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Water under high voltage

  1. Mar 17, 2009 #1
    Hi, I am currently investigating stream of water under high voltage. My setup is using two aluminium plates connected to high voltage power supply with a burette running water between two plates. I found out that the water stream only deflects to one polarity ( i.e. positive/ negative ONLY ) even when i switch the polarity of the plates. Why is it so? Anything like a stream of water beside a charged ruler?
  2. jcsd
  3. Mar 17, 2009 #2
    It's hard to say based on info given. Is it pure water? How high is the voltage? How large is the deflection?

    Water is a polar molecule, so when placed in an electric field a dipole moment (positive and negatively-charged ends) in the water molecule can be induced. For a parallel plate capacitor it is reasonable to assume a constant electric field between the plates, so the only effect on the water molecule should be a small torque (rotation) to align the dipole moment with the electric field. This does not explain the deflection observed, so look carefully at your setup for other factors.
  4. Mar 18, 2009 #3

    I use tap water from the science lab. Voltage is from 0 to 5000V. Deflection is measured by taking photos with a camera and observed on the computer.
  5. Mar 18, 2009 #4


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    It should deflect towards whichever side happens to be closer. Yes it's the same as with a statically-charged comb or whatever.

    buffordboy23: Water has a permanent dipole moment because it's a polar molecule. Induced dipole moments is what you'd see with a nonpolar substance (although polar ones have an induced moment as well, but it's much smaller).
  6. Mar 18, 2009 #5
    :smile: Hi, the distance between the water stream and either of the plates should be accurately equal, else the water will deflect more towards the plate which is nearer to the water stream irrespective of the polarity of the plate. So plz check ur setup for all these factors too!

    Kind Regards,
  7. Mar 18, 2009 #6
    Thanks. Yes, I know that (it's in the name) and had a major brain crash on that one.

    EDIT: I am confused about why you guys are saying that there would be deflection. Water although charged at both ends, has a net charge of zero. So the electric force, F = QE, is zero. What else is going on? The only way that makes sense to me is that the electric field has a large enough deviation from the approximation of a constant field.

    EDIT3: Ignore EDIT2. It's an incorrect application of Coulomb's Law.

    EDIT2: Okay. My mistake was to assume a net charge of zero and base quick calculations off of that. Here's my new look at it. Assume the following setup for a parallel-plate plate capacitor: Charge +Q is on the left plate and -Q on the right plate. Let the dipole moment be oriented parallel to the electric field and the water molecule nearest to the right plate. Case 1: The dipole moment points to right (-q to left of +q), and the net force is towards the "right" plate. Case 2: The dipole moment points to the left (-q to right of +q), and the net force is towards the "left" plate. The reason for the deflection comes from the applied torque to the water molecules by the electric field, which should cause the water droplets to tend toward Case 1.
    Last edited: Mar 18, 2009
  8. Mar 18, 2009 #7
    thanks for all the help

    I just got a few questions..

    1. Why a stream of water has a more observable deflection than a single water droplet? Is there anything to do with more water molecules in a stream of water, hence more attraction?

    2. How the deflection will vary if I change the height of the tip of the burette. i.e. the height of the burette above the "first entering" of the parellel plate (or u can say the electric field). The higher I put the burette, the less deflection...or the other way round? What's the reasoning?
  9. Mar 18, 2009 #8
    EDIT: This is post is wrong too and is based on incorrect application Coulomb's Law as mentioned above.


    1. This may be because a stream of water has less overall collisions with air molecules than say a spray or mist. Water-air molecule collisions likely cause a torque on on the water molecules, which cause the dipole moment to fall out of alignment and reduce the net force on the water molecule.

    2. You can try changing the height of the burette. The higher you raise the burette will result in a greater downward velocity of the water stream due to the acceleration of gravity. As a result, the water spends less time between the charged plates, so deflection should be less with increasing height.

    By the way, I am curious to know what size plates and applied voltage used. Do you know of any video links that show similar experiments to yours?
    Last edited: Mar 18, 2009
  10. Mar 18, 2009 #9


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    I'm not so sure about that. The water should be deflected towards a region of higher field. If these are flat plates, with a uniform field in between, the water should not be deflected.
  11. Mar 18, 2009 #10
    Yes, this what I said originally. This is correct. I tricked myself and used Coulomb's Law, which is an incorrect application in this case. You can model the parallel plate capacitor as two infinite plane sheets approximately. The net forces cancel out. If you want proof, just ask and I will provide it when I have the time. In the mean time, see the link: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesht.html
  12. Mar 18, 2009 #11
    It would be cool to see a couple pictures to try and figure out what is really going on.
  13. Mar 21, 2009 #12
    Actually in theory, how will the deflection vary according to
    1. Voltage across the plates
    2. Starting Height of the water stream

    Because I got linear relationship between them and I doubt it's really true...
  14. Mar 22, 2009 #13
    Voltage determines the strength of the electric field between the plates. This electric field will be constant at any location between the plates, so there should always be no deflection. Deflection can only occur if there is some external field (not from the plates) that you have yet to uncover which causes a variation of the field between the plates.

    This is harder to answer. You see deflection, but you really shouldn't. The downward speed of your water determines the duration (time) that any water molecule can be affected by some electric field. For example, the higher the burette, the less time that water spends in the electric field between the plates; this is due to the acceleration by gravity.
  15. Mar 24, 2009 #14
    I would also recommend using ultrafiltered DI water from a commercial system at 18 Megaohm resistivity to rule out the effects of ions and other contaminants in tap water.
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