8 ms·
What’s a fire and why does it burn? (2016)
- jeromesalimao 5y agoPutting this here after reading the title. https://youtu.be/SXKlJuO07eM https://youtu.be/SXKlJuO07eM
- marethyu 5y agoOh, did Qiaochu finally returned to math after long hiatus?
- Smaug123 5y agoThis post is from 2016, it says at the top.
- thaumasiotes 5y agoThere's a home button above the top that will show the most recent post, from November 2020.
- jiggawatts 5y agoCompare a bunch of chemical formulas and equations in this blog with Richard Feynman's explanation of fire: https://www.youtube.com/watch?v=N1pIYI5JQLE https://www.youtube.com/watch?v=N1pIYI5JQLE
- deleted 5y ago[deleted]
- jbay808 5y agoOr Michael Faraday's masterpiece "the chemical history of a candle", which deep-dives not only into what every part of the fire is, but also how we know what it is. https://www.gutenberg.org/cache/epub/14474/pg14474.txt https://www.gutenberg.org/cache/epub/14474/pg14474.txt The whole book is a delight to read, but I really like this part: > And now I want you to follow me in this explanation. You would hardly think that all those substances which fly about London, in the form of soots and blacks, are the very beauty and life of the flame, and which are burned in it as those iron filings were burned here... > I want you now to follow me in this point,--that whenever a substance burns, as the iron filings burnt in the flame of gunpowder, without assuming the vaporous state (whether it becomes liquid or remains solid), it becomes exceedingly luminous. I have here taken three or four examples apart from the candle, on purpose to illustrate this point to you; because what I have to say is applicable to all substances, whether they burn or whether they do not burn,--that they are exceedingly bright if they retain their solid state, and that it is to this presence of solid particles in the candle-flame that it owes its brilliancy. >... This flame has carbon in it; but I will take one that has no carbon in it. There is a material, a kind of fuel--a vapour, or gas, whichever you like to call it--in that vessel, and it has no solid particles in it; so I take that because it is an example of flame itself burning without any solid matter whatever; and if I now put this solid substance in it, you see what an intense heat it has, and how brightly it causes the solid body to glow. This is the pipe through which we convey this particular gas, which we call hydrogen, and which you shall know all about next time we meet. And here is a substance called oxygen, by means of which this hydrogen can burn; and although we produce, by their mixture, far greater heat[8] than you can obtain from the candle, yet there is very little light. If, however, I take a solid substance, and put that into it, we produce an intense light If I take a piece of lime, a substance which will not burn, and which will not vaporise by the heat (and because it does not vaporise, remains solid, and remains heated), you will soon observe what happens as to its glowing. I have here a most intense heat, produced by the burning of hydrogen in contact with the oxygen; but there is as yet very little light--not for want of heat, but for want of particles which can retain their solid state; but when I hold this piece of lime in the flame of the hydrogen as it burns in the oxygen, see how it glows! This is the glorious lime-light, which rivals the voltaic-light, and which is almost equal to sunlight. I have here a piece of carbon or charcoal, which will burn and give us light exactly in the same manner as if it were burnt as part of a candle. The heat that is in the flame of a candle decomposes the vapour of the wax, and sets free the carbon particles--they rise up heated and glowing as this now glows, and then enter into the air. But the particles when burnt never pass off from a candle in the form of carbon. They go off into the air as a perfectly invisible substance, about which we shall know hereafter. > Is it not beautiful to think that such a process is going on, and that such a dirty thing as charcoal can become so incandescent? You see it comes to this--that all bright flames contain these solid particles; all things that burn and produce solid particles, either during the time they are burning, as in the candle, or immediately after being burnt, as in the case of the gunpowder and iron-filings,--all these things give us this glorious and beautiful light. >... That, however, is not the only thing I wish to mention. If I take a flame sufficiently large, it does not keep that homogeneous, that uniform condition of shape, but it breaks out with a power of life which is quite wonderful. I am about to use another kind of fuel, but one which is truly and fairly a representative of the wax or tallow of a candle. I have here a large ball of cotton, which will serve as a wick. And, now that I have immersed it in spirit and applied a light to it, in what way does it differ from an ordinary candle? Why, it differs very much in one respect, that we have a vivacity and power about it, a beauty and a life entirely different from the light presented by a candle. You see those fine tongues of flame rising up. You have the same general disposition of the mass of the flame from below upwards; but, in addition to that, you have this remarkable breaking out into tongues which you do not perceive in the case of a candle. Now, why is this? I must explain it to you...
- greenyoda 5y agoI also really enjoyed Bill Hammack's series of videos based on Faraday's lectures: http://www.engineerguy.com/faraday http://www.engineerguy.com/faraday
- ThePowerOfFuet 5y ago> This is the glorious lime-light Well holy heck. TIL.
