Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Richard Feynman's description of Fire

  1. Jun 26, 2011 #1


    Okay, I am a total fan of Richard Feynman, but too bad he was alive before my time. According to what he says, I can assume that a piece of wood can "spontaneously" combust if put into a container with pure oxygen then compress that container. So in doing so, the volume of the container would decrease while the pressure increases. So as the pressure increases the kinetic energy of the gas molecules increase since the work is done on the gas. Then would the wood catch on fire? Is that how it works?
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Jun 27, 2011 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Yes. Compressing a gas increases its internal energy (temperature). You wouldn't need pure oxygen. You should be able to use normal air. That is how fuel is ignited in a diesel engine. You would have to compress it quickly or use a thermally insulated container.

    AM
     
    Last edited by a moderator: Sep 25, 2014
  4. Jun 27, 2011 #3
    http://en.wikipedia.org/wiki/Fire_piston

    I'm REALLY new at this forum but I think that can explain a little bit what you are asking for.
    I don't know if you have ever used a bicycle pump. If you have, you may have noticed how the area that's closer to the air exit gets hotter as you keep pumping and pumping. This is because of the heating of the air given by the general equations of gases P.V=n.R.T, so, using this, as you decrease the volume of the chamber, the temperature must increase, as the volume keeps relatively even as the air comes out the pump and balances the system. (Remember that R is an universal constant and n is the amount of matter in the system, which of course will remain constant too).
    The same thing happens in that system, with the difference that the air cannot scape the "pump". Therefore, as you rapidly decrease the volume, you are increasing BOTH temperature and pressure.
    The kind of ignition you are describing is called ignition by detonation, which is the same used in diesel engines. Here, the excesive compression generates very hot areas that can ignite a combustible mix.
    I'm not really sure that a piece of wood could be ignited that way, as it's not the same trying to burn a solid than a fluid (gases or luquids). It is however very possible to use it to combust fuels or alcohol, where as any part of the mix has a very big chance of finding itself with a very hot part of the container, the enormous pressure guarantees that the combustion will keep going through the whole mix. Remember that this principle is used to run diesel engines, which have no spark plugs, and have a greater compression relationship than regular engines which DO use spark plugs.
    Hope this was helpful to you!
     
  5. Jun 27, 2011 #4
    Oh! That is why boats and bikes that run on diesel engines do not have a battery; they don't have spark plugs. I see.
     
  6. Jun 27, 2011 #5
    They do, however, have a "firestarter" (I don't know how to call it in English, I speak spanish xD), which is a semiconductor that gets REALLY hot when it receives electricity, so what you can ignite the mix at the first stages with a cold engine as it gets enough combustion temperature by itself.
     
  7. Jun 27, 2011 #6
    It's called a "glow plug" but not all diesel engines need them to start.
     
  8. Jun 27, 2011 #7

    Drakkith

    User Avatar

    Staff: Mentor

    I've never seen a vehicle with an engine that didn't have a battery other than manual start bikes. The battery is needed to turn the starter initially. Also, with my truck and my old car, you can "push start" them if your battery ever dies and both of them are gasoline not diesel. Just push them as fast as you can, hop in, shift to 1st or 2nd and drop the clutch and VROOM the engine starts.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook