Is Fire a Plasma? | Physics Debate

[Oblef]

Hey everybody.

I heard a physics professor at Berkeley say that normal fire (from a lighter) is plasma. My science teacher at high school says that it isn't. So my question is, is normal fire in state of plasma or not?

Thanks.

 

[Doc Al]

http://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97"

 

[Oblef]

Sorry I'm not from an English speaking country. Is there something you don't understand? When I say that, I mean plasma like there also are gas, liquid and solid. That kind of plasma.

 

[Doc Al]

It's a link. Click it.

 

[Oblef]

Oh thanks, nice experiment he made :D I'm writing my 3rd year assigntment on "light perceptions throughout history" in physics and history. I hope it's okay if I return to ask more questions some other day?

 

[dst]

Do you know what temperatures/charges would be needed to turn air into a plasma?

 

[Oblef]

There would be needed a high enough temperature to kick the electrons all the way of the gas, to create ions. I would assume a couple of thousand degrees C. probably 2-3000.

 

[Sojourner01]

How much dissociation is required before an ionic cloud like a flame is considered a plasma? Since a 'true' plasma is in equilibrium, there must always be a very small proportion of ions that momentarily carry at least one bound electron; so at what point does 'ionised gas' stop and 'plasma' begin?

 

[Danger]

I hope it's okay if I return to ask more questions some other day?

We insist upon it, and hope that you will help to answer the questions of others who have lesser knowledge. The beauty of PF is that you can learn and teach at the same time.

 

[iantresman]

http://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97"

I'm afraid I must disagree with http://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97" in which flames (ie. fire) is shown as a plasma. In his book, Introduction to Plasma Physics and Controlled Fusion, Francis F. Chen notes:

Any ionized gas cannot be called a plasma, of course; there is always a small degree of ionization in any gas. A useful definition is as follows: A plasma is a quasineutral gas, of charged a neutral particles, which exhibit collective behavior. (Ref)​

He further goes on to describe three parameters that need to be satisfied in order to identify a plasma. These are probably better summarzed on Wikipedia (plasma) as: (1) the plasma approximation (2) Bulk interactions (3) Plasma frequency.

In his book, Chen goes on to assess whether certain phenomena are indeed plasma, based on the three parameters, and concludes that a typical flame does indeed meet the criteria of being a plasma.

In practice then, fire is a highly-collisional, partially ionized plasma in which the collisions might mask some of the collective behavior.

 

[Astronuc]

I'm afraid I must disagree with http://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97" in which flames (ie. fire) is shown as a plasma. In his book, Introduction to Plasma Physics and Controlled Fusion, Francis F. Chen notes:

Any ionized gas cannot be called a plasma, of course; there is always a small degree of ionization in any gas. A useful definition is as follows: A plasma is a quasineutral gas, of charged and neutral particles, which exhibit collective behavior.​

He further goes on to describe three parameters that need to be satisfied in order to identify a plasma. These are probably better summarized on Wikipedia as: (1) the plasma approximation (2) Bulk interactions (3) Plasma frequency.

In his book, Chen goes on to assess whether certain phenomena are indeed plasma, based on the three parameters, and concludes that a typical flame does indeed meet the criteria of being a plasma.

Please show where Chen says a 'flame' meets the three criteria for being a plasma.

Chen does state that "the weakly ionized gas in a jet exhaust, for example, does not qualify as a plasma because the charged particles collide so frequently with neutral atoms that their motion is controlled by ordinary hydrodynamic forces rather than by electromagnetic forces." Section 1.6, Criteria for Plasmas.

In practice then, fire is a highly-collisional, partially ionized plasma in which the collisions might mask some of the collective behavior.

No!

An acetylene-oxygen flame at 3300 K is too cold to have sufficient ionization to be considered a plasma!

 

[roguengineer1]

Either way the question was not answered... is a regular fire plasma or not... of course it's not hot enough but is it considered to be in that form of matter or is it a gas...??

