Which candle will go out first? Why? On Monday I'll post a picture of the same candles taken 10 seconds later.
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ANSWER
We got all kinds of answers with all kinds of good explanations! Here is what actually happens:
The top candle went out very very quickly -- in about 5 seconds. I didn't wait to see how long it took for the lower candle to go out. My first thought was that the lower one would go out first, reasoning that carbon dioxide would sink. But then I remembered that in fire safety class, they teach that in case of a fire you should drop to the floor and crawl out of the building -- the good air is near the floor.
I don't understand what the mechanism is. Now that we know the answer, is it easier to reason our way to a correct explanation?
Thanks to wasoxygen ButterflyEffect reddirecpthoo blackfox026 camarillobrillo b_b AdMan707 cgod francopoli ironpotato mitvit MDZX and mk for playing!
wasoxygen has put up some nice physics quizzes under the tag #quiz, and I've got a couple more nice questions I'll put up soon.
The lower one. CO2 will sink, extinguishing the lower candle as it's oxygen supply runs low. O2 will rise allowing the taller one to keep burning. Unless something went horribly wrong and you needed to use that fire extinguisher (I hope not).
Just like with the Feynman Sprinkler, I was able to convince myself in advance of two contradictory outcomes. This shows that I don't understand the mechanism either. We need more data! From Boy Scouts I recall the fire triangle showing that the candle needs heat, oxygen and fuel to continue burning. From everyday experience we see that burning candles provide their own heat and do not usually go out in the absence of strong wind. The photo shows that the fuel supply was not used up. So I feel confident (consistent with our predictions) that oxygen starvation was the cause, either because oxygen was used up or carbon dioxide displaced the oxygen. We could try attaching some threads, spiderwebs, foil strips, dandelion seeds or something similar to the inside of the glass next time and see if we can detect air movement in the upper, middle and lower sections. We could pay attention to any smoke that appears after the candle goes out, to see if it follows any established pattern of air flow. We could try experimenting with a cardboard tube to make a chimney which would draw fresh air up from the bottom and see if we can put the lower candle out first. To test for carbon dioxide, we could try using a sensor. It would be interesting to see how the values change at different heights. Thanks for the quiz!Now that we know the answer, is it easier to reason our way to a correct explanation?
Yes, of course! Well, maybe not the correct explanation. Hindsight devalues science.
I'm gonna say the smaller one. It has less wax..
Upon second inspection, I also took the flame into consideration. If the flame's length is any indication of how fast its burning, then this is how tall the longer candle will be when the smaller one goes out
If this is what it comes down to, I'm going to be very happy.
I'll bite. The taller candle will go out first. Smoke rises so as the glass fills the taller candle will be snuffed out by it's own demise. I base this on my past expertise with pulling things out of my ass. Great idea by the way. I like puzzles.
I LOVE your evidence! Reminds me of the South Park where the doctor on TV is saying that the Institute of Health has decided that rather than eating through your mouth it's healthier to put food up your butt. The doctor explains: "We're basing this on... absolutely nothing."
If I had responded without reading other comments I would have said that the hotter, less dense "exhaust" gas from both candles — atmosphere enriched with carbon dioxide and soot and mostly depleted of oxygen — would accumulate at the top and snuff out the taller candle first. The idea that carbon dioxide is denser and therefore sinks is intriguing, but we do not observe a carpet of pure carbon dioxide on the ground in normal environments. Carbon dioxide molecules weigh, what, 12 + 16 + 16 units, so 44, and most of the atmosphere is diatomic nitrogen, which I think is a good bit lighter per molecule. (I am refraining from outside research.) The dynamic churning of fluids in motion maintains a pretty homogenous mixure, and only extreme gases like hydrogen and helium, or chlorine as in the Bhopal incident, would not be mixed in at all levels. My prediction: the top candle goes out first. I would expect it to burn longer than ten seconds, though. May I suggest a "quiz" tag? I have some ideas to contribute.
On reflection, I realize that I did not make use of a clue mike has given us: he asked the question. If the top candle goes out first, as one might expect due to convection, it would not make a very interesting demonstration worth photographing and posing as a quiz. The shape of the glass is significant. The hot gas from the tall candle will spread out at the top and begin to cool, and momentum will carry it down the sides of the glass, cooling it further. There will be a current based on the updraft of the hot, new exhaust gas in the center which will draw the older, cooler exhaust gas down the sides. The downdraft will be focused by the neck of the glass at the bottom, where it loses momentum, collects, and snuffs out the lower candle. I still don't think the density of carbon dioxide is significant; there will still be plenty of nitrogen in the gas at the bottom. The main mechanism is creating a flow of oxygen-depleted gas toward the bottom. This could be tested by performing the demonstration using a container with a different shape.
I'm going to say the lower one. Got your O2 rising and a larger area on top. That tall sucker will probably burn quite a bit longer than the bottom one.
I think the taller candle will go out first. The cooler, more dense air will sink to the bottom of the container, and so as the concentration of oxygen becomes less and less the smaller candle will still have more fuel to burn than the taller candle.
I'm going to say due to convection loops that are inevitable here that the oxygen is "well mixed" and that they, therefore, go out almost simultaneously. Edit: I actually only posted this, because it hadn't been picked. I'm really of the mind that the warm, oxygen depleted air will rise and snuff out the taller candle first.