to fix this limit to the velocity it acquires from the field, have the macroscopic effect of viscous friction, which here determines the resistivity of the ionized gas.
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