Is Fire a Plasma? | Physics Debate

[brianhurren]

plasmas contain free electrons so should be influenced by a magnet...so, you could get a candle and a magnet from say, a speaker, and see if the flame is affected by it...if no affect, not a plasma

 

[Abstractness]

Ok, here's the proof that fire is a plasma:

 

[OmCheeto]

Ok, here's the proof that fire is a plasma:

And here's proof that water turns air into plasma!

https://www.youtube.com/watch?v=8OuonluJPw8

One thing they didn't mention in your video is the amount of H₂O generated by the flame. I wonder how much that contributes to the conductivity surrounding the flame.

I'm not a plasma expert, but I'm going to go out on a limb here and say your video is not proof that fire is a plasma. It only proves that science is interesting.

 

[Bandersnatch]

Ok, here's the proof that fire is a plasma:

From the video description:

Is a flame really a plasma? Well it depends on your definition of plasma, but there are certainly ions in a flame, formed as molecules collide with each other at high speed, sometimes knocking electrons off of their atoms.

And from the video itself:

Some may argue that flame is not truly a plasma because it's not hot enough and it doesn't have high enough density of ions. One thing is for sure: it does contain ions.

 

[meBigGuy]

My impression is that a flame (say from burning gas or wood) is hot gasses emitting light caused by changing energy states. Different molecules have different characteristic colors consistent with their energy transitions as they cool. (there were many statements that the flame was burning soot, etc, which is circular).

As a non-expert I cast my vote with the statement that the common characteristics we attribute to a flame would not be different if we somehow removed all the ions. Not to say there are no ions, and obviously what ions are there can be exploited to demonstrate limited plasma like behavior.

 

[lunaloca]

I would also kindly like to add that "plasma" does not always refer to near to equillibrium plasma, but could also be non-equillibrium, more commonly weakly ionised plasmas at low pressures or a little more above atmospheric.
In that case, non-thermal plasmas are low temperatures (300-1000 K) and certainly can be related to flames.

In regard to "hot" near to equillibrium plasmas, I personally think fire is still not a plasma. I agree with Astonuc above that any kind of plasma is sustained from an electromagnetic field, while fire is sustained by hydrodynamic forces. I could imagine a a flame is a energetic plume containing ions caused from thermal dissociation and chemical reactions (any flame is a form of combustion), however the electron density or the density of the charged particles is so small that cannot sustain an electromagnetic field.
The glow is light emission by photons release during the different energetic transitions during the chemical reactions. It does not mean that it is plasma just because it glows.
The yellow emission is characteristic of incomplete combustion and perhaps related to the H Balmer series emissions. I hope that was helpful, Would be interested to read your comments. Thanks

 

[lunaloca]

And here's proof that water turns air into plasma!

https://www.youtube.com/watch?v=8OuonluJPw8

One thing they didn't mention in your video is the amount of H₂O generated by the flame. I wonder how much that contributes to the conductivity surrounding the flame.

I'm not a plasma expert, but I'm going to go out on a limb here and say your video is not proof that fire is a plasma. It only proves that science is interesting.

I agree. Flame is certaintly conductive, but it does not mean it is plasma. The so-called buttrflies could be formed by the surrounding "ionic wind" which is mainly formed between the two plate electrodes. The fact that they create steamer channels with the flame means again that it is conductive, as the same would happen id you would add an extra piece of metal in the middle.

 

[lunaloca]

I agree. Flame is certaintly conductive, but it does not mean it is plasma. The so-called buttrflies could be formed by the surrounding "ionic wind" which is mainly formed between the two plate electrodes. The fact that they create steamer channels with the flame means again that it is conductive, as the same would happen id you would add an extra piece of metal in the middle.

But I must admit, the video is very interesting and cool!

 

[OmCheeto]

But I must admit, the video is very interesting and cool!

Both video's are interesting and cool.

I would very much like to recreate that experiment, so I pulled the coil out of one of my spare cars, hoping that I can construct a similar system. (I'm too lazy to build a Van Der Graaf Generator.)

I want to place the two plates over a pot of boiling water and see if my steam theory is correct.

If the steam deflects, then we'll have proved that steam is also a plasma!

I'll have to video that, and send it to the plasma/flame science guy.

 

[lunaloca]

Both video's are interesting and cool.

I would very much like to recreate that experiment, so I pulled the coil out of one of my spare cars, hoping that I can construct a similar system. (I'm too lazy to build a Van Der Graaf Generator.)

I want to place the two plates over a pot of boiling water and see if my steam theory is correct.

If the steam deflects, then we'll have proved that steam is also a plasma!

I'll have to video that, and send it to the plasma/flame science guy.

Yes, the water droplet electrostatic generator is very cool!

Your experiments will be interesting as well to see how stream reacts under certain electrical field.
I believe you are going to create humidified air plasma. As air breaks down under electrical field, steam will break down as well. I am interested to see if stream will increase the breakdown energy needed to ignite the plasma. From my knowledge, there are researchers looking at the effect of humidity on different gases breakdown voltage. In some cases humidity conductivity can enhance the electrical field and decrease the breakdown voltage. There are other cases that above a critical point humidity can increase the breakdown energy needed. So you might want to bare in mind that "steam" (highly humidified air) could be difficult to breakdown, or maybe in a sorter gap only. However, I would be very interested to see how the ionic wind could affect "steam" diffusion. So I will wait for your video! :-)

 

[gtsimpedes]

Can electricity be harnessed directly from fire in an efficient manner therefore doing away with the old standard of conversion of fire to mechanical energy which then is used to generate electricity?