 

[Astronuc]

Flame and exhaust are simply hot gases, as Chen indicated in his text "the weakly ionized gas in a jet exhaust, for example, does not qualify as a plasma because the charged particles collide so frequently with neutral atoms that their motion is controlled by ordinary hydrodynamic forces rather than by electromagnetic forces." Section 1.6, Criteria for Plasmas.

DocAl pointed to a NASA site in which a scientist measured the electrical conductivity of a flame. The flame did NOT conduct electricity.

 

[iantresman]

Please show where Chen says a 'flame' meets the three criteria for being a plasma.

Here in his Problems Item No.6, "A typical flame" in which he asks rhetorically, "Convince yourself that these are plasmas". Perhaps Chen's KT=0.1 is too high for a typical plasma.)

The flame did NOT conduct electricity.

If you wish to accept this as a criteria for defining a plasma, then flames would not be a plasma. And while conductivity is a typical characteristic of plasmas, as far as I know, it is not used to define one. I suspect that a flame is a borderline plasma according to Chen's criteria, one that is not sufficiently ionized to be able to measure any electrical conductivity.

It's also possible that Dr. David P. Stern was unable to detect the conductivity of a flame. I note the following papers which suggest otherwise:

• The Electrical Properties of Flames and of Incandescent Solids (2003), by Harold A. Wilson, ISBN 1929148038
• The Electrical Properties of Flames Containing Salt Vapors for High Frequency Alternating Currents, (1924) A. B. Bryan, Phys. Rev. 23, 189 - 194 (1924
• The Electrical Conductivity and Luminosity of Flames Containing Salt Vapours, Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Math. or Phys. Character (1896-1934)
• Measurement of the Electrical Conductivity of Seeded Flame Plasma, Koyama, K. ; Sekiguchi, T., (1967)

I concede that many of these are not typical flames (and I think a flame plasma is one produced from a plasma, rather than a burning flame).

 

[Sojourner01]

of course it's not hot enough but is it considered to be in that form of matter or is it a gas...??

I think, if you interpret the opinions voiced here in a certain light, it would be most correct to say that a flame is not a thermodynamic phase at all in this sense. An ordinary candle or bonfire flame contains hot particles of solids, ionised gases, non-ionised gaseous volatiles and probably many more transient objects besides. Certain instances of flame may begin to show some of the electromagnetic properties characteristic of true plasmas. In other words, there isn't a 'yes' or 'no' answer.

 

[iantresman]

"We also subjected the candle flame to very strong horizontal electrical fields. The flame splits into two horizontal flames" (ref)

I recall doing something like this in physics class.

I am not sure whether this suggests that the flame is ionized (and hence a plasma), or whether the electric field contributes to the ionization process?

 

[Astronuc]

Here in his Problems Item No.6, "A typical flame" in which he asks rhetorically, "Convince yourself that these are plasmas". Perhaps Chen's KT=0.1 is too high for a typical plasma.)

Actually kT=0.1 would be too low for a typical plasma, but is correct for a typical flame at about 1160 K, however in addition to the energy/temperature one has to look at the particle density.

If one applies the basic relationship between pressure (P), particle density (n) and temperature (kinetic energy) kT for a plasma or gas, namely P = nkT, then we can write

n = P / kT, and for a typcial flame, P ~ 1 atm (10⁵ Pa), and kT = 0.1 eV, then one obtains

n = (10⁵ J/m³)/ (0.1 eV * 1.602x10^-19 J/ev) = 6.2 x 10²⁴ m^-3.

Even an order of magnitude less, the atomic density in gas is ~10²³ m^-3.


A gas with a particle density of 10²⁴ m^-3 would be essentially a vacuum, and in fact is less than a fusion plasma, and is certainly not a 'typical flame'.

It appears that the example in Chen's textbook is in error, and instead n should equal 10²⁴ and not 10¹⁴ m^-3.

I believe that typical flames are considered weakly ionized gases, weakly affected by static magnetic and electric fields. Certainly intense electric and magnetic fields (and electric currents) can be used to ionize a flame or gas, but that is not the issue here.

