5 replies to this topic

[mooeypoo]

Notice: This experiment is incomplete, and unclear. There were several attempts to correctly state the situation, but at the moment, a new re-make is planned to explain exactly and thoroughly what is happening to create this phenomenon.

Well, this is going to be sweet, short and to the point: Fire in closed spaces can really suck.

Ha, I was dying to use that pun for a while now, and here I had the chance. This experiment is a really short and sweet one, and can join your mental arsenal of “party tricks” for the partying geeks. It can really impress anyone, and from now on - you are going to know what makes this happen.

Ready?

(Read more and watch the video...)

Do you have anything to say? Wish to discuss the experiment? Ask questions or criticize the method? Post and debate here!
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[Joatmon]

Over 50 years ago when I was a young airman we used to play a trick on the N.A.A.F.I. girls. We would fill a saucer with coffee dregs, roll up a small ball of paper, float it on the top, set the top of it alight and place the upside down cup over it. The coffee would be drawn up into the cup and then we would wait for the girl to clear the table!

[John Cuthber]

Nice video, pity it's the wrong explanation.
Each molecule of oxygen consumed oxidising carbon is replaced by a molecule of CO2.
With a wax candle you will get some net contraction because the oxygen that combines with hydrogen will be converted to water which condenses.
However, as Joatmon points out, this works with paper
Paper is mainly cellulose.
let's have a look at the equation
(C6H10O5)n, + 6n O2 ---> 6n CO2 + 5n H2O

In this case, there's no net reduction in volume.
CO2 isn't that soluble in warm water, so that's not the reason.

At about 2:50 in that video you can see bubbles escaping from under the glass as it's put over the flame.
That's because the air is heated and expands.
When it cools down again it contracts and that's what causes the reduction in pressure.

[stopandthink]

Nice video, pity it's the wrong explanation.
Each molecule of oxygen consumed oxidising carbon is replaced by a molecule of CO2.
With a wax candle you will get some net contraction because the oxygen that combines with hydrogen will be converted to water which condenses.
However, as Joatmon points out, this works with paper
Paper is mainly cellulose.
let's have a look at the equation
(C6H10O5)n, + 6n O2 ---> 6n CO2 + 5n H2O

In this case, there's no net reduction in volume.
CO2 isn't that soluble in warm water, so that's not the reason.

At about 2:50 in that video you can see bubbles escaping from under the glass as it's put over the flame.
That's because the air is heated and expands.
When it cools down again it contracts and that's what causes the reduction in pressure.

She corrected herself in the info under the video.


"CORRECTION: The pressure inside the glass increases as the fire heats up the molecules. Oxygen is being "consumed" by the fire, that produces Carbon Dioxide (the matter itself remains, no matter is mysteriously 'vanishing' or 'created' out of nothing!). But now, the pressures are different and therefore the water outside the glass are pushed inwards — the lower pressure of the INSIDE 'sucks in' the liquid around it under the pressure stabilizes."

[John Cuthber]

It misses the point that a hydrogen flame would "suck" because the water vapour produced would condense on the cold glass.
Wax (as in the candle) will really produce some contraction in volume.
And, when I was at school, you would lose marks in a physics exam for saying that anything sucks.
There is still air in the glass, the only thing it can do is exert a pressure outwards: it can't "pull".

It's the external air pressure that pushes.

[Consistency]

This thread is old but... Is the water attracted to the heat and absorbs it?