# Why can't capillary action be harnessed as a power source?

**URL:** <https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991>\
**Category:** Factual Questions\
**Created:** [December 9, 2004, 8:42pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991 "2004-12-09T20:42:02Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\
**Post date:** [December 9, 2004, 8:42pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/1 "2004-12-09T20:42:02Z")

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Well, that pretty much says it all. I know there’s GOTTA be a good reason why not. What is it??

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**Author:** ![Gangster\_Octopus](https://avatars.discourse-cdn.com/v4/letter/g/a8b319/32.png) [@Gangster\_Octopus](https://boards.straightdope.com/u/Gangster_Octopus)\
**Post date:** [December 9, 2004, 8:43pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/2 "2004-12-09T20:43:59Z")

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I think you need to be more specific (or at least I need you to be). Do you mean electricity?

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**Author:** ![NurseCarmen](https://avatars.discourse-cdn.com/v4/letter/n/5daacb/32.png) [@NurseCarmen](https://boards.straightdope.com/u/NurseCarmen)\
**Post date:** [December 9, 2004, 8:55pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/3 "2004-12-09T20:55:08Z")

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I suspect it’s due to inneficiency.

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**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\
**Post date:** [December 9, 2004, 8:59pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/4 "2004-12-09T20:59:13Z")

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yes, sorry. I mean, if you take a long, skinny tube and put the end of it in a pool of liquid, the liquid will rise up and fill the tube. This seems like it should be impossible as you have a liquid rising against gravity with to additional input of energy.

Now, I have no idea how you’d make a machine to harness this effect, i am more interested in the principle behind this action and if it is theoretically possible to use it for energy production.

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**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\
**Post date:** [December 9, 2004, 9:28pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/5 "2004-12-09T21:28:26Z")

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No, you can’t extract energy from it. It’d be like trying to extract energy from a refrigerator magnet. If you hold a magnet above a paperclip, the paperclip will “defy gravity” and fly up. But unless you can turn off the magnet and let the paperclip drop, the paperclip doesn’t do any work. Same with capillary action - water molecules want to stick to a surface so it will climb up a capillary, but you can’t make that water flow down on its own.

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**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\
**Post date:** [December 9, 2004, 10:05pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/6 "2004-12-09T22:05:25Z")

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You are confusing force with power. **scr4** ’s magnet example is a good one but I differ in that lifting the paper clip the first time is doing work. You have potential energy but only by letting it drop, not a simple option with a permenent magnet. With capilary you may lift some water up a pipette but how do you release it without expending more energy than the KE of the falling drop of water?

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**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\
**Post date:** [December 9, 2004, 10:30pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/7 "2004-12-09T22:30:18Z")

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Once the capillary tube is full, all you have to do is touch the end to a dry surface or absorbent material. It will all come out.

What is the basis for capillary action?

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**Author:** ![CJJ](https://avatars.discourse-cdn.com/v4/letter/c/ecc23a/32.png) [@CJJ](https://boards.straightdope.com/u/CJJ)\
**Post date:** [December 9, 2004, 10:52pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/8 "2004-12-09T22:52:31Z")

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The OP is not the first to have this idea; WIlliam Congreve attempted but failed to create a perpetual motion machine based on capillary action in 1827.

Capillary action occurs because of the intermolecular attraction between, say, water and the glass in the sides of a tube, in this case the attraction is greater than the intermolecular forces holding the water itself together (the force that explains surface tension). This is roughly the reason for the meniscus that forms when water is placed in a glass graduated cylinder; the water nearest the side of the tube is more strongly attracted to the glass. By contrast, a fluid such as mercury is _less_ attracted to the glass than to itself, leading to an inverted meniscus and a reverse of capillary action (mercury will tend to flow out of a capillary-sized tube).

If a glass tube is placed vertically into a pool of water, the glass-water capillary interaction draws the liquid upward. This force is competing with the natural weight of the water pulling downward, so the tube must be thin enough to keep the gravitational force at a minimum. Since the force is proportional to the inside surface area of the tube, while the weight is proportional to the volume of the tube, at some point the two forces will balance out, say at some tube height h.

The problem comes in actually getting the raised water to flow out of the tube, where it can do some physical work. For this to happen, the tube must be less than h in height, but using such a tube reduces the amount of glass available for the capillary force; the meniscus continues to sit at the top of the tube. Another ingenious idea (if it worked) would be to use a tube \>h, but drill a hole in the side at a height \<h. Unfortunately, the water will never flow out because the pressure of the water in the tube must be less than atmospheric (the surface of the pool of water being drawn from is at atmospheric pressure, pressure decreases with height in the tube, ergo pressure at the hole above the surface must be less than atmospheric).

To summarize, capillary action can raise the water, but attempts to extract work from this potential energy are futile.

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**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\
**Post date:** [December 9, 2004, 10:58pm UTC](https://boards.straightdope.com/t/why-cant-capillary-action-be-harnessed-as-a-power-source/278991/9 "2004-12-09T22:58:26Z")

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Wow! Thanks, CJJ\*! Damn, I love this message board!