 

[Paulanddiw]

I remember a demonstration with a tall gas flame that had two electrodes, one near the top, the other near the burner. The electrodes were excited with the output of a audio amplifier which had been passed through a transformer so that its voltage was on the order of 1000 volts (maybe more). You could hear music, from the audio source, coming out of the flame.

The explanation they gave was that the small ion concentration in the flame conducted current from the electrodes which caused the flame to contract/expand generating music.

 

[pixel01]

I remember a demonstration with a tall gas flame that had two electrodes, one near the top, the other near the burner. The electrodes were excited with the output of a audio amplifier which had been passed through a transformer so that its voltage was on the order of 1000 volts (maybe more). You could hear music, from the audio source, coming out of the flame.

The explanation they gave was that the small ion concentration in the flame conducted current from the electrodes which caused the flame to contract/expand generating music.

Very unique and expensive to operate loud speaker.
Flame does conduct electricity. You may have heard the FID : flame ionization detector

 

[iantresman]

Very unique and expensive to operate loud speaker.
Flame does conduct electricity. You may have heard the FID : flame ionization detector

Which would be consistent with flames being a plasma. I note more information can be found on Wikipedia on the flame ionization detector.

 

[Astronuc]

Very unique and expensive to operate loud speaker.
Flame does conduct electricity. You may have heard the FID : flame ionization detector

What is the voltage?

Which would be consistent with flames being a plasma. I note more information can be found on Wikipedia on the flame ionization detector.

Not necessarily.

Doped flames will have anions and cations within the combusted gases, and that will certainly conduct electricity, particularly with an impressed voltage.

Usually 'plasma' implies free electrons with + ions (cations). I've seen a term 'weakly ionized gas', but what is not clear is whether the 'weakly ionized gas' is a plasma or not, and in addition to the three critieria previously applied, if there is some threshold of free electrons required to meet the definition of 'plasma'.

 

[dst]

Could fire be considered a plasma if we can run an electrical charge through it? In the case of the two electrodes @ 1000V, would that flame running between them be considered a plasma?

 

[iantresman]

It appears that the example in Chen's textbook is in error, and instead n should equal 10²⁴ and not 10¹⁴ m^-3.

I believe that typical flames are considered weakly ionized gases, weakly affected by static magnetic and electric fields.

Ten orders of magnitude are somewhat out, though I do not know where Chen obtained his figures from.

I found one paper which quotes an ion density of an unseeded flame as 10⁸-10⁹ ions/cm³ (= 10¹⁴ - 10¹⁵ m^-3) (ref). A book on flames and combustion found that CO had an ion density at 3000K of 10⁹ ions/cm³ (= 10¹⁵ m^-3) (Ref)

I also just found some images of flames influenced by electric fields, on a site at the Institute of Physics of University of Latvia.

 

[iantresman]

Actually kT=0.1 would be too low for a typical plasma, but is correct for a typical flame at about 1160 K, however in addition to the energy/temperature one has to look at the particle density.

If one applies the basic relationship between pressure (P), particle density (n) and temperature (kinetic energy) kT for a plasma or gas, namely P = nkT, then we can write

n = P / kT, and for a typcial flame, P ~ 1 atm (10⁵ Pa), and kT = 0.1 eV, then one obtains

n = (10⁵ J/m³)/ (0.1 eV * 1.602x10^-19 J/ev) = 6.2 x 10²⁴ m^-3.

Even an order of magnitude less, the atomic density in gas is ~10²³ m^-3.


A gas with a particle density of 10²⁴ m^-3 would be essentially a vacuum, and in fact is less than a fusion plasma, and is certainly not a 'typical flame'.

It appears that the example in Chen's textbook is in error, and instead n should equal 10²⁴ and not 10¹⁴ m^-3.

I contacted Francis Chen for clarification, who replied as follows;

I have never measured a flame, but the density I quoted is quite reasonable. The plasma density 10^14 m-3 is 10^8 per cc, a very low laboratory density but one which is used in some experiments. Atmospheric density is 3 x 10^19 per cc, so only one molecule in more than 10^11 is ionized. It is a very low degree of ionization, which is expected, since the electron temperature is so low. What makes it a plasma is that the Debye length is 0.25 mm, smaller than the flame.​

I think he's saying that the ion density is only the partial density of the overall "gas" density, because a flame is a partially ionized plasma, and we're interested only in the ions, not the neutrals.

