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What’s a fire, and why does it burn?
- joeevans1000 10y agowell, then... https://www.youtube.com/watch?v=edQ08Eze0jM https://www.youtube.com/watch?v=edQ08Eze0jM .
- jimmyswimmy 10y agoThat was awesome. Reminded me of the Simpsons episode where the PTA disbanded and Professor Frink teaches a nursery school class: playing with a baby toy, he tells the children they can't play with it because they won't appreciate it on as many levels as he does. I had a quantum mechanics professor explain to us the difference between a classical mechanics fireplace and a quantum one. Because in classical mechanics energy is distributed continuously, it would emit gammas,x-rays,etc and would kill you if you could see it. The quantum fireplace would behave as fireplaces do, a much less destructive manner. Fun read.
- RangerScience 10y agoThis is definitely the most detailed description of fire I've ever seen. Note - from an artistic perspective, and AFAIK, wood (and kerosene, and other sooty fuels) produce a "deep"-looking flame, while propane (and other 'clean' fuels) produce a "shallow"-looking flame, due to the black colors introduced by the soot. Compare the look of the fire in these propane effects: http://www.effectspecialist.com/special_effects_Propane_flame_cannons.htm http://www.effectspecialist.com/special_effects_Propane_flam... to the look of the fire in this kerosene effect: http://mentalfloss.com/article/71663/watch-mesmerizing-fire-tornado-spin-slow-motion http://mentalfloss.com/article/71663/watch-mesmerizing-fire-... This is also (AFAIK) why you generally don't see "sooty" colored flame effects - the amount of the coloring agent that is needed to overcome the "darkening" of the soot is impractical, at least for large-scale effects. Source: That one time I got really excited the talked to people who'd made a (gigantic) flame tornado art piece.
- JshWright 10y agoThe differences in the examples you provided have more to do with the exposure settings of the camera than the carbon (soot) content of the flame. It's actually the heated carbon particles that give flames their yellow color. The soot itself is what's glowing.
- RangerScience 10y agoHmm. I mean, your first point is straight up wrong - I've spent far too much time looking at (and taking pictures of!) different kinds of fire to think the difference in appearance between propane and kerosene is just the camera settings. But your second point seems pretty reasonable; I'm still not sold. Can you go into more detail as to how the presence of glowing soot in one kind of fire - and absence in the other - would create the appearance of solidity (or lack thereof)? Edit: Sorry, that came out real aggressive. I mean, can you throw some more words at how the glowing soot changes the look of the fire - regardless of the artistic interpretation of the difference?
- JshWright 10y agoSorry, I was referring to the specific examples you provided. While those flames would indeed look different, those differences would be completely overshadowed by the differences in exposure. In one case you have flames that are _completely_ blown out (due to the dark surroundings), and in the Slow Mo Guys video the camera has been set to capture the details of the flame. There are absolutely differences between various flames burning in various conditions. When it comes to burning hydrocarbons (propane, methane, kerosene, gasoline, etc), that difference has more to do with the fuel/air mixture than it has to do with the specifics fuel. It is a lot easier to get a good fuel/air mixture when burning a gas like Propane. When a pool of liquid is burning, it is almost impossible to get a good mixture of fuel and air (even with a bunch of fans), so it is burning fuel rich, resulting in a lot of unburned fuel intermixed with the flame. I would suggest Periodic Videos recent video about the bunsen burner. They achieve both sorts of flames you're talking about with a single fuel source (methane). https://www.youtube.com/watch?v=in9BGz958jg https://www.youtube.com/watch?v=in9BGz958jg
- JshWright 10y agoTwo videos I'd like to share (well, one video, and one playlist)... The first is a 'flashover simulator'. This is a training tool for firefighters to help them recognize the signs of a 'flashover'. A flashover occurs when the radiant heat of the fire is sufficient to cause pyrolysis and ignition of more or less every surface in the room simultaneously (the floor, furniture, etc). I'm mostly sharing this because sitting in one of these is where it first clicked for me that fire burns gasses, not solids. Watching the fire snake lazily through the smoke overhead, until it felt like I was sitting on the bottom pool, looking up at the surface. https://www.youtube.com/watch?v=-Z4xxObrTlI&t=40s https://www.youtube.com/watch?v=-Z4xxObrTlI&t=40s The second is Bill Hammack (The Engineer Guy) delivering Faraday's lecture series on The Chemical History of a Candle. This is a truly excellent lecture series, despite its age (they did choose to keep most of the original language, which can take a little getting used to) https://www.youtube.com/playlist?list=PL0INsTTU1k2UCpOfRuMDR-wlvWkLan1_r https://www.youtube.com/playlist?list=PL0INsTTU1k2UCpOfRuMDR...
