Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Rokitansky on 15/05/2004 21:33:03
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Does anyone know why is the flame of the candle the hotest on the top of it?
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I would hazard a guess that because heat rises , it accumulates at the tip of the flame where it gets concentrated......I'm probably soooo wrong but I'd like to know if my guestimate is anywhere close. Good luck in getting your answer Darko.
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Actually I remember hearing that the hottest part is one of the layers in the flame. I think it was the yellow layer that's outside the blue layer. Hope that makes sense.
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It is logical that the hotest part of the flame is the brightest. But why is it on the top? Blue layer? There is no blue color in the flame, as far as i can tell. At least i can`t see it.[:p]
Nice guess, Neil. Anyone else ?
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A flame is the combustion of a fuel vapour in an oxidising agent (usually oxygen). The place where this occurs most efficiently, i.e. the tip of the flame, will be the hottest part. This is because at this point all of the fuel has been heated to beyond its ignition temperature and is well mixed with oxygen. There is also a 'thermal blanket' of heat from the lower portion of the flame below rising up so all of the heat coalesces at this point.
You can demonstrate this for yourself. Take some thin copper wire. Hold the wire in a pair of pliers or tweezers.
Insert the wire into a flame and it will glow hot. If you hold the wire at the tip it will heat quickly and glow at the site where it is closest to the flame. Now put the wire through the middle of the flame. It will only glow red hot at the edges where the flame meets the air. That's because the centre of the flame is starved of oxygen and is relatively cool. But it contains superheated fuel that is rising and will combust at the flame tip.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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What exactly is a flame? I've been asking for ages and no one has ever answered me. My guess is particles getting energy and jumping off and letting off light (maybe that's the origin of ash) - different elements different heights and different colours of light - but I have absolutely no idea ....
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I think a flame is just a plasma of gas, which forms due to the combustion process releasing large amounts of thermal energy. I may be wrong though!
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
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Right qpan, and as Chris said, the flame is caused by the chemical reaction of a fuel vapor with oxygen (usually from the atmosphere).
In a candle, the wax is melted by the heat of the flame, and then moves up the wick due to capillary action. Then the heat causes it to vaporize and float upward. As it moves upward and away from the wick, it is mixing with oxygen from the surrounding air. Once the mixture is right, the combustion reaction will start, which releases heat. This reaction breaks the hydrocarbons and produces carbon and other compounds (I'm weak here, refer to the chemistry people). The carbon is heated enough that it becomes incandescent. As Chris pointed out, the inside of the "flame" is cooler because there the hydorcarbons have not yet mixed with oxygen and are not burning. As they move higher they do burn.
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John - The Eternal Pessimist.
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...and the candle wants to burn because the products of the conmbustion are predominantly carbon dioxide, water and heat so there is a huge entropy increase making the reaction very favourable.
Next you need to explain why the flame is orange.
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Wait, what do you mean? You mean that the initial heat makes the wax float upward and then it burns which makes it give off light? (what does it really mean to 'burn'; besides a chemical reaction with oxygen?)
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Quantum, a liquid/solid can never be burnt, it's always the thin layer or sometimes huge layer of gases above it depends on which fuel you use.
And as with 'burn' I think we normally associate it with as giving off energy in the forms of heat and/or light.
If you blow fluorine on to a each of metal and it start to combust e.g. Mg does it count as burn or just react?
Tom
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Burning: reaction of oxidation in wich energy is released
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I don't know enough chemistry to get into combustion and burning.
But to explain why the flame is orange has to do with quantum mechanics. The molecules are heated, which causes the outer electrons to be boosted up to a higher energy state. When they release this energy, it is given off as a photon of energy, the wavelength (color) being dependent on exactly which orbitals are involved. If you look at a candle flame (or any other) with a diffraction grating (or a prism) you will see distinct lines of bright light. These correspond to certain materials that are giving off light. This is how scientists determine the composition of stars - by looking at the emission lines in their spectra.
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John - The Eternal Pessimist.
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The flame is orange because it contains soot (carbon particles) produced by incomplete combustion of the candle wax. These soot particles produce visible heat (glow) when they get hot due to the mechanism explained by John, above.
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Why does the way things give off light have anything do do with quantum mechanics?
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Well, the light given off is caused by electrons falling from from a higher energy shell to a lower one, emitting a photon in the process. The colour of light depends on the reactant being burned, as every element has different energy levels between shells, and thus, the emitted photons also have different energies. The energy of a photon determines it wavelength and therefore its colour, which is why different metals produce different coloured flames when placed in a flame.
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
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Actually, even though that is the mechanism, don't think its actually quantum mechanics!
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
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That is the mechanism, and it is quantum mechanics, because the wave properties of the electron are what govern the "shell" that it can reside in. QM predicts the energy (wavelength) of the photon that will be generated when the electron transits from one shell to a lower energy shell.
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John - The Eternal Pessimist.
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Ah, ok- I stand corrected!
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
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To summarize this thread, the hottest part of the flame is near the top where the gases that are burning are optimally mixed with oxygen. The chemical reaction of the hydrocarbons (from the wax) with oxygen from the air is an exothermic reaction that generates a lot of heat. The heat causes the electrons in the outer shells of the reaction products (primarily carbon aka soot) to be excited and kicked up into higher orbits around their atoms. The electrons will spontaneously jump back down to their lower energy state and emit a photon of energy in doing so. The energy of the photon is the energy difference between the higher and lower energy states. This energy is quantized and takes certain values that are predicted by quantum mechanical calculations based on the wave properties of the electron.
This is really short. If you'd like more, I might get some more time to write sometime next week.
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John - The Eternal Pessimist.