# Why is fire conductive?

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## Main Question or Discussion Point

Hi

Why is fire conductive?

Have encountered several examples where fire is used as a conductive material in high voltage demonstrations. However I've never gotten a straight answer as to why.

Some sources claim fire is a plasma, but that's not true. The flame of a candle is not hot enough to make plasma out of air.

Other sources give other explanations and contradict each other.

Can anyone here tell me why is fire conductive?

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jedishrfu
Mentor
Watch this video and maybe it will clarify your questions:

and this one on plasmas in microwaves:

Thanks!

Do you know if the ions are generated by collisions, or if they come from some part of the chemical reaction?

Certain chemical reactions create ions during the reaction when electrons move around, even though both the reactants and products have neutral charge.

Since the flame goes out if the field is strong enough, could that suggest that the reaction is interrupted when charged particles are pulled in opposite directions?

https://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97
Here NASA claims fire is not plasma. So that should mean a candle flame is not hot enough to generate ions from collisions alone... Or did I misunderstand something?

In a flame, hundreds or thousands of chemical reactions occur. For the reaction between methane and air, we learn that $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$, but the reality is much more complex. Many intermediate species are formed that react with each other (hydrogen, carbon monoxide, ...). Most of these species eventually react to carbon dioxide or water. Some of the intermediate species are charged species. In methane, this is mostly $H_3O^+$ and of course $e^-$. The concentration of the charged species is usually very low and they usually react with each other again to form neutral species. When you put a very high potential difference over a flame, you can separate the positive and negative species, and in this way create an electrical current through the flame.

The concentration of the ionized species (the plasma density) is very low in flames and they usually do not affect the flame in any way. Ions are therefore often neglected in flame studies and we usually do not refer to a flame as a plasma. However, when you want to study the behavior of flames with high applied voltages, you will need the same equations as when you study a plasma (Maxwell's equations). The flame is then considered to be a weak plasma.

jim mcnamara
Mentor
Major electric utility outages can and do occur when there is a grass fire or forest fire in close proximity to a transmission line. Basically the entire system experiences a short at the place where smoke surrounds all of the conductors.

I was at PNM then. They indicated a large grass fire was the primary cause. 2000 megawatt transmission lines from the San Juan power plant were rendered unusable for an hour.

anorlunda
Mentor
Major electric utility outages can and do occur when there is a grass fire or forest fire in close proximity to a transmission line. Basically the entire system experiences a short at the place where smoke surrounds all of the conductors.
That's true, but it is unclear how much to blame ionization as compared to soot. The critical points are the gaps in the suspended strings of insulators. Salt spray near the seashore does the same thing.

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sophiecentaur
Gold Member
That's true, but it is unclear how much to blame ionization as compared to soot. The critical points are the gaps in the suspended strings of insulators. Salt spray near the seashore does the same thing.
View attachment 227301
I neer understood how those things are 'self cleaning' in the rain.

jedishrfu
Mentor
This brings new meaning to the classic Gene Kelly song "Singe'ing in the Rain"

anorlunda
Mentor
I neer understood how those things are 'self cleaning' in the rain.
The glass parts you see are like the wide brim of a hat. They keep the rain off the important part. In the image below, I colored the metal parts in red, and the air gap where actual insulating occurs as blue.

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