- Maursault 5y agoJames Burke's Connections episode 9: Countdown [1], covers the invention of limelight, gives a history of the technology and places it in context. The entire series is intertwined (thus the title, Connections), but if you start watching Ep. 9 at 18:30 it gives plenty of preamble for the need for the invention before introducing it and covering its adoption. [1] https://archive.org/details/james-burke-connections_s01e09 https://archive.org/details/james-burke-connections_s01e09
- h2odragon 5y agoSee also bone fires (bonfires)
- qiskit 5y agoHere is feynman on AI: https://www.youtube.com/watch?v=ipRvjS7q1DI https://www.youtube.com/watch?v=ipRvjS7q1DI Feynman turned his focus to computing ( AI, heuristics, quantum computing ) later on his life.
- philipfweiss 5y agoWhat if... the author wasn't aiming this post for a completely lay audience, but rather someone with a undergraduate/graduate level of mathematics/physics background who was interested in learning more about some phenomenon?
- fennecs 5y agoNope everything needs to be completely intuitive, explaining in detail is completely wrong /s I think both can complement each other, but it is definitely nice to go into the details. Watch the Feynman explanation then try to use that as you go through the actual details. The initial more hand wavy, but morally correct explanation, gives you something to latch onto, and then you can use that to guide your reasoning in the more complicated steps. Honestly the idea or expectation that things need to be read linearly is also wrong, you should include some reasonable level of detail and elaboration in your writing, but organise it so it can be picked over. You can switch betweeen levels too.
- Graffur 5y agoWow. I read the initial blog post (well I read the start, and skimmed the rest) and I thought it was interesting but I am just not motivated to dive into something so complex. I learned nothing. Then I watched Richard Feynman's explanation and feel like I learned something! And was motivated about it!
- ravi-delia 5y agoFeynman's explanation is obviously great, but it and the blog have different goals. There is really no sense in which fire is best explained by Planck's Law. Obviously the goal of the blog post is to explore how physics is nested, with complex phenomena the result of simpler laws which are the result of even simpler laws. I mean, the part that's about chemistry specifically, which most would call the part that's about fire, is only the first 1/5 or so. It is easier to learn and feel motivated to learn less physics rather than more, and Feynman's gift was imparting the most essential information with the least physics possible, a sort of distillation of physicist-intuition for the layman.
- kamaal 5y ago>>I watched Richard Feynman's explanation and feel like I learned something! I don't know the link to the video on YouTube, but there was one interview where they were discussing why Math appears hard to so many people. Basically people look at solving Math problem like an elaborate heuristic needed to be memorised to solve the problem. He proceeds to explain that's for people who don't understand what they are doing. To understand anything at all, is to start with basic assumptions/axioms and logical rules of manipulating them and moving forward from there.
- mortenjorck 5y agoSometimes tangential HN comments are a gift. Thank you for this one.
- kurthr 5y agoI would note, that I think the units of the last two equations of 0.5um to 0.3um for a nuclear fire, should probably be 0.5nm to 0.3nm to be Xrays, since they should be a bit more than 1000x hotter.
- dataflow 5y agoDifferent colored fires: https://www.youtube.com/watch?v=DHtNrH3P2zU https://www.youtube.com/watch?v=DHtNrH3P2zU Black fire: https://www.youtube.com/watch?v=5ZNNDA2WUSU https://www.youtube.com/watch?v=5ZNNDA2WUSU
- agumonkey 5y agoThat's how you double the rate of physics/chemistry students.
- giantg2 5y agoUntil they realize that most of the work is much more boring.
- agumonkey 5y agoMost work is structurally boring, from obligation, low risk and repetition. At least you're bored AND educated :)
- giantg2 5y agoThat could be said of me as a dev. I don't see (formal) education itself as an asset. It's a means to an end. Most of the education is not used and forgotten. The type of people who are able to learn, might be better off learning what they want without an education.
- agumonkey 5y agoIt's a mean to an end but also a way to alter/improve your perception of things and your existence outside the daily duties.
- giantg2 5y agoIt's possible that in many areas of life ignorance truly is bliss. Being educated might mean you see through the BS but there's nothing you can do to change it.
- philipfweiss 5y agoQiaochu, unrelated, but I really enjoy your contribution to MathOverflow. This article is equally solid :)
- deleted 5y ago[deleted]
- fennecfoxen 5y agoWhy does the submitter hate The Little Mermaid (1989)? Original article title is "What’s a fire, and why does it – what’s the word – burn?".
- danaris 5y agoMaybe that's too long for the title char limit?
- selimthegrim 5y agoThey must be fans of Malá mořská víla (1976) instead.
- jotm 5y agoIt kind of blew my mind to learn (and internalize) that everything burns under the right conditions (mostly temperature). Metals, bodies, water, everything.
- aidenn0 5y agoDepending on your definition of "burn" the noble gases might not qualify; they'll form stable compounds with fluorine as the oxidant but not oxygen
- willis936 5y agoIf burning requires breaking chemical bonds: loads of substances don't burn. If burning is the act of ionizing, then everything burns. Humans like to put things in neat boxes, but nature doesn't really care about our boxes.
