How does capillary action of a liquid not violate energy conservation?

[Nick tringali]

I am learning about capillary action of water. As water moves up paper. How is that not violating energy conservation as it is going against the force of gravity. This obviously can't be infinite energy.

 

[anorlunda]

No violation of energy conservation. Gravity is not the only force involved.

https://en.wikipedia.org/wiki/Capillary_action#Phenomena_and_physics
Capillary penetration in porous media shares its dynamic mechanism with flow in hollow tubes, as both processes are resisted by viscous forces. Consequently, a common apparatus used to demonstrate the phenomenon is the capillary tube. When the lower end of a glass tube is placed in a liquid, such as water, a concave meniscus forms. Adhesion occurs between the fluid and the solid inner wall pulling the liquid column along until there is a sufficient mass of liquid for gravitational forces to overcome these intermolecular forces.

 

[Vanadium 50]

How is carrying water up a flight of stairs not violating conservation of energy? Or more relevantly, taking it up an elevator.

 

[phinds]

How is carrying water up a flight of stairs not violating conservation of energy? Or more relevantly, taking it up an elevator.

I don't think that's a fair analogy. In those cases it is obvious what the force is that is opposing gravity. In capillary action it's NOT obvious, thus the OPs question.

 

[etotheipi]

I think the best way to understand this is to derive it explicitly. What capillary action is doing is minimising the Helmholtz free energy ##F## of the system. If the liquid rises by an amount ##\delta h##, then you increase the surface energy due to the liquid-solid contact by ##2\gamma^{LS} \pi r \delta h## however you also decrease the surface energy due to the gas-solid contact by ##2\gamma^{GS} \pi r \delta h##, where ##\gamma^{LS}## and ##\gamma^{GS}## are the surface tensions at the liquid-solid and gas-solid interfaces respectively [defined by ##\gamma = \frac{\partial F}{\partial A}##].

The gravitational potential energy of the system is also raised by ##\pi r^2 \rho g h\delta h## [N.B. technically the ##\rho## here is the difference between the liquid and gas densities, because after all we're also displacing air and changing the potential energy of that little parcel of air as well]. So, your overall change in energy is$$\delta F = (2\pi r[\gamma^{LS} - \gamma^{GS}] + \pi r^2 \rho g h) \delta h$$At equilibrium ##F' = 0##, so we must have $$2[\gamma^{LS} - \gamma^{GS}] + \rho g r h = 0$$i.e. the liquid rises to a height$$h = \frac{2[\gamma^{GS} - \gamma^{LS}]}{\rho gr }$$Does that help?

 

[DaveC426913]

If one simply examines a puddle of water on the kitchen counter one can trivially see that water is capable of resisting the pull of gravity (otherwise it couldn't maintain a meniscus, and would instead spread until it is one molecule thick).

Capillary action is just the same force embiggened.

 

[A.T.]

I am learning about capillary action of water. As water moves up paper. How is that not violating energy conservation as it is going against the force of gravity.

A magnet can pull metal up against gravity too. You have to look at the total potential energy, not just the gravitational one.

 

[Nick tringali]

How is carrying water up a flight of stairs not violating conservation of energy? Or more relevantly, taking it up an elevator.

Because we would be using energy in the form of chemical energy (ATP), no?

 

[Nick tringali]

Thank you all, makes more sense now.

 

[etotheipi]

Because we would be using energy in the form of chemical energy (ATP), no?

No, certainly not ATP!

The conversion is from surface energy to gravitational energy.

 

[Nugatory]

No, certainly not ATP!

The conversion is from surface energy to gravitational energy.

I think the comment you were responding to belonged to the short digression about carrying something up a flight of stairs

 

[etotheipi]

I think the comment you were responding to belonged to the short digression about carrying something up a flight of stairs

Whoops

 

[hutchphd]

I think OP means in the legs of the water bearer on the stairs (already noted I see).
The question is, IMHO, a very good one, but troublesome only because it is unusual to our us: we don't much see it at our quotidian scale. But it is just another physical/chemical event driven by entropy.
If I open a bottle of beer, bubbles form and spontaneously rise to the surface! This is every bit as "odd" as capillarity, but somehow not as creepy. So the answer is that no rules are broken or laws impinged. It was already baked in the cake when the seas separated from land.

 

[russ_watters]

It's always a game of Find the Power Source.

You could, in theory, construct a device that utilizes capillary action for continuous energy production. You simply run a wick through a vertical pipe, hanging out the top, then immerse the bottom of the pipe in a pool of water (the bottom of the wick just touches the water). Water will continuously flow up through the pipe/wick, and you could locate a turbine/generator at the bottom of the pipe.

But capillary action is not the power source. What is?

 

[DaveC426913]

It's always a game of Find the Power Source.

But capillary action is not the power source. What is?

The Sun. Ultimately, it's always the Sun. Everything else is a battery.

 

[Vanadium 50]

You could, in theory, construct a device that utilizes capillary action for continuous energy production.

This sounds essentially like a Sipping Bird, only using capillary action instead of a phase change. I suspect it would be less efficient.

 

[jbriggs444]

In addition to the sun, you need the sky. A heat source to power the evaporation part of the cycle is only helpful temporarily without a cold sink to make the condensation part work.

 

[DaveC426913]

OK,

• the sky and
• either
  • the sun, or
  • geothermal energy - since technically, geotherm energy cannot ultimately be traced back to the Sun - it preceded the Sun.)

 

[hutchphd]

But capillary action is not the power source. What is?

While @DaveC426913 is certainly correct for the power source when a wick is included to connect the Earth (and sky) into the energy cycle, I think the answer does not directly address the proximate OP question:

As water moves up paper. How is that not violating energy conservation as it is going against the force of gravity

The answer is that somewhere in the deep distant (or not) past when glass and water were individually created (or they somehow were separated) energy was stored just as surely as if spring had been stretched. It may have been sunlight or maybe not. Given the proper geometry we can use some of that energy to lift the water a certain distance up the tube.
Have you ever seen a soap-powered boat?

Same deal. Neither the capillary tube nor the boat can be repeated without putting the energy in again (for the capillary it is easier then for the soap)

 

[sophiecentaur]

Water will continuously flow up through the pipe/wick, and you could locate a turbine/generator at the bottom of the pipe.

This sounds essentially like a Sipping Bird, only using capillary action instead of a phase change. I suspect it would be less efficient.

It's great when these type of questions come up. Forget SR and QM, there are still a lot of classical problems to make your brain hurt.
If the wick (or sipping bird) are operated in a thermally insulated, sealed bell jar, evaporation will eventually stop. The height of wet wick will be at a maximum and the bird will stop sipping.
There's a heat engine lurking behind many of these questions.