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Perpetual motion?

  1. May 8, 2007 #1
    There is a U-tube which contains pure water of about half of its volume. And let's put a capacitor around one surface of the water in the tube. If we apply a high dc voltage to the capacitor, the surface of the water in one side of the tube rises slightly because the permittivity of water is eighty times greater than that of air in the room temperature.

    And here is the question. If we make small holes on both sides of the tube and connect the holes using a small tube so that the water can circulate through, then will the water really circulate? If we ignore the viscosity of the water and the friction with the tube, will it work perpetually?
  2. jcsd
  3. May 9, 2007 #2
    Even if it does run continually it is not perpetual motion, you are driving with electricity. That is no different to putting electricity into an electric motor.
  4. May 9, 2007 #3
    But, the electricity doesn't have any load. It seems that it supplies no work, i.e., no energy at all.
  5. May 9, 2007 #4
    Same old basic premise. (No, the resolution is not in maintaining the charge.) You've changed the effective potential of the water, sure. But the effective potential is changed on both sides of the hole. If the water is forced to rise up to the hole, it won't be able to fall on the other side.
  6. May 9, 2007 #5


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    I must admit that I've never seen this one before. Enthusiasm and wishful thinking know no bounds.
  7. May 9, 2007 #6
    It is the first time that I see a perpetuum mobile which uses electrical properties. I like it!

    The part of water that is "sucked" by the condenser is at a pressure smaller than the atmospheric pressure. The line of hydrostatic pressure equal to the atmospheric pressure is at the level of the water in the other branch of the tube. Then, when you make a hole in this part of the tube, that is under atmospheric pressure, you must not ask if the water will pour out, but if the air is not going to bubble inside the tube.
    I have not still worked out a simple reasoning to demonstrate that the air does not enter. A more complicate reasoning is that when you put some air to replace some of the water in the capacitor the electrical potential energy increases (the capacitance diminishes). Worded otherwise, to enter the air bubble, it must push out some water.
    I'll continue to think about this to see if there is a more direct and simple reasoning.
  8. May 9, 2007 #7


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    While a perfect capacitor would not consume power, there is no such thing in the real world.
    In general dialectics consume power.
    One that moves will use more.
  9. May 9, 2007 #8
    Yes, dialectics consume power, much more than dielectrics.

    I think that you have not understood what is the fun with "perpetumm mobile".

    It is not to say "it won't work". Of course we all know that it won't work. The fun is to demonstrate, without using the conservation of energy, that it is in equilibrium or, that once you have used the energy fournished for the initial state, you won't get any more.

    Water is not a good dielectric because it is ionic and has a residual conductivity, even when it is pure. But there are others dielectrics, non ionic, whose conductivity is really small. So small that it is very hard to measure. You can replace water by one of any such good dielectrics and it does not change the fun of the problem.
  10. May 9, 2007 #9
    First simplification: Consider a U-tube of ferromagnetic fluid, with a magnet at one end of the U, so that the fluid level differs on each side. Now, make a hole in the higher-level tube, and imagine whether the fluid will flow out (and down another tube to the lower-level side of the U)?

    Second simplification: Consider just a U-tube of water, tilted slightly sideways so that the water fills further along one side than the other. Now connect another tube between those two arms; will the water start circulating (from the fuller, lower arm, up the connecting tube, into the higher, emptier arm)?

    A little while ago this idea was presented on PF, with the U-tube of ferrofluid, and further obfuscated by (rather than having the fluid itself circulate) threading a chain of light buoys through the tube, which circulate if archimedes principle is naively applied inappropriately. I look forward to seeing coriolis forces worked in somehow as well.
    Last edited: May 9, 2007
  11. May 9, 2007 #10
    << insulting comments edited out by berkeman >>

    First: the idea is to find a simpler reasoning for this problem and not another one.
    Second: replacing everyday water with a fluid that very few have ever seen and less still have had the possibility to play with, is not a simplification.

    I do not think that someone who is interested in the problem of this thread would be << deleted by berkeman >> to have a confusion with the non-problem you are exposing.

    Once again, the fun is not to replace this physics problem with another more complicated or more << deleted by berkeman >> . The fun is to give the exact reasoning for this problem.
    Last edited by a moderator: May 10, 2007
  12. May 9, 2007 #11
    I was somewhat serious when I was posting it if the water will really circulate. However, now I get to know it will not, thanks to the cesiumfrog's explanation. :smile:
  13. May 9, 2007 #12
    If we ignore the viscosity of the water and the friction with the tube, will it work perpetually?

    I think friction is the key word. Without friction, many things would move perpetually :)

    In the future, use ms paint or something to draw out your experiment.
    Last edited: May 9, 2007
  14. May 9, 2007 #13
    Nobody has mentioned the physical mechanism that draws the water up. Toward (the edge of) a capacitor (or more obviously, to a single electrostatically charged rod), the water molecules (neutral free dipoles) align with the varying field, and (since the field strength now differs between the the two 'poles of the molecule) the molecules have a net attraction in the direction of stronger field.

    This being understood, we can reduce the problem by replacing that mechanism with something else (conceptually simpler or just more familiar) that has an exactly equivalent effect. For example, we can replace the charged capacitor with a very dense mass, which also causes the water to rise higher on that side of the U (but by the mechanism of gravity). With no remaining "herrings", it should be obvious that the water would not flow through a connection between the U-arms.. indeed, if it would, then it have already done so through the bottom of the U-tube. Friction is also clearly irrelevent. If the water is forcefully attracted to between the plates, why wouldn't that force also oppose it from flowing elsewhere?

    Another way of looking at things would be to imagine an O-shaped tube full of water. One probably wouldn't expect a capacitor around one section to cause the water to start circulating? Though with some electric current leakage..
    Last edited: May 10, 2007
  15. May 10, 2007 #14


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    :rofl: Fun with spell checkers. Spell something wrong right.

    Yea! I doubt I'm a candidate for Funniest Member :wink: :smile:

    How about a real world system.
    The Nuclear Fast Breeder Reactor.
    Not only does it make significant power.
    But once started it makes its own fuel. o:)

    Small yes, but not small enough to keep capacitors from going bang if operated outside the design limits.

    In this case the motion of the dielectric consumes power.
    Independent of any direct dielectric loses.
    They make some very nice speaker systems that are just big capacitors.
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