The Weight of Fire

[Shadow]

Well, the title speaks for itself really. This is a reaction to a post made some time ago about what element fire was. Well, it's plasma. But how much does it weight? Or, to be more precise, what is it's density? How do you measure it? If it even has a density, does that mean that when you blow at a candle and the flame gets smaller, it's density increases? Or does the energy somehow dissipates? Thanks in advance for any and all answers.

[MrMongoose]

Well, for starters, it's not plasma;)

 

But it'll have a weight corresponding to its mass which is just the sum of all the particles that it is made of. As fire's generally got a fairly low density (being a warm gas), it will generally be quite light, but to put numbers to that, you'd need to specify what is being burnt under what conditions.

[insane_alien]

MrMongoose, it IS a plasma for the last time.

 

take a oxygen rich natural gas flame, burns bright blue. is it hot enough to emit primarily in the blue spectrum? no. nowhere close. this is because of plasma interactions. much like in a fluorescent bulb.

[John Cuthber]

When Mr Mongoose gets bored of the pretty colours he can measure the electrical conductivity of a flame too.

Anyway, since any flames are very much hotter than the air round them they are bouyant. Their weight, as measured in air, would be negative.

[Shadow]

So, that would mean, disregarding burns etc., that if I had a stick weighing, I don't know, half a kilo or something, and a 10 km high flame on it, I could still pick it up?

[thedarkshade]

So, that would mean, disregarding burns etc., that if I had a stick weighing, I don't know, half a kilo or something, and a 10 km high flame on it, I could still pick it up?

Well first there's no flame 10 km high, but even if there was the fact that it has gone so high tells that it is extremely light, a lot lighter than the air, so the possibility to pick it up cannot be dismissed.

[MrMongoose]

MrMongoose, it IS a plasma for the last time.

 

take a oxygen rich natural gas flame, burns bright blue. is it hot enough to emit primarily in the blue spectrum? no. nowhere close. this is because of plasma interactions. much like in a fluorescent bulb.

 

Oxygen rich is specific to some flames, and fluorescent bulbs irrelevantly do contain a lot of plasma. Last time though:( I was hoping for a convincing argument before you gave up.

[Mr Skeptic]

I'd say a flame contains only a little bit of plasma.

[MrMongoose]

I'd say a flame contains only a little bit of plasma.

 

THANK YOU.

 

I thought the whole world was against me:mad:

[insane_alien]

Oxygen rich is specific to some flames, and fluorescent bulbs irrelevantly do contain a lot of plasma..

 

not irrelevantly, it is by the same process in which both glow.

 

see the bit of the flame that glows? that is plasma. if it were merely hot gas you would be able to see right through it. even air at 3000*C (considerably hotter than a bunsen flame) is perfectly transparent.

[swansont]

I'd say a flame contains only a little bit of plasma.

 

Plasma, like a vacuum (or pregnancy), is a binary state which has a continuum of conditions where you designate it as such; in the case of plasma it's the ratio of charged/neutral particles. Even when the fraction is relatively small you still call it a plasma.

[Mr Skeptic]

THANK YOU.

 

I thought the whole world was against me:mad:

 

Well you got to be careful whether the whole world is against you or you're against the whole world

 

In any case, as swansont said, a little plasma is still plasma. Kind of like being only a little pregnant is still pregnant.

[MrMongoose]

The world started it... I just turned a playground fight into a nuclear war:o

 

I never claimed that there was no plasma in a flame, just that the largest part of a flame will generally be made of gas, which in my opinion makes it more proper to speak of a flame as gas than as plasma.

 

I still don't see the relevance of the fluorescent tube though. The two have some things in common, but a whole lot more is different.

[Sisyphus]

Whether you want to call it plasma or gas is not really relevant to the OP. It's still going to behave just like a very hot gas. Since the density of gases is inversely proportional to temperature, the matter in the flame will be very light, lighter than the air that surrounds it.

 

So, that would mean, disregarding burns etc., that if I had a stick weighing, I don't know, half a kilo or something, and a 10 km high flame on it, I could still pick it up?

 

Yes. That flame would not make the stick any harder to pick up. Just like the 10km of air over ANY stick doesn't make it any harder to pick up.

[Mr Skeptic]

Yes. That flame would not make the stick any harder to pick up. Just like the 10km of air over ANY stick doesn't make it any harder to pick up.

 

The updraft of a 10km flame would probably be enough to pick up anything that you can

[iNow]

Well, it's plasma. But how much does it weight? Or, to be more precise, what is it's density?

Is it possible you are actually asking how massive flame is? How much mass? I don't know the answer, but that would be different than weight (the effect of gravity on that mass).

[Shadow]

What I'm asking is, how is it that fire, which is matter as far as I can tell, has no weight, as in gravity having no effect on it. So yes, I'm also asking if it has any mass, or anything that would make it "heavy". Cause, as far as I know, photons are the only particles that have a mass of zero....so how does fire, or plasma in general, fit into all this?

[Mr Skeptic]

Fire is lighter than air. It has mass all right, but if you put it in air it will float, like a hot air balloon. It has weight, but it is less than the weight of air.

[Shadow]

I see....thanks Mr. Skeptic. Now, for my next question, what is it's exact mass? Or does it vary with the type of material being burned? Or do we even know for that matter?

[insane_alien]

to get the exact mass we would need(at the very least for a rough estimate) the flames height, width, burn rate, composition, pressure and temperature.

[Shadow]

Well, how about density?

[insane_alien]

still going to need most of that info.

[MrMongoose]

The density is the mass per volume, and one form or the perfect gas equation is p=rRT (r being the density, normally denoted by rho)

 

So r=p/RT

 

The density will depend on the gas that the flame consists of which will typically be a mixture of oxygen and the fuel youre burning. If you know the density at a certain pressure and temperature then you can work it out at any other pressure and temperature. The simple answer is that finding a reference state purely from theory is impossible to do accurately for all but the simplest cases, so it is measured from experiments.

[Shadow]

I see. Okay, thanks all ;-)

[brainfart94]

to get the exact mass we would need(at the very least for a rough estimate) the flames height, width, burn rate, composition, pressure and temperature.

 

thats not much:D

 

height: 10km

width: n/a

burn rate: n/a

composition n/a

pressure: n/a

temperature: n/a

 

we're getting close...