physics help

[gwnn]

Surface tension...

 

Apparently it's always modeled by the little round molecules on the surface being attracted by the inside molecules and not that strongly attracted by the glass/air molecules. Now, that should mean the outer molecules just accelerate to the middle of the liquid! Wikipedia dismisses this nonsensical argument and just says "this is resisted by the liquid's resistance to compression". That sounds like gibberish to me, isn't that also another kind of intermolecular force? I don't get it.

[Al_U_Card]

And here I thot it was what I felt when I finally got her clothes off..... ;)

[hotShot]

You are a little unspecific about the liquid, surface tension depends a lot on the liquid.

 

If the liquid is water, the molecules are dipolar and have electric attraction to each other, while the attraction to molecules in the air is of a much weaker different kind.

And yes the molecules from the surface are pulled inside. This is why a drop of water tends to be round.

 

Now imagine a crowd of people around someone who is giving away money. Everybody wants to get to the middle to reach for the money, but is not easy to get from the edge to the middle because the is already someone, you have to push aside.

This is similar to the situation a water molecule at the surface is in. It wants to be covered from all sides, but to move to the middle other molecules have to be moved away.

[hrothgar]

Different types of forces...

 

Most liquids are very difficult to compress because the molecules are packed quite closely together

 

Surface tension has to do with hydrogen bonding

[matmat]

think of it this way....

 

three charges Q1, Q2 and Q3 are equally spaced in a line. they are all positive (other permutations are possible). Q2 and Q3 are connected by a compressive spring. Q1 represents the glass, Q2 and Q3 liquid molecules. Q1>Q3. Q2 will be pushed away from Q1, closer to Q3, but the spring will prevent it from being indefinitely accelerated.

 

I'm not saying this is a very realistic model, but it is a way to think about it.

[pigpenz]

think of it this way:

if you jump off the golden gate bridge hitting the water is like hitting concrete

 

VanderWalls forces:

strong WFF will create tension like water surface tension

Low WFF will create weak bonds like graphite, when used as a lubricant

[Trinidad]

The difference in "hardness" of the water when jumping from the Golden Gate bridge and when stepping into the water has nothing to do with surface tension. This difference is due to the fact that the "fluidity" of a material is rate dependent (it depends on the time of the process):

 

If you hit something fast, it will appear hard because the molecules in the material aren't given the time to make room. If you deform the same material slowly the molecules get the time to move and the material appears soft. You can see that in water, but it is most notable in Silly Putty: When you press it slowly, you can deform it and roll it into a ball. If you then throw that ball against the wall, it bounces back, pretty much undeformed. The ratio of the characteristic time for the molecular movement in the material and the characteristic time for the experiment is called the Deborah number.

 

http://en.wikipedia.org/wiki/Deborah_number

 

The surface tension is not time dependent. It is about molecules that attract each other wanting to be surrounded by the most attractive molecules. Molecules are most attracted to other molecules that look exactly like themselves. They don't like different molecules.

 

If you want to call molecules racist, that would describe it very well. Creating a stable droplet of oil in water is like creating a stable white neighborhood in a black suburb (or the other way around). You don't need a degree in sociology to see that that is going to be difficult. :(

 

Rik

[whereagles]

Wikipedia dismisses this nonsensical argument and just says "this is resisted by the liquid's resistance to compression". That sounds like gibberish to me, isn't that also another kind of intermolecular force? I don't get it.

It's another kind of force, yes. As the top layer of a liquid is pulled into the bottom by vertical van der Walls forces, it wants to penetrate the next-to-top layer. This attempt increases electronic cloud repulsion between the layers and, at some stage, that repulsion is enough to compensate vertical van der Walls attracion.

 

As those two forces cancel, the result is that the liquid stabilizes, with a highly compressed, very thin layer at the top. Due to the compression, this layer has a high density of horizontal van der Walls forces, which keep it packed and resistant to breaking. This is why it resembles a sort of "elastic blanket" when bugs walk over it.

 

Hope that helped.

[kenberg]

"It's another kind of force". I need help with this.

 

There are gravitational forces and electromagnetic forces. There are some short range forces (I think). Weak attraction is some sort of nuclear force independent of emf, is that right?

 

 

I am assuming that van der Wall forces are some sub-category of these more basic forces, is that right?

 

What I am asking, I think, is: Is vdW really "another kind of force" or is it fully explainable in terms of the rather small number (4?) of basic forces?

 

 

I switched my major from physics to math fifty years ago this fall, so forgive me if I am hopelessly out of date about the nature of forces.

[hotShot]

You can have electrical charges, dipolar objects with a partial charge and you can have neutral objects that polarize each other in a way so that there are very weak electrical charges that can interact. Te latter is called van-der-Waals interaction.

[helene_t]

Yes, the van der Waal forces are explainable in terms of electromagnetic forces. They are not an elementary force.

[whereagles]

kenberg, there are 4 fundamental forces

 

1. strong nuclear force (keeps nucleus together)

2. weak nuclear force (featured in stellar main sequence and beta decay)

3. electromagnetic

4. gravity

[Al_U_Card]

kenberg, there are 4 fundamental forces

 

1. strong nuclear force (keeps nucleus together)

2. weak nuclear force (featured in stellar main sequence and beta decay)

3. electromagnetic

4. gravity

You forgot jump-shifts.... :rolleyes:

[pdmunro]

I like this explanation of surface tension:

http://wwwchem.csustan.edu/chem2000/Exp5/BKG.HTM