- aidenn0 5y agoBurning certainly does not require breaking chemical bonds (or else Hydrogen is not flammable). I always considered burning to be oxidation; possibly only when oxygen is the oxidizing agent.
- pdonis 5y agoHow would you burn water?
- ncmncm 5y agoYou would need stuff that steals the hydrogen away from the oxygen, or the oxygen away from the hydrogen. It probably involves fluorine or sodium.
- weberer 5y agoIf you pass water vapor through a high voltage spark gap, it creates hydrogen peroxide. Not sure if that would count as "burning".
- santiagobasulto 5y agoThanks for this. Almost 20 years ago I was in high school and I asked my physics teacher "what is fire?". He looked at me with an expression that clearly said (but without words): "this kid is stupid". After I re-affirmed my question and my desire to know what "actually" was fire, he just couldn't answer. Everybody laughed, of course. Now, almost 20 years after, I have a partial answer.
- anitil 5y agoI had a similar experience with static electricity. The explanation was something like 'When you rub wool on a glass rod you develop a charge between them'. Ok, so.... why? And why does the charge develop with one polarity and not the other? Now I'm a bit older I understand that the explanation is somewhere between "the geometry of pointy things" and "get a PhD and you'll understand some of it"
- x3n0ph3n3 5y agoI'm reminded of this video from a few years ago: What is a flame? [1] 1. https://www.youtube.com/watch?v=5ymAXKXhvHI https://www.youtube.com/watch?v=5ymAXKXhvHI
- dboreham 5y agoBah. The article doesn't appear to answer my long-standing question which is : where exactly do the photons come from? It says black bodies emit photons, but...how exactly? It says maybe its emission spectrum photons which aren't black body radiation. I have a slightly better idea where those photons come from: an electron changes energy state in an atom and as a side-effect a photon is released. Still don't really know where the photon came from though..
- dataflow 5y agoIIRC (from high school chemistry?) electrons go into higher energy levels due to the release of chemical energy, then emit photons as they fall back to lower energy levels (the energy of the photon being the delta between the energy levels). Is that accurate/am I missing anything?
- whatshisface 5y agoTake an electron and shake it back and forth. Everything that feels its force will shake too. That's a photon.
- etrautmann 5y agoI had not quite thought about it in this way, thank you!
- femto 5y agoAnd if we get two photons above a certain energy and slam them together we get an electron and a positron. So where did they come from and where did the photons go? The electron emits a photon: photons from photons. And all those virtual photons that make the electron hang around near the the nucleus. It seems like its photons all the way down...
- dbsmith83 5y ago> It says black bodies emit photons, but...how exactly? When an object has any temperature above 0K, that means the chemically bonded particles are moving around/vibrating. This movement of charged particles (release of internal energy) creates electromagnetic radiation (photons). The other radiation you are talking about is also energy being released as photons, but it is from electrons in an excited state falling to a lower energy state. Molecules can take in energy differently. Some frequencies of incoming energy produce bond movement/vibrations, and other frequencies can excite an electron's state. Consequently, when the energy is released, it can be in different frequencies (infrared/heat, visible light, UV, etc.) depending on the amount of energy lost. Since electron orbitals are discrete states, the release of energy from an electron going to a lower orbital is a discrete frequency of light.
- bawolff 5y ago> [ways to extinguish a fire] remove the heat (e.g. spraying a fire with water) I've always wondered about this. To me i assume water smoothers not makes cold. After all, water doesn't extinguish an oil fire, (which is i presume because oil floats) which i would expect it to if its solely about cooling things down. Are there any experiments i could convince myself its really about cold and not smoothering?
- jameshart 5y agoThere's definitely cases where water can be used to smother fire (think: plunging something burning into a large body of water). But if you're throwing water onto a fire.. Consider that in order to smother a fire - to form an actual barrier between fuel and oxygen - water would have to remain liquid, which it can only do if its temperature stays below 100°C. For water to stay below 100°C, whatever the water is in contact with also needs to be below 100°C. So you can't actually permanently smother something with liquid water without also cooling it down to below 100°C. An experiment you could try would be to maybe take a matchstick and soak it in some water. Then dry off the outside of it, so you can tell there's definitely no water covering the fuel of the stick. Then try to set fire to it.
- Doxin 5y agoRight, but wouldn't the steam produced assist in smothering the fire as well? Water expands rather a lot as it turns to steam.
- dTal 5y ago>Consider that in order to smother a fire - to form an actual barrier between fuel and oxygen - water would have to remain liquid Why? Steam can still displace oxygen. We smother fires with inert gases all the time.
- EricE 5y agoMuch of that I was familiar with - the very end did throw me a curve ball: "Planck’s law doesn’t just stop at the maximum, so nuclear explosions also produce even shorter wavelength radiation, namely gamma rays. This is solely the radiation a nuclear explosion produces because it is hot, as opposed to the radiation it produces because it is nuclear, such as neutron radiation." I have heard that in different ways but I guess I didn't grok it until reading that last sentence in particular. I always love unexpected insights.