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Bending distilled water with static electricity

  1. Dec 12, 2011 #1
    You probably know how to bend water: Use a comb and rub it with wool (or just comb your hair) and hold it next to thin jet of water coming out of a tap.
    Here is a video: Comb with wool
    An explanation is offered here (the water molecule is polar).

    However, I remember my physics teacher telling me that this effect does not occur with distilled water because the ions are actually causing the bending.

    Can anyone with access to distilled water confirm this?
     
  2. jcsd
  3. Dec 12, 2011 #2

    Borek

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    Interesting.

    First of all - distilled water won't be enough, as if it is in contact with air it becomes saturated with carbon dioxide and becomes acidic - which means it contains ions. What you will need is the ultra pure water (sometimes called 18MΩ water), and the experiment has to be done in the inert atmosphere. Working with ultra pure water is quite difficult, as it dissolves everything and becomes very easily contaminated; preparation and transport are an art it itself.

    Now, a lot depends on the mechanism. I have heard two versions. One is that water molecules are dipoles and they get ordered in the electric field, which is enough for the effect to be observed. Second is that small amount of ions dislocate to the water surface which becomes slightly charged - and this charge is responsible for bending. In the first case effect should be observed even in ultra pure water.
     
  4. Dec 13, 2011 #3
    well, i had a go with de-ionised water and that bent like the video. but then maybe it wasnt de-ionised enough? though when i tried isopropanol and then methanol, they both bent too. at which point alas, i ran out of solvents to try since i am not at the lab!
     
  5. Dec 14, 2011 #4

    Borek

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    Isopropanol and methanol are less likely to contain lots of ions. Molecules have small dipole moment. But it is so small I started to wonder if induced dipoles are not enough to bend water. It could be checked with carbon tetrachloride, which has no dipole moment, and doesn't dissolve ionic substances.
     
  6. Dec 14, 2011 #5
    Thank you for your effort! That's very interesting. Could the bending of isopropanol and methanol be caused by water since alcohol is hygroscopic?

    Also if anyone else wants to try some other materials you are welcome to test it :biggrin:
     
  7. Dec 14, 2011 #6

    DrDu

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    I see three possible explanations:
    1. dipole orientation polarization of the water molecules
    2. ionic charge displacement polarization
    3. water getting actually charged due to contact with the vessel or tab when poured out

    The third possibility should also lead to a deflection in a homogeneous field whence it can easily be ruled out experimentally.
    The relative importance of the second and third mechanism can be inferred from the dependence of the dielectric constant on concentration of the ions.
    The contribution of mechanism 2 can also be calculated via the Poisson Boltzmann equation, see http://en.wikipedia.org/wiki/Poisson–Boltzmann_equation
    My first guess is that it is insignificant in remotely diluted solutions.
     
  8. Dec 15, 2011 #7

    DrDu

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    I changed my mind. The Debye length is microscopically small as compared to a drop of water even in quite diluted solutions. On the other hand already a tiny concentration of ions corresponds to a immense amount of charge. Hence the ions will completely screen the electric field inside the drop and the attraction is due entirely to the ions in the surface layer.
    Would be nice to try this out with a solution of some salt in a unpolar liquid. Maybe some crown ether complexes.

    Edit: On the other hand, the ion mobilities in electric fields are very small so that time may be insufficient for a gradient of ionic charges to form.
     
    Last edited: Dec 15, 2011
  9. Dec 15, 2011 #8
    ... entirely possible alas, for the purity, or lack of it, is something i have no way of assessing.

    but anyway, thank _you_ for your original question, it has been a lot of fun messing about in the kitchen trying to find something that _wouldn't_ bend, but unless i can figure out some way to siphon out some petrol from the car fuel tank i dont think i shall have much success -- and not contribute much to the discussion i fancy. for it would seem that this water deflection business is in fact rather popular in the classroom demonstration area. e.g:

    http://dev.physicslab.org/Document.aspx?doctype=5&filename=Compilations_NextTime_WaterStream.xml

    http://www.physicsclassroom.com/class/estatics/u8l1e.cfm

    and as to the mechanisms involved, there is a fair amount of discussion as well:e,g.

    Why Does a Stream of Water Deflect in an Electric Field?
    G. K. Vemulapalli and S. G. Kukolich
    http://www.jce.divched.org/hs/journal/issues/1996/Sep/clicSubscriber/V79N09/p887.pdf [Broken]


    Electrical Deflection of Polar Liquid Streams: A Misunderstood Demonstration
    Maryam Ziaei-Moayyed and Edward Goodman:
    http://www.jce.divched.org/journal/issues/2000/Nov/PlusSub/V77N11/p1520.pdf [Broken]

    what larks!!
     
    Last edited by a moderator: May 5, 2017
  10. Dec 16, 2011 #9

    DrDu

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    Ok, so deflection seems to be in fact due to droplets getting charged as I proposed as mechanism 3!
     
    Last edited by a moderator: May 5, 2017
  11. Dec 16, 2011 #10
    Thank you f.c., those are excellent papers. The conclusion of the second paper is interesting, suggesting to stop giving the wrong explanation for that experiment in school.

    I found another nice experiment called the floating water bridge (video). See what happens if you apply 10kV to distilled water.

    Elmar Fuchs who thought of the experiment has more videos, one showing a hot bridge (60°C).
    He also gave a talk at SETI explaining in detail his experiments.
     