 

[Paulanddiw]

What is the voltage?

Not necessarily.

Doped flames will have anions and cations within the combusted gases, and that will certainly conduct electricity, particularly with an impressed voltage.

Usually 'plasma' implies free electrons with + ions (cations). I've seen a term 'weakly ionized gas', but what is not clear is whether the 'weakly ionized gas' is a plasma or not, and in addition to the three critieria previously applied, if there is some threshold of free electrons required to meet the definition of 'plasma'.

I saw the demonstration years ago. Now that you mention it, I believe he had doped the flame with salt or something.

 

[roguengineer1]

ok...

 

[MaWM]

I contacted Francis Chen for clarification, who replied as follows;

I have never measured a flame, but the density I quoted is quite reasonable. The plasma density 10^14 m-3 is 10^8 per cc, a very low laboratory density but one which is used in some experiments. Atmospheric density is 3 x 10^19 per cc, so only one molecule in more than 10^11 is ionized. It is a very low degree of ionization, which is expected, since the electron temperature is so low. What makes it a plasma is that the Debye length is 0.25 mm, smaller than the flame.​

I think he's saying that the ion density is only the partial density of the overall "gas" density, because a flame is a partially ionized plasma, and we're interested only in the ions, not the neutrals.

Ok, I can see his point. However, we can reinterpret the original question. If we removed the charged particles from a flame would it look any different? No, probably not. So, what we can say is that the appearance of the flame, its glow, its shape, its heat, the qualities that most people think about when they think of a flame, are not due to its plasma properties.

 

[roguengineer1]

interesting enough. so in your opinion it's not a plasma...??

 

[MaWM]

not if that question is intended in a practical commonsense definition of "flame".

Seems claiming that flame is a kind of plasma is like claiming that a Ferrari is a kind of office desk. Which is to say, you *can* put your computer on it if you really want to, but that's not usually its primary function.

 

[iantresman]

Ok, I can see his point. However, we can reinterpret the original question. If we removed the charged particles from a flame would it look any different? No, probably not. So, what we can say is that the appearance of the flame, its glow, its shape, its heat, the qualities that most people think about when they think of a flame, are not due to its plasma properties.

Many plasma, to a first approximation, behave as a gas, so this is not surprising. More so as a (candle) flame is not subjected to strong magnetic or electric fields; but if it were, it behaves as a a plasma, and it is because a flame is a plasma, that it is able to do so.

pixel01 mentioned that flames do conduct electricity, hence the flame ionization detector, and this is a property of plasmas, not gases.

I would also argue that you can see a flame because it is a plasma; plasma are prodigious produces of electromagnetic radiation, and the reason why 99.999% of visible universe is plasma.

 

[MaWM]

I would also argue that you can see a flame because it is a plasma; plasma are prodigious produces of electromagnetic radiation, and the reason why 99.999% of visible universe is plasma.

Nah.. while the plasma element of a flame may produce some light, the vast majority comes froms excited neutral atoms.

 

[iantresman]

Nah.. while the plasma element of a flame may produce some light, the vast majority comes froms excited neutral atoms.

Yes, I'm wrong. That would make sense because as Chen said, "only one molecule in more than 10^11 is ionized", so the small proportion of plasma would contribute next to nothing.

 

[chefcrsh]

OK I'll add my own argument from authority here, just to make things more tidy.

I just read in Tyson's "Death By Black Hole" where he claims normal fire is a plasma.

 

[marinebiopink]

hello i am doing an indapendant research project for class on the question "is fire a gas, liquid or plasma?" but the problem is i seem to be finding sevreal diffrent answers telling me that its nither so now i just want to know what is fire? as in what state of matter? so far i know for a fact its not solid seeming how my brother would wave his fingers thru the flame its not THAT hot so what is it?

 

[russ_watters]

Welcome to PF.

Post #2 contains a good link with an explanation. Is there a more specific question you have about it?