- developer2 10y agoThat first video is visually stunning, and also gives a complete noob a good idea of what to expect from fire. It's one thing to have some generalized idea that "fire spreads quickly"; it's quite another to see how a seemingly low-key flame will almost instantly burst into an unmanageable fireball. It's interesting to see that there is a pre-emptive warning to a "flashover" - you can see the edge of the fire, for lack of better terms, "licking" or "curling" along the edge. Of course that visual warning is of little consolation if it comes with less than a couple of seconds of time to react; especially without a fire retardant suit. Imagine how many lives would be saved if every citizen could experience this kind of training first hand. I think inexperienced individuals are likely to think there is time to gather belongings "while the fire is still scaling". How many of us, upon seeing a fire "slowly" taking over a corner of the room, would believe we have plenty of time to collect our most prized or expensive possessions before running out the door? Random side thought: I wish we'd come up with a better way of testing evacuation drills. All drills do is teach people that every alarm is fake, and so everyone spends minutes packing up their laptops, grabbing their coats, etc. It's disconcerting to watch people "finishing a block of code", drinking their coffee, or texting on their phone at an office when an alarm goes off. And I've seen that at a company where the executive team planned an unannounced drill 2 months after the scheduled annual drill. As in, nobody knew it was a test. And yet it took 15 minutes to evacuate a small building. facepalm
- falcor84 10y agoI came here for the Little Mermaid reference and definitely got some answers. Thank you for writing this!
- fennecfoxen 10y agoI'm disappointed in whoever effaced the Little Mermaid reference from the title of this article. I thought we were supposed to use the titles the source article used. :b
- padobson 10y agoFor the uninitiated[0][1]: And ready to know what the people know / Ask 'em my questions / And get some answers / What's a fire and why does it / What's the word? burn? [0]https://play.google.com/music/preview/Tgiolwxoj2rmb6sqjvlnphb6cum?lyrics=1&utm_source=google&utm_medium=search&utm_campaign=lyrics&pcampaignid=kp-lyrics https://play.google.com/music/preview/Tgiolwxoj2rmb6sqjvlnph... [1]https://youtu.be/SXKlJuO07eM https://youtu.be/SXKlJuO07eM
- leetrout 10y agoWhen I was a volunteer firefighter circa 1999 we were forced to learn the definition of fire that would appease a lawyer / court in case we got subpoenaed. Apparently somewhere else there had been an arson fire and the fire chief was on the stand and the defense asked him what fire was and all he could come up with was "it's hot" or something similar. I don't know if it caused the case to be won or lost but our Chiefs decided we wouldn't chance the embarrassment. I'll now forever remember that "fire is the rapid oxidation of a combustible material resulting in the release of energy in the form of heat and light". Still waiting on that subpoena and my chance to put this to good use. Or at least trivia night or something...
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- sandworm101 10y agoHeat and radiation, some of which is called light. Infrared energy isn't really heat, nor is it generally called light as it isnt visible. Similarly, many chemical fires release substantial amounts of UV, but i doubt anyone would see much of it in an emergency through all the smoke and ir. Think rocket fuels.
- nsxwolf 10y agoI've frequently seen "light" used to refer to the entire electromagnetic spectrum in physics discussions.
- swift 10y agoYeah, I typically hear visible light referred to as, well, "visible light". It may depend on the whims of the particular textbooks and professors you learned from, but in my physics classes we always used "light" to refer to the entire electromagnetic spectrum, and I only realized relatively late in life that some people don't use the terminology that way.
