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The water battery

  1. Mar 3, 2013 #1
    Can someone please explain to me how water drops generate electricity? I saw this in a Walter Lewin lecture and became very curious to know :)
  2. jcsd
  3. Mar 3, 2013 #2


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    It sounds like you are talking about a Kelvin water drop generator.

    There is an interesting video on another web site that shows the effect. Though I don't much care for their explanation.

    Actually, I just read the wiki entry, and didn't care for their explanation either.

    Which tells me I probably don't understand how these things work.

    A quick google finds that it has been talked about at least once here: Kelvin water dropper generator

    An alternate name is Lord Kelvin's Thunderstorm. I like this name because this is how I imagine the device to work. Its simply a controlled mini-me thunderstorm.

    Though looking at the wiki entry on lightning, it appears that there is some debate on how lightning is formed.

    Weird..... I always assumed I knew how lightning formed. It struck me as obvious. I guess I'll have to go back to school. :grumpy:
  4. Mar 3, 2013 #3


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    Ah ha! I found Lewin's video.

    https://www.youtube.com/watch?v=oY1eyLEo8_A ​

    I like professor Lewin.
  5. Mar 3, 2013 #4
    Thank you :)
    Yeah, looks very similar to lightning, but I still don't get how water drops get charged lol Is it because of friction with air or something?
  6. Mar 3, 2013 #5
    Doesn't lightning form when two oppositely charges clouds collide with each other? Clouds get charged when... never mind lol
  7. Mar 3, 2013 #6


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    That's how I perceived it. Kind of like rubbing a balloon you head. As the raindrops fall through the air, electrons are removed, charging the upper atmosphere with an excess of electrons. The positively charged raindrops fall to the ground. This creates an electrostatic potential, which results in lightning.

    But the wiki article says that other factors may be involved. They list two hypotheses:
    Cloud particle collision hypothesis
    Polarization mechanism hypothesis

    But the Cloud particle collision hypothesis does not appear, to me anyways, to be able to explain the effect in the water battery.

    And the academic references don't seem to help, as there are no ice phase particles involved with the water battery.

    ps. Please don't assume that I have any working knowledge beyond that a mere layman. I subscribe to threads like this because:
    A. I would like to know the answer
    B. Posting automatically subscribes me to the thread
    C. A freakishly smart PF member will answer the question for us
    D. A freakishly smart PF member will steer us to the correct answer


    This reminds me, that I no longer understand how batteries work and need to post a question in the chemistry section.
  8. Mar 4, 2013 #7
    Thank you!
    You are very nice :)
  9. Mar 4, 2013 #8


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    I have built several Kelvin Water Droppers that generate high voltage by induction.
    The best technical explanation of the theory of operation I’ve been able to find is by Bill Beaty here:

    Cheers, Bobbywhy
  10. Mar 4, 2013 #9
    Thank you :)
  11. Mar 4, 2013 #10


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    Well if that's the best explanation, I have a lot to unlearn and re-learn!

    I know water molecules are polarized, but that statement says they are ions.
    Is liquid water actually equal parts of H2O, OH+ and H3O- ?
    In my "How do batteries work?" thread, Borek says that H2SO4 breaks down into three different ions. Borek also said that people don't always include the details.

    ps. The author sounds just like me!

  12. Mar 4, 2013 #11


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    Tap water normally has dissolved salts and other chemical impurities that make it electrically conductive by ion flow.

    http://www.tmasc.com/basic di info systems.pdf
  13. Mar 4, 2013 #12
    Even pure water has a concentration of 10^(-7) OH- and H+
  14. Mar 4, 2013 #13


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    This is the only "induction" I'm familiar with. (I'm somewhat familiar with transformers and induction motors.)

    The drops are electrostatically charged. I suppose that charged particles traveling though the loop create a changing magnetic field, but not in a way as to separate charge within the system, as far as I can tell. (scratches head...)

    I must say, that studying peculiar simple devices is very educational. I'm currently studying the wiki entry on the "Triboelectric effect".

    These tendencies were explained to me, in terms of full, and not full, outer electron shells.

    hmmmm.... Do they still speak of outer electron shells?

