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07-31-2012, 11:25 PM   #16
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by PairTheBoard What happens if you lift the tube out of the source water? Does the water in the tube just stick in there rather than flow out the bottom of the tube? If so, how do they get the water out of the tube? With a little plunger? PairTheBoard
That most definitely depend on the geometry of the tube. Some very small thin ones are very different than a big radius one.

Just spend a lot of time on this link and any books on fluid mechanics you can get. Its an interesting topic.

http://en.wikipedia.org/wiki/Surface_tension (will offer better mathematical perspective of the problem)

Just observe what happens to a drinking straw next time. It does hold some water in it. Now imagine the radius 10 times smaller and its a lot easier to keep more in relative terms (maybe even absolute ones).

You can ask a related problem for example. How big does a drop of water need to be to break and fall?

Last edited by masque de Z; 07-31-2012 at 11:36 PM.

08-01-2012, 12:57 AM   #17
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Re: Why won't this, ahem, perpetual motion machine work?

Originally Posted by PairTheBoard
"What happens if you lift the tube out of the source water? Does the water in the tube just stick in there rather than flow out the bottom of the tube? If so, how do they get the water out of the tube? With a little plunger?"

Quote:
 Originally Posted by masque de Z That most definitely depend on the geometry of the tube. Some very small thin ones are very different than a big radius one.
I'm talking about a tube thin enough to have sucked the water up via the capillary action discussed in the OP.

PairTheBoard

08-01-2012, 01:25 AM   #18
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by ganstaman If you drill the hole, will water leak out?
I'm pretty sure it doesn't, hence why this doesn't work. I don't think you can actually "collect" the potential energy you've stored in the water in the tube. That seems anti-thermodynamics.

08-01-2012, 01:32 AM   #19
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by RLK We have to be very clear here. If you drill a hole below the level of the water without any surface interaction, then of course it pours out. The energy you get is the energy due to lifting the water to make the column in the first place.
I don't think this is right. I think any tube that is narrow enough to cause capillary action will hold the water if poked a hole in the side.

Quote:
 What gets complicated is the drilling of a hole in the wall above the water's level before the capillary effect occurs. The drill is a problem also because the material of the drill could have a surface interaction with the water which would wick it out as it punctures through the glass.
I think you're overcomplicating this. I had envisioned that you have a very tall tube, you watch the water come up, and then you come in with a drill and make a hole in the side. Some might think that the water then spills out. I think this is wrong.

Quote:
 Imagine that the glass has a precut plug being held in place. If the plug was below the pre-capillary water level, then you would have to push on it to hold it in place because the static pressure of the water would try to push it out to create a hole. But if it was above the pre-capillary level but was wetted as the capillary effect of the water pulls the water up, then you would have to pull on it to hold it in place. The water interaction would be pulling it in slightly as the water is pulled up the sides of the container. If you let go, the plug would fall into the container and the water would drop back below the hole, pulled by gravity and the surface tension of the water in the center.
I'm not sure this is right, either. I'm unconvinced that this accurately reflects the behavior or water at a size where capillary action happens. In particular, capillary action (I believe) is pulling inward on the plug, and I don't think that there's an outward static pressure due to the water.

08-01-2012, 01:40 AM   #20
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by PairTheBoard Originally Posted by PairTheBoard "What happens if you lift the tube out of the source water? Does the water in the tube just stick in there rather than flow out the bottom of the tube? If so, how do they get the water out of the tube? With a little plunger?" I'm talking about a tube thin enough to have sucked the water up via the capillary action discussed in the OP. PairTheBoard

And i responded that this still depends on the geometry because what the water does depends on what is energetically more favorable and the geometry determines that. My guess is the thinner will retain the most water in relative terms and maybe even in absolute terms in some cases. The material may also play a role. What happens when you wash Tupperware? Doesnt water tend to remain in droplets still in the surface? Imagine a very thin plastic tube now like a very thin drinking straw. Cant you imagine it having still lots of water once removed from the eg big container. My answer about the droplet size is also very relevant if you think about it.

 08-01-2012, 01:42 AM #21 veteran     Join Date: Aug 2009 Location: Stanford, CA USA Posts: 3,322 Re: Why won't this, ahem, perpetual motion machine work? On the drill a hole on the side question i suppose what happens also depends on the size of the hole and the radius of the tube and the overall resulting geometry.
08-01-2012, 03:05 AM   #22
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by masque de Z And i responded that this still depends on the geometry because what the water does depends on what is energetically more favorable and the geometry determines that. My guess is the thinner will retain the most water in relative terms and maybe even in absolute terms in some cases. The material may also play a role. What happens when you wash Tupperware? Doesnt water tend to remain in droplets still in the surface? Imagine a very thin plastic tube now like a very thin drinking straw. Cant you imagine it having still lots of water once removed from the eg big container. My answer about the droplet size is also very relevant if you think about it.
I doubt any significant amount of water would come out. If it did then with a very light tube and a very short lift costing little energy you could harvest excess energy from the falling water over and above the little it costs you for the minimum lifting of the tube out of the source water. The same capillary action that got the water up the tube to begin with ought to hold it in the tube once the tube is lifted out of the source water.

