OmCheeto said:
I'm starting to understand why you are known as Enigman, as both of these puzzles puzzle me.
In the meniscus experiment, the cork floats to the higher level. But why? At that position, it would have the highest potential energy. I always thought systems tended towards the lowest level of energy.
It's true that when the cork is at a higher elevation, it has a higher potential energy. But that's just the cork. The system as a while has a lower potential energy when the cork is at the highest level.
When the cork is placed in water, the cork displaces its own weight in water. In other words, when the cork sinks into the water a little bit, an equal amount of water -- in terms of weight (not volume) -- rises up a little. The potential energy of the cork decreases slightly as it sinks a little, and the potential energy of the displaced water rises a little. Energy is not conserved though as the oscillations die out (at first the cork will do some "bobbing," but that doesn't last). Eventually an equilibrium is reached.
Since the water is denser than the cork, the potential energy of the whole system is lowest when the water is at its lowest level possible, which means the cork is at the highest level possible.
The water will "cling" upwards slightly at the sides of the glass due to capillary action.
http://en.wikipedia.org/wiki/Capillary_action
That means the cork has the tendency to go to the sides of the glass where the water level is highest.
So the solution to the problem is fill up the glass of water as much as possible, such that the surface of the water fills slightly higher than the rim of the glass, and is held in a convex shape (rather than concave as it was previously) and held together by surface tension. Now the highest point in the water is in the center of the glass, rather than the edges. The cork moves to the center.
And now this matchstick problem. The cellulose burning, consumes free oxygen, but emits both CO2 gas, and H2O vapor, and obviously is heating up the whole system. It doesn't make sense, if you know as little as I do.
I'm glad it's Friday. I will contemplate, or perhaps google, why these two enigmas baffle me.
The fact that one of the byproducts is water probably adds a significant boost. But even just hot air cooling will draw much of the water into the glass.
If the glass is lowered into the water while the fire is still burning, a net amount of gas is actually being produced. But pressure doesn't build up inside the glass because when the pressure above atmospheric pressure, the gas simply exits the bottom of the glass and bubbles to the surface outside of the glass. There's no mechanism to keep the pressure inside the glass much above atmospheric pressure. In the end, at the point in time that the reaction stops, all that's left in the glass is hot gas, whatever that might be. (The fact that a good part of it is water vapor might play a significant role, but it's not essential.)
Neglecting vapor pressure for the moment, as the gas cools, it simply becomes a
PV = nRT system. [Edit: with the pressure times area (force) equal to the weight of the net water in the glass.] The volume and pressure inside the glass decrease, causing the water to get "sucked" into the glass. (Or better worded, the water gets "pushed" into the glass by the pressure differential, the pressure difference between the pressure in the glass and atmospheric.)
Now let's consider that a small but significant fraction of the original gas might be water vapor. A good fraction of it probably is because it is one of the byproducts of the flame oxidation (a.k.a. the "burning" reaction). Vapor pressure is dependent on temperature alone (volume plays no role on the vapor pressure). So as the gas cools, the overall pressure can decrease more dramatically the more of the original gas is water vapor. This water vapor part isn't essential for the experiment to work, but it might play a significant role in bringing more water into the glass. (Instead of using a match, if you could pipe some steam into the upside down glass, all else being the same, you could draw *lots* of water into the glass -- as much as a water barometer would allow.)
don't know why edit's not working. 
