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Candle question

  1. Jun 13, 2010 #1
    Hi all,

    I have been wondering how much energy does a candle produce per second. I know, that the heat energy can be measured, but what about the ionization energy? By ionization energy I mean the energy which is consumed when neutral atoms in the air are ionized.
    How can the amount of ions generated by candle per second be counted?

    Thanks for answers
     
  2. jcsd
  3. Jun 13, 2010 #2

    Andy Resnick

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    Candle flames are not energetic enough to ionize atoms. Perhaps confusingly, the light output by a single candle is given by the unit 'candela':

    http://en.wikipedia.org/wiki/Candela

    A candle emits about 1/683 W per steradian; an isotropically emitting candle emits about 0.2 W.
     
  4. Jun 13, 2010 #3

    russ_watters

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    According to the wiki on candles, a typical candle emits a combined total (light and heat) of about 77 watts:
    http://en.wikipedia.org/wiki/Candle

    ....though, unless I'm missing something it doesn't quite jive with another part of the article:
     
  5. Jun 13, 2010 #4

    Andy Resnick

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    Ah... photometric units.

    http://en.wikipedia.org/wiki/Lumen_(unit)

    1 lm = 1 cd*sr, so an isotropic candle emits 4*pi lm ~ 13 lm. Converting photometric (lm, lx, phot, nit..) units to radiometric (W, J, W/cm^2...) units is an exercise in frustration, but converting luminance to radiance can kinda-sorta be done by the NIST conversion I gave above. The candle article you quote gives the same conversion, but they left out the fact that the conversion occurs at the maximum response of the model human eye. Band-averaged over the spectrum, 13 lm will give less than 13* 0.17 W.
     
  6. Jun 13, 2010 #5

    russ_watters

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    Ok, good to know where your number comes from. To clarify, though, the OP didn't ask how much light a candle produces, but rather asked how much energy it produces. So it would be on the order of 40 or 77 watts total, 38.8 or 76.8 of which as heat and the other .2 as light.
     
  7. Jun 14, 2010 #6

    Andy Resnick

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    Actually, if you want to pick nits, the OP asked how many ions a candle produces, and how much energy is associated with the ionization process.
     
  8. Jun 14, 2010 #7
    Exactly
    Thanks for answering my first question. Andy, you have claimed in your first post that
    According to my knowledge, plasma is ionized gas and candle flame is plasma... also if you put a candle between two charged plates, the flame divides into two ion streams. So if flame does not ionize the air around candle, what does?
     
  9. Jun 14, 2010 #8

    Andy Resnick

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    Candles produce light and heat by combustion, which is an electrically neutral process, AFAIK. The soot, on the other hand, can apparently pick up charges from the air. I am not sure how this occurs.

    The bottom line, a candle flame is not energetic enough to ionize atoms or molecules. Proof of this is the fact that candles don't produce UV or X-ray radiation.
     
  10. Jun 14, 2010 #9
    So those ion streams generated when a candle is between charged plates are primarily caused by the voltage difference? But the heat makes the ionization process easyer, doesn`t it? (lower voltage is required)
     
  11. Jun 14, 2010 #10
    The combustion of paraffin wax releases about 42 kJ of energy per gram. Assuming that a candle combusts 100% of its wax (none is lost as unburnt vapour etc.) then the power output of such a candle = rate of energy released from combustion.

    I've had a look at various paraffin candles on the internet and compared their claimed burn times with the estimated mass of wax they comprise. I got figures which varied between about 60 W for the slowest-burning tealights and 100 W for the brightest pillar candles.
     
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