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- shmerl 10y agoIt could also mention that fire can be considered plasma[1]. 1. http://www.askamathematician.com/2013/05/q-is-fire-a-plasma-what-is-plasma/ http://www.askamathematician.com/2013/05/q-is-fire-a-plasma-...
- nishs 10y agoThoroughly entertaining read. This article does not cover the history of scientific theory around fire, which is quite fascinating too. Scientists in the 17th century theorized (incorrectly, of course) that fire was an intrinsic property of "phlogisticated" materials [0], which could be burnt to release the phlogiston contained in them. The theory prevailed for over 100 years, until Lavoisier proved that a gas (oxygen) must be present for combustion by measuring the increased weight of phosphorus upon burning [1]. [0] Phlogiston theory: https://en.wikipedia.org/wiki/Phlogiston_theory https://en.wikipedia.org/wiki/Phlogiston_theory [1] Oxygen theory: https://en.wikipedia.org/wiki/Antoine_Lavoisier#Oxygen_theory_of_combustion https://en.wikipedia.org/wiki/Antoine_Lavoisier#Oxygen_theor...
- roywiggins 10y agoI just made the connection between phlogiston and the "magic smoke" theory of electrical devices :)
- hexane360 10y agoI love the sheer inertia that theory showed. Even after we knew burning added mass, many scientists believed that it didn't disprove phlogiston, only prove that phlogiston has negative mass.
- jcranmer 10y agoIt should be noted that, for a time, the phlogiston theory survived even after Lavoisier's experiments. It only showed that phlogiston had negative weight, what's wrong with that? :-)
- shmageggy 10y ago> Combustion is in some sense the opposite of photosynthesis, an endothermic reaction which takes in light, water, and carbon dioxide and produces hydrocarbons. This is a neat idea that I hadn't heard before!
- bcbrown 10y agoI've heard it said that Buckminster Fuller described fire as "sunlight unwinding from tree trunks", which struck me as a very poetic way to phrase it.
- photogrammetry 10y agoQuite beautiful. Thank you for the quote.
- rcthompson 10y agoThe other interesting aspect is that it's more or less the same overall reaction that we perform in our cells to derive energy from organic materials (though the mechanism is very different). I think this is partly why we say things like "exercise burns fat".
- JoachimS 10y agoFeynmann had some really nice description of this: https://www.youtube.com/watch?v=N1pIYI5JQLE https://www.youtube.com/watch?v=N1pIYI5JQLE
- myself248 10y agoThat's interesting, I've always described metabolism and combustion as the opposite of photosynthesis. You even get back the water that the plant consumed, in the form of steam or condensation dripping out the car's tailpipe, or desert rodents that don't need to drink liquid water because they're so good at keeping the water they metabolize from food! We just use the atmosphere as a temporary place to store the oxygen, and keep it diluted with enough nitrogen that things don't immediately unphotosynthesize themselves. ;)
- mncharity 10y agoMy "What is flame?" - a contest entry: http://www.clarifyscience.info/part/L1Uko http://www.clarifyscience.info/part/L1Uko Skimming OP, one thought. My very fuzzy understanding is that, while flame has a spectra shaped like a blackbody curve, it isn't one. Instead, that's the soot emission spectra (due to its diverse mess of bonds). The BBR spectra is much redder and dimmer. This misconception appears widespread. Meta-observation: Creating fun and correct science education stories is hard, requiring multi-year collaborative effort from many people. It seems a pity there's nowhere on the web to build such. ... Wikipedia has "no original research". The US national science education wiki was/is? just a search engine for high-barrier low-quality paywalled science education magazines. Here in the wretched past, the best we manage to do is accumulate scattered blog and discussion threads. Disrupt, please?
- jcranmer 10y ago> ... Wikipedia has "no original research". But they do have a "List of common misconceptions" page.
- mirimir 10y ago> The heat produced by combustion can be used to fuel more combustion, and when that happens enough that no additional energy needs to be added to sustain combustion, you’ve got a fire. Sure, but what does "sustain combustion" mean? Well, it provides activation energy for reactions. And here's the thing: lack of activation energy is all that keeps us and other combustible stuff from burning.