 

[bigfooted]

Can electricity be harnessed directly from fire: yes.
in an efficient manner: yes, you can potentially capture all of the charged particles.
Use it as an alternative to generate electricity: no.

Premixed methane-air flames have charged particle mass fractions of the order of 10^{-10}. That will not generate a lot of electric current (order of a couple of mA), considering that flames have very high electrical resistance (order of MOhms).
It is more efficient to use the released heat to generate electricity.

 

[stedwards]

I don't believe there is proof or disproof of the statement: "a flame is a plasma". All material above absolute zero temperature will have a fraction of free electrons. At what degree of ionization one wishes to define something as a plasma, due to heat, is a matter of convenience to a particular application.

 

[Hercuflea]

Yes, fire is a low-temperature plasma that is not in thermal equilibrium. This is a chart from Lawrence Livermore National Laboratory.

 

[stedwards]

read some of the 63 previous posts. everybody has an opinion, where there is no strict dividing line.

 

[OmCheeto]

read some of the 63 previous posts. everybody has an opinion, where there is no strict dividing line.

Yah! And I'm still of the opinion that I don't know.

All I can do is show experiments, that demonstrate, that the Veritasium video is not proof that a flame is a plasma:



Now, if you were to take the ionized tape up to the ISS, chop them into bits, and place them in between two charged plates, I can imagine that they would demonstrate the same properties as the flame. So then, is ionized cellophane tape a plasma?

I'm still leaning towards "no". I just got out my multimeter, did the experiment referred to in post #2, and measured: resistance = ∞

...
Before writing to you, just to make sure, I took an electric meter and measured the resistance between two metal contacts separated by a small distance, putting both in the flame of a gas oven, which gets pretty hot. No electric current could be detected, both inside the flame and away from it, meaning the flame did not conduct any observable electric current.

You can get electrons to flow in a vacuum. Is the vacuum a plasma? I don't think so.

 

[stedwards]

Well, this thread is about plasma due to heat; the net charge should be zero. I'd challenge all to show me that there is a hard division line between plasma and non-plasma of the heat generated variety. A logarithmic scale might serve better, such as pH, or earthquake intensity, Perhaps log(free electrons over total atoms). Of course, this wouldn't do for a solid metal.

 

[bigfooted]

I'm still leaning towards "no". I just got out my multimeter, did the experiment referred to in post #2, and measured: resistance = ∞

We did a series of experiments in the lab where we created a quasi-onedimensional setup using a flat flame burner and we placed it between a cathode and an anode. Depending on the applied potential difference we can measure a current and estimate that in our situation, the diodic resistance of the flame (lean methane-air flame) is between 5-25 MOhm.

A detailed chemical mechanism describing a methane-air flame can have more than 300 reactions. There are only 1-4 reactions involving charged species like electrons and H3O+. The concentration of these species is very low and they don't affect the other species much so they are usually neglected. But when you ignite a flame, the charged species become very important and if you want to study flame ignition you cannot ignore this.

 

[OmCheeto]

... the diodic resistance of the flame (lean methane-air flame) is between 5-25 MOhm.

...

This would seem to confirm that a flame is not a plasma.

I just realized that this thread is ancient.
hmmmm...
Let me synopsisize the opinions over the last 8 years:
username(mm/yy)

OP: Oblef; "Is fire a plasma?"

No                 Yes            Partially?
Doc Al(12/07)                     lantresman(12/07)
Astronuc(12/07)                   Sojourner01(12/07)
MaWM(01/08)        chefcrsh(07/08)
russ_waters(05/10)
Drakkith(05/11)
jetwaterluffy(10/11)              juanrga(10/11)      
BadBrain(10/11)    Abstractness(10/13)
OmCheeto(10/13)
Bandersnatch(10/13)
meBigGuy(10/13)
lunaloca(10/13)
bigfooted(06/15)   Hercuflea(06/15)

It appears that most people believe that fire does not fit the definition of plasma.

ps. In my forays into the quest for an answer this morning, I had some weird questions, which generated more questions:

Is the interior of a fluorescent bulb an example of a plasma?

Fluorescent lamps are negative differential resistance devices, so as more current flows through them, the electrical resistance of the fluorescent lamp drops, allowing for even more current to flow. Connected directly to a constant-voltage power supply, a fluorescent lamp would rapidly self-destruct due to the uncontrolled current flow. To prevent this, fluorescent lamps must use an auxiliary device, a ballast, to regulate the current flow through the lamp.

+

Usually very high: For many purposes, the conductivity of a plasma may be treated as infinite.

Or is it a borderline, coincidentally similar effect?

Touching a fluorescent bulb, the surface temperature is obviously not a bazillion degrees, as seems to be required for a "proper" plasma.

 

[russ_watters]

Can electricity be harnessed directly from fire in an efficient manner therefore doing away with the old standard of conversion of fire to mechanical energy which then is used to generate electricity?

If you loosen your definition of "fire" to cover the same chemical reaction at any temperature, what you are asking for is a fuel cell:
https://en.m.wikipedia.org/wiki/Fuel_cell

Their current efficiency is not typically good enough to prefer them over the old fashioned way for most applications.