Coal is Glowing here

  1. Coal is Glowing here!!!

    I got this question in Resnick Eisberg:
    Pockets formed in a coal fire seem brighter than the coals themslves.Is the temperature in such pockets appreciably higher than the temperature of an exposed glowing coal?

    Strictly speaking,I have never observed this...however, it looks to me it is not very much essential to have appreciable difference in temperature.The factor amplifies the intensity to a higher value.Again, it may be for the difference in the value of emissivity of glowing coal and coal-pocket(I am not sure...and I do not know what is a coal-pocket).

    Can anyone say something about it?
  2. jcsd
  3. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    Coal on the surface is losing heat to both convection and radiation - into the room and you, otherwise there wouldn't be much point.
    In a cavity (either in a piece of coal or formed by a few pieces) the surface is mostly pointing at other burning coal at the same temperature so isn't losing much heat.

    There may also be volatiles evaporating out of the coal that are burning giving a blue flame.
  4. Also, 'brighter' doesn't always correlate with 'hotter'.
  5. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    Coal is pretty much a black body!
  6. turbo

    turbo 7,063
    Gold Member

    It's not just coal. Burning wood releases volatiles that burn very vigorously and give a very blue color in our wood stove. The blue color is a sign that the volatiles are burning at a very high temperature, though the total heat output of those gases may be less than the cooler solids that are burning.
  7. I agree that volatiles should be present.It is not very much essential to have appreciable difference in temperature. The reason may be that in the pockets, the free Carbon particles undergo atomic transitions. However, in the coal-interior, heat energy is dissipated to rupture the bonds, to make effective reactions etc;but here the atomic transition will be less. There is a slight difference in temperature but the factor T^4 amplifies the intensity to a higher value in the pockets.
  8. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    I suspect it is more due to a temperature difference.
    You would only need a few hundred K difference between black coal and glowing red cavities.
    You are seeing brightness more as a function of wavelength as the peak emmission moves from the deep red to white (proportional to T) rather than absolute power ( proportional to T^4) your eye is very bad at measuring absolute power.
  9. In a properly tended coal fire, the interior is hotter and brighter than the surface. But, it's complicated. Resnick and Eisberg is a Quantum Physics text, I believe, and the higher temperature in the interior of a fire is due to several combustion effects: (1) the interior fuel is largely coked coal rather than coal; (2) the interior is receiving the most air, either by convection or blast; (3) the surrounding coal and ash are fairly good insulators for the interior; and (4) the radiation pattern of the interior looks somewhat like a black-body cavity.

    Empty pockets (hollows) by the way are much cooler than actively burning pockets of fuel.

    Anyway, the proof of the higher temperature may be seen by placing pieces of iron in the two locations and watching to see which heats up the most. You can tell that by the color of the iron.

    Now that I've said all that, I'm baffled why Resnick and Eisberg had that in a Quantum book.
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