- janvdberg 10y agoI thought this would be a link to a Richard Feynman video where he 'explains' fire. But it isn't, so here you go: https://www.youtube.com/watch?v=N1pIYI5JQLE https://www.youtube.com/watch?v=N1pIYI5JQLE
- JoachimS 10y agoDang you beat me to it.
- falsedan 10y ago> Thermal radiation is produced by the motion of charged particles: Agreed. > anything at positive temperature consists of charged particles moving around This does not follow: infrared radiation != temperature. Neutron stars have positive temperature…
- falsedan 10y agoDear downvoters, please comment to let me know what I should improve. Thanks, falsedan
- philipkglass 10y agoYou are being downvoted because you appear to be trying to correct a mistake that does not exist in the source material. I can make sense of your attempted correction by reading "thermal radiation" as "infrared radiation," but those terms don't mean the same thing. Thermal radiation refers to radiation emitted due to a body's temperature, but the peak emission frequency can go up into the visible and beyond if the body is hot enough. (Like the sun and larger stars.) Examples of non-thermal radiation would be e.g. excited state transitions in molecules that produce light in fireflies or the blue light from the lower portion of a candle flame. (The blue part of a candle flame is hot relative to the surrounding air and liquid wax but not hot enough that its thermal emissions would peak in the blue.) Fluorescent and LED light sources also emit visible light by non-thermal processes.
- falsedan 10y agoI think the error does exist; infrared radiation is a type of electromagnetic radiation and relies too n charged particles (electrons) absorbing energy, going through various excitation levels, and then emitting photons with particular frequencies. The error in the source is claiming that charged particles are necessary for a body to have a positive temperature (i.e. what I quoted). The error in the downvoting is a kneejerk correction without explaining + not reading in good faith.
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- SagelyGuru 10y agoParticles is the wrong word to use when trying to explain radiation heat of fire. Particles usually mean subatomic particles. Particles in general sense are too vague. Molecules would be better.
- pstch 10y ago"Particle" is used at all scales, most usually subatomic, but also microscopic and macroscopic. A car can be viewed as a particle, so I'm not sure it's so wrong to label a group a molecules as "particles". Also, the author was only referring to particles when trying to explain the concept of thermal radiation where he is actually speaking of subatomic particles, not of molecules.
- adriand 10y agoThis reminds me of growing up in a really conservative, religious household (think of it as a typical Bible Belt upbringing, except that instead of the south-east part of the US, I grew up in Canada, of Dutch heritage). God created the universe 6,000 years ago, etc. Anyway, to the detriment of their desire for me to grow up believing the same things they did, my parents were big readers and really encouraged me to read, and as a budding nerd, mainly what I read was science fiction and science non-fiction, which led me to have intense arguments with my parents. I vividly remember my father saying to me at one point, as his response to me making some point about the validity of modern science, that "no one knows how fire works", and that therefore science really wasn't so knowledgeable after all. I remember thinking, "surely if they've figured out atomic fusion, they know how fire works", but I really didn't have a proper response at the time. I had no idea how fire worked. It still seems magical! Some 30 years later, I finally have the answer...perhaps I should send it to him.
- flexie 10y agoThis post is a good reminder that innovations are so often made by people with zero or little education. Scientists don't agree on when humans learned to control fire. Some say 200,000 years ago, some almost 2 million years ago. But clearly, man learned to control and to make use of fire for heating and cooking, for agriculture and as weapons and even for machines long before anyone understood what it is or why it behaves like it does.
- 3chelon 10y agoI remember when I was about 10 I asked my teacher this question. He had always been a good teacher, encouraging open-minded debate, so I was quite shocked when he told me not to ask stupid questions. (In retrospect, he'd probably just had his fill of me asking unanswerable questions that day.) Glad to finally get a detailed answer, 39 years later!
- castratikron 10y agoIf this was written by someone who "isn't a physicist", this is an excellent summary of statistical physics.
- loeg 10y agoHe's a pure math guy.
- freebeeme 10y agoFor me fire is something nice, sitting around it sharing ideas, it burns to remind us things can have two faces. The negative burning or the positive warmt.
- Pica_soO 10y agoI expected to be Feyn-mannered. TL,DR; Stored chemical energy can be released, by adding to its jiggling- and produces enough energy to cause a jiggling chain reaction to other storages nearby.