PairTheBoard

08-01-2012, 09:08 AM   #23
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by Aaron W. I don't think this is right. I think any tube that is narrow enough to cause capillary action will hold the water if poked a hole in the side.
You misunderstood me. I specified a hypothetical "without surface interaction" to separate the capillary effect from the bulk fluid dynamics effects. I agree that the water will not go out of the hole.

Quote:
 I think you're overcomplicating this. I had envisioned that you have a very tall tube, you watch the water come up, and then you come in with a drill and make a hole in the side. Some might think that the water then spills out. I think this is wrong.
I did not overcomplicate it, you did with the drill. That is why I went to the preexisting plug to get the complication of the drill out of the problem. Again, I agree that the water does not spill out of the hole.

Quote:
 I'm not sure this is right, either. I'm unconvinced that this accurately reflects the behavior or water at a size where capillary action happens. In particular, capillary action (I believe) is pulling inward on the plug, and I don't think that there's an outward static pressure due to the water.
There are two contributions to the total force on the plug. The net contribution is inward. There is a hydrostatic pressure due to the height of the column, but that is less than the capillary action. That is why the column does not drop back down. I probably made it too confusing by looking at a case without capillary effect for contrast.

To restate, if you put a capillary tube with a preexisting plug into a beaker of water, the water will climb into the tube due to the surface attraction between the water and the walls. The water in the center of the tube is lifted by surface tension with the water at the walls, kind of like a spring supported at the edges and hanging in the middle. If you measure the force on the plug, it will be inward due to the force required to support the column of water. If you release the plug it will move into the capillary tube and the column of water will drop slightly because there is less wall interaction supporting the capillary rise. No water will flow out. That is all I was ever saying.

Last edited by RLK; 08-01-2012 at 09:16 AM.

08-01-2012, 09:29 AM   #24
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by masque de Z And i responded that this still depends on the geometry because what the water does depends on what is energetically more favorable and the geometry determines that. My guess is the thinner will retain the most water in relative terms and maybe even in absolute terms in some cases. The material may also play a role. What happens when you wash Tupperware? Doesnt water tend to remain in droplets still in the surface? Imagine a very thin plastic tube now like a very thin drinking straw. Cant you imagine it having still lots of water once removed from the eg big container. My answer about the droplet size is also very relevant if you think about it.
The droplet size question is very well known in fluid mechanics. The size of a droplet is determined by the density of the material forming the drop and the surface tension of the material. Surface tension results because it takes energy to form an interface, depending on the nature of the material forming the interface. Water has a permanent dipole moment so it reduces its internal energy by forming rapidly shifting hydrogen bonds even in the liquid state. (That is why the freezing point of water is so much higher than a nonpolar fluid of similar molecular weight, say methane or butane). When you make a water surface the water molecules at that surface can no longer form hydrogen bonds with neighbors so their energy is increased because of that loss of stability. That effect causes the water to try to form a geometry that minimizes the existing surface. A drop falls when the weight of the drop is to great to be supported by this surface tension effect. I have had to solve these types of problems back in my chem engr fluid dynamics courses.

All of those other questions can be answered and are well known. The way a drop spreads on a surface depends on the surface tension of the liquid and its interaction with the surface.

 08-01-2012, 03:10 PM #25 veteran     Join Date: Jun 2009 Location: Live from StL It's Sat Night Dead! Posts: 2,891 Re: Why won't this, ahem, perpetual motion machine work? Seems like this could work if we found a material that changed it's surface tension properties in response to something, like an electrical signal. After the tubes were filled with water, the signal could be turned on or off causing the fluid to fall and release it's energy.
08-01-2012, 04:25 PM   #26
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by FoldnDark Seems like this could work if we found a material that changed it's surface tension properties in response to something, like an electrical signal. After the tubes were filled with water, the signal could be turned on or off causing the fluid to fall and release it's energy.
Interesting. It seems like water should change its surface tension in response to an electric field. Water's high surface tension is due to hydrogen bonding which occurs due to water's inherent dipole moment. If you applied an electric field you would cause those dipoles to change their alignment slightly so that the hydrogen bonding would be slightly weaker. That should reduce the surface tension. That would cause the water at the walls of the tube to rise slightly and the water in the center of the tube to drop slightly since it would take less energy to form the water/air interface. It is not clear that there would be a net change in energy though. Plus, it would take energy to create the field that caused the water to change alignment. All in all I am sure that you would end up with no gain.

