A Quick Physics Question

Hi SDMB Members,

For years a question has been puzzling me and finally I believe I have found the forum where I may find an answer :wink:

When you dip a sponge or some paper into water (for instance) the water will rise up the paper/sponge through Capillary Action.

My question is, since the water in the paper now has more potential energy than before - where did it come from? Is the water now cooler than it was before?


Can’t give a definate answer to that, it’s an odd question. I think the energy to draw water up for example a capillary tube would come from the surface tension of the water and charge on the glass it’s going up, and traveling up paper or a sponge would be similar. I don’t think it would loose heat energy in the process, but it possibly would to the atmosphere due to a larger surface area if there is a diference in the water and air temperatures… but that has nothing to do with the physical movement of the water - someone with a physics textbook in front of them could answer that better.

That’s a damned interesting question. I’m speculating here, but I’m guessing that part of the energy is work done by surface tension and part is elastic potential energy released due to relaxation in the sponge (maybe the presence of water allows some of the residual stress to decrease)

A really, really damned interesting question there! I think City Gent has the answer, from what I recall from Fluid Dynamics class, but I need to find my book to verify.

Surfaces have energy, and a dry paper towel has lots of surface area. Since water wets paper, the energy of a water-paper interface is lower than that of an air-paper interface. When water is absorbed in a paper towel, the gravitational potential energy increases, but no more than the decrease in surface energy which results from replacing air-paper interfaces with water-paper interfaces.

What does “surfaces have energy” mean? I’m not challenging you, I just don’t understand what this means. I also don’t understand about water-paper interfaces having less energy - why? Is it just relaxation of cellulose or other materials in the paper/sponge?

If the structure of the sponge relaxes due to water softening it, then it can indeed provide the necessary energy to ‘suck’ the water up.

This is one of those great questions that no one thinks to ask. Nicely done, ianlyte. If no one comes up with the answer soon I may have to dig into my books as well.

In his book “Perpetual Motion”, Orde0Hume reprints a proposed Perpetual Motion machine consisting of two discs that can rotate about their centers. They are placed close to each other, but not toushing, with their centers of rotation less than a radius apart (like a Venn diagram). They are then placed less than half a diameter deep in a container of water.

The theory is that capillary action pulls water up into the slight gap between the discs. This added weight causes the discs to rotate, bringing the water down. But capillary action then pulls the water level back up again, so you have perpetual motion.
Only you don’t, of course. But to tell the truth, I don’t know why not. (Saying it’s due to friction in the hubs dooms it won’t work. The machine should be flawed even WITH frictionless joints) I suspect that contemplating this device might help answer thev OP.

Molecules in water (or any other liquid or solid) have a mutual attraction. There is also some degree of attraction or repulsion between water molecules and those of other substances. The surface, or interface, energy is related to the difference in the atraction between like molecules in the liquid and that between molecules of the liquid and those of the other substance.

For example:

Clean glass has a high affinity for water so water molecules ‘like’ being adjacent to glass - giving the glass/water interface a relatively low energy. That is why water will happily form a thin film to ‘wet’ a clean glass surface easily.

Oily substances are repulsive to water molecules so oil/water interfaces have a high energy. You can see this when water forms raised droplets on an oily surface. Although the raised drop has more potential energy than a this film, this is counteracted by the reduction in energy gained by reducing the contact area at the interface.

The problem is this statement:
My question is, since the water in the paper now has more potential energy than before - where did it come from? Is the water now cooler than it was before?

There is no change in potential energy, just a transformation. Water in a test tube has a rim, the meniscus, that is higher than the average water level, so why doesn’t that water flow downhill, making a perfectly flat surface? Because surface tension is a real force acting on the water. You can’t evaluate the potential energy of a system without looking at all the forces. No enegy is generated by water going up a capillary tube, just as no energy is required to hold up the meniscus.

You can think of this as a weight hanging from a spring. The spring (which is at the molecular/electomagnetic level in the case of surface tension) doesn’t perform work by holding up the weight, there is a balance. When you introduce a capillary tube to a container of water you are modifying the system, and the forces (gravity and surface tension) establish a new balance.

If you remove the capillary tube with the water held in it you must add more energy to lift the water, only then has any “work” been performed.

Capillary action works in glass tubes, the process of “wetting” an absorbant material is a seperate issue (though related of course).

Except that the energy before was not potential energy, it was interface energy.

Capillary action is the phenomenon whereby a liquid rises up any narrow tube-like structure by converting interface energy into potential energy. Narrow tubes work better because they have more surface area per unit volume, thus enhancing the effect. Abosrbant materials, on a microscopic level, have a structure which resembles many narrow tubes in many respects so calling it capillary action is quite appropriate.

Surface tension is the result of an electrical attraction on the molecular level - in water it is because one end (the O side) of the H20 molecule is negatively charged, and the other end (H side) is positively charged. Adjacent molecules are then attracted, positive to negative, called hydrogen bonding in water. This sticking together is called cohesion and is why droplets form on your well waxed car - the cohesive forces are greater than the gravitational forces trying to flatten out the water.

Similarly, there are adhesive forces between a liquid and a solid, that are sometimes greater than the cohesive forces, hence capillary action. If the water in a glass were not subject to gravitation, it would climb right out of the glass and cover as much of the glass as possible. With gravity, it rises until the adhesive force and the gravitational force are balanced. Same thing with a sponge or piece of paper.

So, when you dip your paper into the water, the surface tension or surface energy, which is a form of potential energy, is overcome by the adhesive forces which draw the water up, i.e., input of kinetic energy, and increase BOTH the surface energy and the gravitational potential energy. Surface energy wants to decrease the surface area, but you are in fact increasing the surface area, therefore increasing the surface energy.
I got most of this information from “Understanding Physics” by Isaac Asimov.

DOH! Moderator please…
sorry about that.