Can a negative charge ionize air? Adding to this question, I have studied that positive charge ionizes air but never studied that negative charge ionize air. If possible, please explain how does it ionize air?
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asked Jun 27, 2014 at 9:35
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$\begingroup$ Can you explain what you mean when you say a positive charge ionizes the air. I think you may be misunderstanding the physics of what is going on here. $\endgroup$– nivagCommented Jun 27, 2014 at 9:43
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$\begingroup$ Well, I mean to say that it is generally mentioned that a positive charge in an object ionises air. Have I misunderstood this completely? I'm sorry if I have. I hope you will make this clear to me. $\endgroup$– Kushashwa Ravi ShrimaliCommented Jun 27, 2014 at 9:53
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2 Answers
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Ionization of this sort is not caused by positive or negative charges per say, but by an electric field. We can view the ionization as the electric field trying to accelerate the electrons and nucleus of an atom in opposite directions and ripping them apart.
(In reality, this is a minor effect. Most ionization is caused by a chain reaction as free electrons are accelerated and collide with other molecules, ionizing them).
Therefore to get ionization we need to setup a large electric field. To achieve this generally a positive and negative charge are used. For example in a glow discharge.
I guess if you had a single positive or negative charge in space it could generate a big enough electric field near it to cause ionization. This would be somewhat similar to what happens in lightning.
You can also have ionization due to other sources such as thermal ionization in stars but I don't think that is what is asked about here.
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Both positive and negative charges can produce ionised air molecules, but the mechanisms are different.
A positive charge attracts electrons produced by background radiation. The high field strength accelerates these electrons and the electrons collide with and ionise air molecules. The electrons from the ionised air molecules are in turn accelerated towards the positive charge and they collide with and ionise more air molecules. The result is an electron avalanche. So the positive charge produces positive ions.
A negative charge produces electrons by field emission. If the charge is high enough it reduces the work function of the electrode to zero and electrons are released from the surface. These electrons then bind to air molecules to produce negative ions, so a negative charge produces negative ions.
Both types of ionisation are examples of corona discharge.
answered Jun 27, 2014 at 10:11
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$\begingroup$ Do you have any references that electron avalanche is the result of positive charge and field emission negative charge? Looking at the wikipedia articles they seem to both be due to large electric field and therefore not directly associated positive or negative charge. Also why do you think the electrons from field emission will bind with with air molecules but not those due to electron avalanche. For that matter why are they binding to molecules (which are stable) at all. $\endgroup$– nivagCommented Jun 27, 2014 at 10:32
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$\begingroup$ @nivag: an avalanche only occurs with positive charge. Your own answer states Most ionization is caused by a chain reaction as free electrons are accelerated and collide with other molecules, ionizing them, which is what I meant by an avalanche. $\endgroup$ Commented Jun 27, 2014 at 10:40
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$\begingroup$ @nivag: gaseous O$_2^-$ anions are stable and known as the superoxide anion. $\endgroup$ Commented Jun 27, 2014 at 11:05