The effect of an electric field on water's surface tension is not something I recall seeing. I will look around for that, just curious now.

Edit:

I found this.

Quote:
 Thus, a high-voltage electric field (333 kV m-1) has been shown to raise the water activity in bread dough, so ensuring a more efficient hydration of the gluten [331]. Rather unexpectedly, such electric fields (~1 MV m-1) apparently increase water's surface tension by about 2%.
I am going to try to find the original paper. I have to agree with this comment that an increase would be unexpected. It is probably tough to measure, the abstract I saw said it was done by fitting the drop shape using the surface tension as an adjustable parameter. I am suspicious of such an indirect determination.

Looks like the abstract is all I can get. I am not sure I buy this.

Quote:
 Development of a new methodology for the study of both shape and surface tension of conducting drops in an electric field is presented. This methodology, called axisymmetric drop shape analysiselectric fields (ADSA-EF), generates numerical drop profiles in an electrostatic field, for a given surface tension. Then, it calculates the true value of the surface tension by matching theoretical profiles to the shape of experimental drops, using the surface tension as an adjustable parameter. ADSA-EF can be employed to simulate and study drop shapes in the electric field and to determine its effect on liquid surface tension. The method can also be used to measure surface tension in microgravity, where current drop-shape techniques are not applicable. The axisymmetric shape of the drop is the only assumption made in the development of ADSA-EF. The new scheme is applicable when both gravity and electrostatic forces are present. Preliminary measurements using ADSA-EF suggest that the surface tension of water increases by about 2% when an electric field with the magnitude of 106 V/m is applied.

Last edited by RLK; 08-01-2012 at 04:42 PM.

08-01-2012, 04:59 PM   #27
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Re: Why won't this, ahem, perpetual motion machine work?

My edit time has expired so this continues my last post.

Quote:
 The effect of a uniform electric field on the bulk and interfacial properties of a model dipolar fluid is investigated by using a modified mean-field density functional theory. Particular attention is given to the dependence of the vapor-liquid phase coexistence in a slab-shaped system on the direction of the electric field with respect to the slab surfaces, as well as in the sphere-shaped system on the surrounding dielectric permittivity. For planar vapor-liquid interfaces, the interfacial profiles of the orientation order parameters and components of the dielectric-permittivity tensor are calculated. Analytical expressions for these interfacial profiles and their dependence on the electric field are obtained. When the electric field is normal to the interface we find that the thermodynamic surface tension is lowered compared to that in zero field, and that when the electric field is parallel to the interface the surface tension is enhanced. In contrast, the mechanical surface tension at the equimolar dividing surface is always enhanced by the field regardless of the field direction, and it assumes its highest value when the field is parallel to the interface.
The bold makes sense. When I was thinking about the problem earlier I imagined a field normal to the surface. But if the field is parallel to the interface then it reinforces hydrogen bonding in the interface and an increase in surface tension is expected. With only an abstract I cannot tell how that affects the experimental paper's conclusions.

I am satisfied now.

 08-01-2012, 05:39 PM #28 addicted     Join Date: Jan 2008 Location: vṛkṣāsana Posts: 45,810 I'm pleased that this arguably silly question sparked an interesting discussion. I reported this fact to 'pup, and he's also pleased, though I think he'd have preferred that his idea turn out to work.
08-01-2012, 06:48 PM   #29
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Re: Why won't this, ahem, perpetual motion machine work?

Quote:
 Originally Posted by RLK It is not clear that there would be a net change in energy though. Plus, it would take energy to create the field that caused the water to change alignment. All in all I am sure that you would end up with no gain.
I suspect you're right here. The energy has to come from somewhere, right? It cannot be created from nothing.

I guess I'm not exactly clear how the energy equation balances out when water makes it's way up the capillary tube creating potential energy. Does the system absorb heat in order for the water to climb the tube?

 08-01-2012, 07:08 PM #30 Carpal \'Tunnel   Join Date: Dec 2003 Posts: 6,129 Re: Why won't this, ahem, perpetual motion machine work? You wouldn't really need to get the water out of the tube to harvest its potential energy. Just tip the vertical tube over and let it fall flat. If you could manufacture new tubes cheaply enough you could just discard the used ones filled with water and replace them with new empty ones - then repeat. PairTheBoard

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