Fire

[Erasmus00]

I've been thinking about this for a few minutes, and an answer isn't coming to me. Why does fire have a well-defined outline? Any ideas?

-Will

[Tormod]

The first question that popped into my mind when I read your question was, "does it really"?

[Erasmus00]

The first question that popped into my mind when I read your question was, "does it really"?

 

well, I've been lighting matches and staring at them for awhile, and its not a gradual fall off, but a fairly sharp outline. It flickers a bit, but its sharp.

-Will

[infamous]

The first question that popped into my mind when I read your question was, "does it really"?

If you abserve the shape of the flame given off by a candle, one could reckon a rather distinct shape. This is however only one example, if one chooses to find a recognizable shape for say, a house fire, this becomes less defined. My own opinion is that; the shape is determined by the area of super heated gas which has risen to the point of combustion. Outside this boundry, one may still find combustable gases but tempertures fall below the required flash point of said gas.

[infamous]

Also, maybe as the heat of the fire rises, a vortex is created, kind of like a minature tornado which gives the appearance of a defined surface. Similiar to how we can recognize the shape of the tornado.

[Erasmus00]

If you abserve the shape of the flame given off by a candle, one could reckon a rather distinct shape. This is however only one example, if one chooses to find a recognizable shape for say, a house fire, this becomes less defined. My own opinion is that; the shape is determined by the area of super heated gas which has risen to the point of combustion. Outside this boundry, one may still find combustable gases but tempertures fall below the required flash point of said gas.

 

A quick google image search for house fire reveals that even in the case of house fire, you have a well defined region. It would seem to me that the temperature would drop off gradually away from the center of the reaction. This gradual temperature drop off, I would think, would result in a gradual radiation drop off.

-Will

[C1ay]

The first thing that came to my mind when I saw this was that someone was yelling 'Fire' in a crowded forum :friday:

[CraigD]

Why does fire have a well-defined outline?

The glowing region of a flame does appear well-defined, particularly a small flame, such as that of a small brazing torch or a butane lighter adjusted to its lowest working setting.

 

Flame is plasma, atoms where the electrons have become disconnected from their nuclei. The glow is photons emitted by electrons as they become reconnected to nuclei in the plasma. Because electrons in plasma readily lose energy due to interaction with surrounding air (this is, of course, the source of the energy for convective heating), the plasma containing region is confined to an elongated spheroid region close to the combustion.

 

In a near vacuum, plasma loses far less energy to surrounding molecules, diffusing further and into more irregular shapes. Large fires involve stronger convective air currents, whipping the plasma envelopes into wilder shapes – in a large fire, such as a bonfire, small envelopes of plasma can be observed detaching from the main envelope and existing independently for very brief durations.

[Erasmus00]

Flame is plasma

 

Ahh, there is my error. I had been thinking of the glow of the fire as thermal radiation.

-Will

[Bo]

a flame has a well defined shape in space, for the same reason for example a water droplet as a well defined shape: the flame is kept together by the intermolocular forces the shape of the flame is the shap that minimalises the intermolocular potential.

 

I'm not completely sure if a flame realy should be seen as a plasma. An ionized cloud seems more likely to me.

 

Bo

[Jay-qu]

Flame is plasma, atoms where the electrons have become disconnected from their nuclei.

 

does this mean that flame can be effected by a magnetic field?

 

Large fires involve stronger convective air currents, whipping the plasma envelopes into wilder shapes – in a large fire, such as a bonfire, small envelopes of plasma can be observed detaching from the main envelope and existing independently for very brief durations.

 

I have seen this and been stumped as to why it is possible! ahh the releif of finding answers :friday:

[CraigD]

does this mean that flame can be effected by a magnetic field?

I believe so. Since the positively charged nuclei and negatively charged electrons in a plasma are no longer strongly attached to one another, a moving (either the field or the plasma) magnetic field should deflect them in opposite directions. If this is done before the electrons have lost enough kinetic energy to recombine to the electron-less nuclei, the electrons will lose energy interacting with themselves and other nuclei, changing the frequency of the photons they emit. So a sufficiently strong moving magnetic field should change a flame’s shape and color, and create + ions of whatever’s being burned and free electrons and negative ions of whatever’s around.

[Jay-qu]

I believe so. So a sufficiently strong moving magnetic field should change a flame’s shape and color, and create + ions of whatever’s being burned and free electrons and negative ions of whatever’s around.

 

I think an experiment is in order! :friday: dont worry I will be carefull :friday:

 

but it is going to have to wait right now I'm off to my final physics exam!

[Edge]

Fire is indeed quite mysterious and interesting. It is energy I say, but well, does it have mass?...

[Jay-qu]

ok I tried it - and at first there seemed to be some deformation when I put the magnet close to the flame, but upon futher experimentation I put my finger close to the flame and saw the same defects. conclusion: I could not decern any noticible effects that a magnet had on a flame.

[CraigD]

ok I tried it - and at first there seemed to be some deformation when I put the magnet close to the flame, but upon futher experimentation I put my finger close to the flame and saw the same defects. conclusion: I could not decern any noticible effects that a magnet had on a flame.

I suspect you need a stronger magnet. I’d guess a small electromagnet would be the surest, cheapest, fastest way to get a strong magnetic field near a flame. Ideally, your school has one – I suspect that strong commercial electromagnets are pricey. They’re mechanically pretty simple, though, so building one to use 110 V AC isn’t terribly difficult.

 

Anyway, good luck and here's to you, Jay-qu, for doing real Science! :friday:

[Jay-qu]

I suspect you need a stronger magnet. I’d guess a small electromagnet would be the surest, cheapest, fastest way to get a strong magnetic field near a flame. Ideally, your school has one – I suspect that strong commercial electromagnets are pricey. They’re mechanically pretty simple, though, so building one to use 110 V AC isn’t terribly difficult.

 

Anyway, good luck and here's to you, Jay-qu, for doing real Science! :hihi:

 

Yeah the thought had crossed my mind - I was using a rare-earth magent. When it comes to making an electro-magnet is AC or DC better? the AC would create an alternating field, yeah? also what is ideal a high current, high voltage or both?