# The Naked Scientists Forum

### Author Topic: How Far Outwards Does A candle Flame Give Off Heat ?  (Read 4867 times)

#### neilep

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##### How Far Outwards Does A candle Flame Give Off Heat ?
« on: 21/03/2008 17:05:03 »
Dear Flameologists

See this bunch of candles ?

Nice eh ?

I took that photo...yes me !!...quality eh ?

Anyway....say I had one candle.......and it was isolated in the middle of breeze-less building of infinite size........Say I was ten miles away.....if I had the apparatus ...would I be able to detect the heat ?..NOT the light !!..but the heat ?

Is there a limit to how far I could get away from this regular bog standard candle flame to a point where I would not be able to detect the heat ?

whajafink ?

Be wax lyrical ,  with abandon or quite tame

lol...quality poemage and ewe know it !!

#### Karen W.

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##### How Far Outwards Does A candle Flame Give Off Heat ?
« Reply #1 on: 21/03/2008 17:56:10 »
Very nice poem... Quality Sir Quality!

What about infrared? I wonder does it detect at heat at those kind of distances?

#### lyner

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##### How Far Outwards Does A candle Flame Give Off Heat ?
« Reply #2 on: 21/03/2008 18:36:28 »
It would depend how much money you wanted to spend and, very importantly, how long you were prepared to take doing your measurement.
A quick back of a fag packet calculation;
Assume the candle is producing 1W of heat and it is 10km away.
You would need a large reflector to focus as much of the candle's energy as possible onto a detector - say 2m diameter.
The energy intercepted by the dish would be about 2nW (a nanoWatt is a thousand millionth of a Watt).
I saw some adverts for power meters (for measuring RF power, actually, but there can't be much difference as many of them work on the heating effect of RF). Apparently, they are available with sensitivity of 1nW.
It looks like you just about COULD do it!
Of course, you would have the problem that other things in the vicinity could be producing as much heat as the candle - the old signal to noise ratio problem. I think you might need to do it with the candle hanging from a balloon (not a hot air balloon though!!!!) at 10,000m at night. That would remove most of the stray heat sources. Damn - it wouldn't burn at that altitude. Ah well, any more questions?
« Last Edit: 21/03/2008 18:38:11 by sophiecentaur »

#### Bored chemist

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##### How Far Outwards Does A candle Flame Give Off Heat ?
« Reply #3 on: 21/03/2008 20:16:27 »
According to this site
http://www.post-gazette.com/healthscience/20000828salt2.asp

which includes this text "SALT will be a near-twin to the Hobby-Eberly Telescope, which is housed at McDonald Observatory in Texas and is operated by a consortium that includes Penn State University as a major partner. It will have a giant, segmented mirror measuring 11 meters across that is capable of detecting the flame of a candle on the moon."
The answer is really quite a long way (of course candles don't burn on the moon.)
Purists will say that this telescoope is looking at the light from the candle, not the heat, but a candle dissipates a lot more energy as heat than it does as light (I'd need to check but something like 100 to 1000 times more heat) so I think you could still sense the heat from a candle on the moon.
Nobody said it would be cheap or easy.

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##### How Far Outwards Does A candle Flame Give Off Heat ?
« Reply #3 on: 21/03/2008 20:16:27 »