MadSci Network: Physics |
Dear Daniel: Air becomes a plasma when there is sufficient energy present to remove the electrons from the atoms so that the air is electrically conducting, made up of free electrons and positive ions. Air is a mixture of nitrogen and oxygen, but let's talk just about nitrogen. The electrons are bound to the nitrogen atom with an energy equivalent to 14.534 electron volts (1). To ionize the nitrogen atoms from heating only the temperature would have to be high enough for the electrons to have energy equivalent to these 14.534 electron volts, under which conditions the electrons could then separate from the atoms and become free. For an atomic particle the kinetic energy of the particle is equal to 1/2(kT) where k is boltzmann's constant and T is the absolute temperature in degrees K. At normal room temperature, about 27 degrees C (273 degees K) this calculates to be an energy of 1/40 of an electron volt. This (the electron volt) is the energy the charged particle (the electron) would have if it moved across a potential drop of 1 volt. To achieve this energy by heat alone requres a very high temperature. To achieve a tmperature high enough to have 1 electron volt of energy the temperature would have to be 12,000 degrees K. (A very high temperature indeed!) But we said that to remove the electron from the nitrogen atom we need an energy of 14.534 electron volts, which means the temperature would have to be 14.534 times as high as this, or actually 174,408 degrees K. Therefore if you wished to make a plasma in air using only heat as the source of energy you would have top raise the air temperature this high! If you are interested to know how to make a plasma in air without having to go to this temperatre let me know and I will give you an explanation. [Ref. 1: A Physicist's Desk Reference; American Institute of Physics,1989;Edited by Herbert L. Anderson.] R. Bersin.....
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