    It's been a good 30 years since I've been to university.
  15. Mar 4, 2013 #14


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    True and the excess surface charge of pure DI water is very small. I'm not really sure what would happen if you used it for the generator (pure DI water being a high K dielectric) but I'd think it would charge very slowly. I do know that pure water is a very strange compound that under electrical stress can store charge inside a volume of pure water for long periods of time.


    http://www.lsbu.ac.uk/water/mol-easy.html [Broken]
    Last edited by a moderator: May 6, 2017
  16. Mar 4, 2013 #15


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    In the Kelvin Water dropper the two upper metal rings (or cylinders) are called “inducers” because they charge the falling water drops by electrostatic induction. This can occur across an empty space (as in the dropper) and does not require any contact friction. This is the same mechanism that causes balloons to adhere to surfaces and that cause styrofoam particles to be attracted by a charged rubber pocket comb, for instance. It’s important to not confuse electrostatic induction with electromagnetic induction.

    If there is a better description of the detailed theory of operation of the Kelvin Water Dropper than Beatty’s then it would be great to read it.

    Cheers, Bobbywhy
  17. Mar 4, 2013 #16


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    The drops are already charged by the time they start falling. Once the drops are falling, they don't accumulate any additional charge. As explained in Beatty's article, what the inducers do is cause the water in the upper cans to separate into ionized layers, repelling the ions with like charge away (upwards) from the inducers, and attracting the ions with opposite charge towards the inducers (downwards), so that the drops of water have the opposite charge of the inducers. As the charge increases, the water drops start to scatter due to attaction towards the inducers, until a spark occurs, resetting the system back to a near neutral state.
  18. Mar 4, 2013 #17


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    I do believe, that we have another; "What is 'Magnetism' thread." :tongue2:

    Runs off to bed, pull the covers over his head, and waits for the obligatory line through name.......
    Good night. :redface:

    ps. Will research "electrostatic induction" in the future.

    I know that it's real now, because it has a wiki page.......
  19. Mar 5, 2013 #18
  20. Mar 5, 2013 #19


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    Electrostatic induction always seems to be ignored in 'elementary' explanations of electrostatic effects. The classic one is when a rubbed comb (charged by friction) picks up (uncharged) bits of paper and dust. How is this?
    When the comb is brought next to the particle, there is some movement of charge in the particle (polarisation of the molecules, if it's an insulator). The unlike charges end up closer to the comb than the like charges. They are closer, so the attractive force is greater than the repulsive force on the similar charges, which are further away. Net Force is Attraction without the paper actually having a net charge.

    At School, we were shown the 'Electrophorus' (Google it). This is a metal disc on an insulating handle. You place it on a (+, say) charged mat. '-' charges move slightly towards the mat and + charges go to the far side. You touch it with your finger and the excess + charges flow to earth, leaving an excess of - charges on the metal. Take it away and it is charged 'by induction'. Again, no 'rubbing' involved.

    The Kelvin Water Dropper (and the Whimshurst Machine) work on a similar principle. There is very slight polarisation of the water in the reservoir (one nozzle is slightly positive and the other is slightly negative. As the drops fall through the rings into the tins, the two halves of the apparatus gradually build up an 'induced' potential difference between the two diagonal structures. No friction is involved here - all that's necessary is a very small (unavoidable) imbalance at the start, which just gets magnified. I have heard this described as Positive Feedback.
  21. Mar 5, 2013 #20


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    I think I'm understanding how this machine works now. I re-read the paper again after studying topics hinted at from the various posts: Electrostatic induction, Electrophorus, Gold-leaf electroscopes, etc.

    I ended up thinking that this reminded me of Millikan's oil drop experiment.

    https://www.youtube.com/watch?v=XMfYHag7Liw ​

    One could modify Kelvin's thunderstorm device, say, replace the bottom tubs with glass beakers such that you could use a high speed camera to film the smaller drops approaching and then reversing direction. I think that would be interesting.

    btw, did anyone watch the slo-mo portion of the video from my original post? Many of the smaller droplets went into orbit around the loop! I found that quite entertaining.

    Ah ha! That explains why the hoops are hoops.

    I am definitely going to build one of these devices.
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