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Why does smoke rise upwards?

  1. Mar 13, 2008 #1
    First i thought that smoke used to rise upwards due to a 'projection' created by a chimney.. but lately when I was looking at an essence stick.. the smoke could literally go anywhere. Even the same with a burning piece of coal. Even the smoke generated at the bottom part of the coal 'envelopes' around the piece of coal and rises upwards.

    No matter what orientation we select, the smoke always rises upwards. By 'upwards' I mean 'opposite and along the line of force of gravity'.

    So.. what exactly is the reason of this happening? My best guess is that this is due to another fluid surrounding the burning object as well as acting as a medium for the travel of smoke [i.e. air]. So.. if i burn a piece of coal in vacuum [just believe that for know we have some oxygen bubbles in the coal itself or something].. will the smoke move around in all directions despite the effect of gravity?
  2. jcsd
  3. Mar 13, 2008 #2


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    Smoke rises for the same reason 'hot air' rises, convection currents.

    Nice Avatar by the way :wink:
  4. Mar 13, 2008 #3


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    "Smoke", strictly speaking, consists of particles of ash carried on the hot are from burning. Since hot air is less dense than cold air, it rises as it is displaced by the heavier cold air. I suppose you could say that the "fluid" you are talking about is the cold air. Actually, with the particles of ash, the density is not a great deal less than that of cold air- smoke does not always rise. Depending on the circumstances, it can stay close to the ground.

    You cannot have ordinary burning in vacuum- no oxygen. If you were to use a substance that contained its own oxygen, say sodium peroxide, the vapors that came off in vacuum would not rise.
  5. Mar 13, 2008 #4


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    Hi rohanprabhu! :smile:

    I think you're right - the smoke heats the air, and then the hot air rises, causing a draught, which carries the smoke with it. :smile:

    The draught will seek out the nearest suitable opening, whch is not necessarily vertically upward … so if you have no chimney, but just make a small hole in the roof, the smoke from a fire should head straight for the hole, instead of filling the room.

    I don't think your vacuum suggstion would be very helpful, though - wouldn't anything automatically fill the vacuum?
    Last edited: Mar 13, 2008
  6. Mar 13, 2008 #5
    thanks to everybody.. that cleared it up for me.. thanks a lot to HallsOfIvy...

    totally.. :D
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