In reply to Abd ul-Rahman Lomax's message of Sun, 22 Apr 2012 21:52:35 -0500: Hi, [snip] >I don't know the specific energies involved, but my sense of this is >as follows: > >First of all, yes, it's obvious: gasoline does not ignite just >because an individual molecule is oxidized, as it will be if, say, a >cosmic ray with hign energy hits it. You can have a mixture of >hydrogen and oxygen, an "explosive mixture" and it will just sit >there, even if you expose it to, say, a few high-energy photons or >energetic particles. > >There is obviously a "hump" to get over to allow the oxidation >reaction to occur. Under conditions well below ignition temperature, >if a single reaction occurs, it cannot raise the temperature of the >local environment enough, the products are immediately cooled. >However, at -- or very close to -- the ignition temperature, only a >little extra heat is needed. The closer to ignition temperature, the >less the needed heat, until, at ignition temperature, the oxidation >starts happening en masse, it all heats up and a wave of ignition >passes through the material. > >Badly explained, perhaps. But the basic idea is that at low >temperatures, a little puff of heat doesn't do anything. A single >reaction is just that, it has no observable effect.
This appears to tie in with Dave's reply. The ratio of surface area to volume increases as the radius decreases, until at the molecular level the heat released by a single reaction escapes in all directions and the amount captured by any given molecule in the neighborhood isn't sufficient to ignite it. However when many reactions occur in close proximity to one another the heat has "nowhere to go" and a chain reaction is initiated. > >UV light probably isn't energetic enough, by the way. I don't think >it would be absorbed by the gasoline. But I don't know. I think it is energetic enough, it only has to trigger a chemical reaction (e.g. "sun burn"). (UV-A is 3-4 eV which is as much as you get from most chemical reactions. Production of e.g. H2O yields less.). BTW Jones' point about a magnifying glass is also a good one. Since it will ignite combustible substances, and all it does is concentrate photons in a small region, those photons are clearly energetic enough on an individual basis. > >The gasoline, by the way, doesn't ignite just because you bring a >flame near it, not directly. Rather, the vapors will ignite from >contact with the flame, and the ignition, from that mixture of >gasoline and air, can ignite the gasoline. > > Actually, the liquid gasoline never burns. Only vapor burns. This is true in an engine as well. The smaller the droplets, the more easily they evaporate and provide the necessary vapor. I have seen an oil fire quenched simply by rapidly cooling (water) the outside of the metal container that held the oil, thus stopping the evaporation. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
