Something occurred to me today that's troubling me in an intellectual sense, because I can't even begin to figure it out. A friend and I were talking about science oddities like thixotropic fluids and superfluids. I thought about capillary action, and then it hit me.
Suppose you have a container of water, and a strip of paper suspended vertically above it. If you lower the end of the paper into the water, the water actually begins to climb up the paper as it soaks in; this is the capillary action we all know and love. But what the hell is doing work on the water? The water is higher than it was before, so it has more potential energy...what the hell added that energy to it? What has less energy as a result?
Re: Physics bleg
I'm going to forward this question to my dad, who, while not a physicist, is a retired hydrologist, but in the meanwhile, google turned up this gem:
OBJECTIVES:
Students will explore the
process of capillary action
that causes candles to burn.
Students will connect this
knowledge to their own
spiritual experiences on
Shabbat and compare
themselves to Shabbat
candles.
This sounds dangerously like encouraging self-experimentation with Spontaneous Human Combustion!
"They civilize left, They civilize right
Till nothing is left, Till nothing is right"
Re: Physics bleg
JD-
The water molecules are attracted to molecules on the surface of the paper fibers. Other water molecules, in turn, are attracted to the water molecules that went up the fiber. It's a cohesive effect: Water molecules like sticking together and they like attaching to certain types of surfaces (e.g. paper fibers). Of course, at some point they climb too far, and the force of gravity wins out. But there's a trade-off, and up to some height they can do it.
"the only thing worse than a freeper is a blue state freeper that doesn't realize they're a freeper." -dhex
hoisted by their own waterboard!
-dhex
Re: Physics bleg
Well the water molecules are already next to other water molecules. Except for the ones on the surface, they aren't next to as many. That's what makes surface tension, and capillary action as well. It's the surface molecules that want to be next to more stuff (preferably more water, but anything where they can induce a little local charge imbalance is better than air).
The question of work, I think is one of irrelevant parameterization. Energy is conserved. The water molecules are at a lower energy state for being absorbed into the paper, that's balanced by the higher gravitational potential. Of course if you just dropped the paper onto the water, it would saturate without raising the water. So it must be exothermic.
Hmm Maybe I'm as confused as JD
seriously though, i think you're crazy on this. and you think i'm crazy. everybody wins! - dhex
Re: Physics bleg
Warren-
Yes, the other water molecules are already surrounded by other molecules, but the one that just went onto that fiber is more attractive because it's alone, so it has a larger local dipole moment. Water molecules with neighbors have their dipole moments partially canceled out. Also, it has hydrogen atoms sticking out with nothing else attached, enabling hydrogen bonding. Water molecules with neighbors already have their hydrogen atoms in use.
"the only thing worse than a freeper is a blue state freeper that doesn't realize they're a freeper." -dhex
hoisted by their own waterboard!
-dhex
Re: Physics bleg
Hm...so a water molecule at the surface of the water 'wants' to attach itself to a paper fiber because that's a lower-energy configuration?
Re: Physics bleg
Water molecules have lower energy when they are surrounded by polar molecules and/or things that they can form hydrogen bonds with. (This is simplifying a fair bit, but it's a useful rule of thumb.) A water molecule at the surface of the water is surrounded half by water and half by what is effectively vacuum. (Those gas molecules are at a much lower density than the surrounding liquid, and they mostly collide and bounce off rather than attaching.) If the paper fiber has some polar molecules, or some stuff that it can form hydrogen bonds with, it will prefer that to air. So it will go there. There's an attraction. But now it's created a void. So other water molecules move around, and the net result of all of this is that the interface gets deformed as some of the molecules are drawn to the fiber.
"the only thing worse than a freeper is a blue state freeper that doesn't realize they're a freeper." -dhex
hoisted by their own waterboard!
-dhex
Re: Physics bleg
So, how do water molecules feel about being subjected to my digestive tract?
wah wah waaaaaaah
I CAUTION YOU / IN DEFEATING ORCS WE MAY FIND THE ONLY VILLAIN LEFT TO FACE IS OUR OWN PREJUDICE--qwantz.com
Re: Physics bleg
thoreau,
Right. But the question is, what about work? What about energy? When the molecule forms the hydrogen bond, it goes to a lower energy state. So that could account for it climbing to a higher gravitational energy.
seriously though, i think you're crazy on this. and you think i'm crazy. everybody wins! - dhex
Re: Physics bleg
just to be the pedantic semanticist that I am, the water molecules' attraction to each other is cohesion, the water molecules' attraction with atoms in the cellulose fibers of the paper is adhesion.
I'm guessing (without bothering to look it up) that there may be a greater dipole on the hydroxyl groups on the celluloses, since the hydroxyl oxygens are attached to a single hydrogen and a carbon, and the carbon has electronegativity close to that of oxygen. Thus, the oxygen may be pulling the electrons from the single hydrogen to a greater extent, increasing the dipole and resulting in a greater and more energetically favorable attraction from the water molecules.
of course, I could be wrong, since I just pulled that out of my rectum
Re: Physics bleg
Had it been adhering or cohering before that?
All the world loves a clown.
Re: Physics bleg
a little bit of both
Re: Physics bleg
Innominate one - well, I think you're about right, at least from what I remember from genchem. You're not going to get much of a dipole out of a carbon/oxygen bond, but you'll get a pretty strong one out of oxygen/hydrogen. The negative end of the water molecules is going to be pretty attracted to that, so I think your conjecture makes sense.
JD - The water molecules lose energy when they form hydrogen bonds, so that's where the potential energy comes from when they move up the strip. If you wanted to get really fancy I bet you could calculate the amount of energy given off by the bonds forming (Gibb's Free Energy? science guys? I don't want to embarrass my "woot I know a little" self too badly here) in kJ/mol and then figure out how high the water will climb because you know the force of gravity...I'm not signing up to do that as a homework problem mind you, but it's probably possible. As long as you know the temperature, and the enthalpy change....and the entropy change...yeah....definitely not signing up to do that homework problem because I have this feeling that the state functions of big nasty cellulose molecules aren't exactly hanging around on a chart someplace.
Whenever I catch so much as a glimpse of pr0n, I suddenly turn into a sex-crazed barbarian, slashing and clawing my way through whatever and whomever until I find something to put my weiner into. -- Taktix