    Last edited: Dec 16, 2011
  12. Dec 17, 2011 #11
    yes, that one i have seen before, very odd!. though for sheer mind-boggling gravity-defying weirdness, that quantum locking business must surely wins, hands down
    http://io9.com/5850729/quantum-locking-will-blow-your-mind--but-how-does-it-work

    but never mind! this is drifting way off topic and there is one more thing i'd like to add regarding the bending being due to the water drops getting charged mechanism. namely, that when i try it, it seems to make no difference whether i use a charged glass rod or a charged plastic one (my toothbrush handle actually), the water stream always deflect in the same direction, i.e. towards the rod. which is not at all what one might expect since glass and plastic wind up carrying opposite charges when rubbed (though whether glass is +ve and plastic -ve or vice versa i have no idea)

    now with all apologies to DrDu and i know that one should never let the facts upset a good theory, this observation is a little tiresome to say the least.
     
  13. Dec 17, 2011 #12

    DrDu

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    Are you sure that the charged rods aren't simply influencing an opposite charge of the drops?
    Another important point: It should make a difference whether you study isolated drops or a stream of water. If the water contains ions it will act like a conducting rod in the latter case.
     
  14. Dec 17, 2011 #13
    DrDu, you've been faster with the reply.

    I'm still thinking about the second paper. They explain that the droplets get charged due to charge induction similar to the Kelvin water dropper (video here).
    That would also explain why the sign of the charge does not matter (both glass and plastic attract the stream as you have observed).

    I thought about another issue though:

    1. What if we didn't have droplets but a continuous (distilled) water stream? Then a charge induction (separation) should not occur.

    2. However, we can easily bend a continuous water stream from a tap with a charged comb. The question is what mechanism takes place here. (Ok, no distilled water here)

    3. It would have been interesting to examine the difference between distilled and tap water (or salt water) to check whether ions play a role.

    --

    By the way, you can repel the water stream but you need strong neodymium magnets:
    Video 1
    Video 2
     
  15. Dec 17, 2011 #14
    ah, well, yes, i think i have misunderstood. i thought that the mechanism proposed was that the fluid picked up a permanent charge directly on leaving the end of the dropping funnel, and it was this permanent charge +ve, or -ve, that i hoped to detect by using the differently charged rods.-- but to no avail as i reported above -- always the fluid deflected in a direction toward the rod.

    the position of the rod for best effect was indeed quite important -- maximum deflection occurring when it was placed high up next to the stream at the point where it curved most, and before it broke up into droplets. i suppose it would help to upload some pictures but (a) i have no idea how this might be achieved and (b) i have been summarily banned from any further messing about in the kitchen since i had a small accident and spilt a really _very_ small amount of white spirit all over everywhere and stunk out the place for hours. oops.

    (oh, by the way, the white spirit aka naptha EC-265-185-4 _didn't_ bend. hurrah! that at least behaved as expected!)
     
  16. Dec 18, 2011 #15

    DrDu

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    A continuous stream would behave like a metal wire. Maybe you could try out how a thin earthed wire will behave when you bring close your charged comb to the other end?
    I think the conductivity even of highly pure water is too high for it not behaving like a good conductor in these kind of electrostatic experiments.
     
  17. Dec 18, 2011 #16
    i would just like to make clear that it makes no appreciable difference whether the rate of flow is adjusted to produce either well-defined seperate drops or a continuous stream, the deflection observed is always in the direction of the charged rod, whether plastic or glass, i.e. +ve or -ve.

    the real problem i feel that there are (a) just too many competing effects going on to be able to come to a definitive conclusion with the apparatus available, and (b) i am the only one here actually doing any experiments!! (or reporting the results!!) come on you guys, get out your toothbrush handles and face flannels and start rubbing them together!!
     
  18. Dec 18, 2011 #17
    ok, one last final post on this subject before i get murdered, (or get to spend the night on the sofa), so if small child A holds her toothbrush handle very close to the top of the stream so it deflects to the right (say), then when child B brings up his toothbrush close but further down, the stream will deflect even further to the right, but if i hold the glass rod close to the stream it will deflect left. back towards the rod which, finally, i think, proves that this stream is now carrying a charge opposite to that on the top toothbrush handle. BUT under normal circumstances, i.e. a one toothbrush / glass rod experiment, the stream always deflects towards it. which demonstrates (perhaps) that the stream is not usually charged, and one must resort to the induced dipole/ polarisation explanation? (and this result holds if the flow is reduced to droplets as well). heavens. i need a strong drink. did lord kelvin have all this grief with his buckets you think?
     
    Last edited: Dec 18, 2011
  19. Dec 18, 2011 #18

    DrDu

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    Besides calculating the angle of deflection for all three mechanisms, I have to say that I did this experiment some time ago and that this morning, much to the dismay of my wife, I started to construct some Kelvin apparatus out of some yoghurt cups (before learning from this thread that this apparatus was invented by Lord Kelvin).
    But besides this I came to the conclusion that the competing effects can experimentally only be distinguished in some precision experiments which won't be possible with my toothbrush.
     
  20. Dec 18, 2011 #19

    DrDu

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    No, if you have a continuous stream it will alway deflect in the direction of the rods.
    With drops this should be different, so maybe you can test it in your kitchen.
     
  21. Dec 18, 2011 #20
    This sounds reasonable. I did the following:
    1. I took a small stripe of aluminium and attached it to the edge of a table. The charge comb attracted it.
    2. I took the aluminum piece and attached it to a tap. The piece also moved towards the charged comb.

    Haha! Only scientists use yoghurt cups to produce lightning.
    Let me know if you've actually built the Kelvin water dropper and if it worked.
     
    Last edited: Dec 18, 2011
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