MadSci Network: Physics

Re: At what temperature does air turn to a plasma?

Date: Wed May 3 18:19:36 2000
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 957067037.Ph

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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