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Kelvin's thunderstorm generator

  1. Oct 24, 2013 #1
    I came across this experiment where two flowing streams of water, each falling through a can/inductor and finally falling into a bucket (metallic) creates a huge electrical potential difference! Ok I know that the falling water droplets have to acquire electrical charges, but I just can't figure out HOW IT HAPPENS!! Can anybody please explain it to me? Here's a link to the experiment:

    http://makezine.com/projects/make-31/lord-kelvins-thunderstorm/

    Ok and one more thing, so does that really mean that gravitational potential is being converted to electric potential here?
     
  2. jcsd
  3. Oct 24, 2013 #2

    Bobbywhy

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    From Wikipedia, “Kelvin water dropper, Principle of operation”:

    “Any small charge on either of the two buckets suffices to begin the charging process. Suppose, therefore, that the left bucket has a small positive charge. Now the right ring also has some positive charge since it is connected to the bucket. The charge on the right ring will attract negative charges in the water (ions) into the right-hand stream by electrostatic attraction. When a drop breaks off the end of the right-hand stream, the drop carries negative charge with it. When the negatively charged water drop falls into its bucket (the right one), it gives that bucket and the attached ring (the left one) a negative charge.

    Once the left ring has a negative charge, it attracts positive charge into the left-hand stream. When drops break off the end of that stream, they carry positive charge to the positively charged bucket, making that bucket even more positively charged.

    So positive charges are attracted to the left-hand stream by the ring, and positive charge drips into the positively charged left bucket. Negative charges are attracted to the right-hand stream and negative charge drips into the negatively charged right bucket. The positive feedback[3] of this process makes each bucket and ring more and more charged. The higher the charge, the more effective the electrostatic induction is, so the charges grow exponentially with time.

    Eventually, when both buckets have become highly charged, a few effects may be seen. An electric spark may briefly arc between the two buckets or rings, decreasing the charge on each bucket. Or if this isn't allowed to happen, following Coulomb's law the buckets will start to electrostatically repel the droplets falling towards them, and may fling the droplets away from the buckets. The water drops might also be attracted to the rings enough to touch the rings and deposit their charge on the oppositely charged rings, which decreases the charge on that ring. Each of these effects will limit the voltage that can be reached by the device.”
    Also, don’t forget to check these: “References” and “External links”.

    http://en.wikipedia.org/wiki/Kelvin_water_dropper
     
  4. Oct 24, 2013 #3
    Yes I did see this, and the very 1st line is creating my confusion... “Any small charge on either of the two buckets suffices to begin the charging process." So at the start of the experiment, was one of the buckets charged? But I didn't see this being mentioned anywhere in the setup of the experiment. Please help me out of this confusion.
    Assuming that the bucket is charged at first, the rest of the experiment is easy to understand... opposite charges are induced in the flowing streams thereby creating a circuit with flowing charge and hence a potential difference develops.
     
  5. Oct 25, 2013 #4

    Bobbywhy

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    Here are two descriptions of how the initial charge difference occurs:

    "Although water is full of charged particles - ions from dissolved salts and from the breakdown of water itself, the water that initially falls through the holes in the bucket will be uncharged on the average. However, the randomness of the universe will soon cause there to be a slight charge on one the can/wire systems. Perhaps a drop will fall that is slightly charged, or maybe a cosmic ray will hit the wire and cause a small charge to form on one of the little cans.

    Let's say that the little can on the left (and hence the large can on the right) is slightly positive. Then, this positive charge will have three effects on the falling streams.
    1) Negative ions will be slightly attracted towards the left most hole, so that the water falling through it will, on average, be slightly negatively charged. Thus, the left most large can (and the right side small can) will become negatively charged.
    2) Conversely, positive ions will move to the vicinity of the right hand side whole, making the right hand side stream positively charged.
    3) The positive charge on the can will repel any slightly positively charged drops on the left hand side - this will tend to increase the total negative charge falling into the left hand large can."
    http://outreach.phys.uh.edu/electexperiments.htm

    "SELF-STARTING
    But where does the first charge come from? In fact, if you build such a device, it will usually create voltage all by itself, spontaneously, without being pre-charged. During dry conditions everything near the generator ends up with a tiny electric charge just from being handled. If one of the upper cans is slightly negative, it will cause the water to have imbalanced positive, which will start up the other side of the generator, which will make the charge on the negative side become larger, etc., over and over.

    It's like balancing a penny on edge: it's hard to start out with a perfect balance, and usually it falls one way or the other. Same with this generator. If there's a tiny electrical imbalance at the start, the generator will amplify it over and over, and the voltage will "fall over" to either one polarity or the other. A high voltage will magically appear from nowhere. (But nobody knows which side will start out positive and which will be negative.)"
    http://amasci.com/emotor/kelvin.html
     
  6. Oct 25, 2013 #5
    Ok... Now I understand what's going on! The fact that nothing is perfectly neutral is being exploited here. That's pretty clever actually... :D
     
  7. Oct 25, 2013 #6

    Bobbywhy

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    I have built and operated several Kelvin Water-dropper Generators. Once you've got the technique and materials functioning, the sky is the limit! I've scaled-up my generators to VERY LARGE systems and guestimate have reached 30kV with LOUD and BRIGHT arcs!

    If you go to that "amasci" site above you can see what that experimenter has done in this regard.
     
  8. Oct 27, 2013 #7

    Bobbywhy

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    Here is a new discovery that may allow more efficient charge transfer in Kelvin water-dropper generators. Drops of water are created in the shape of footballs with two sharply pointed ends! I propose some experimenter make these drops and use them in the K. generator and measure the results.

    Stabilizing Liquid Drops in Nonequilibrium Shapes by the Interfacial Jamming of Nanoparticles
    http://www.sciencemag.org/content/342/6157/460
     
  9. Nov 19, 2015 #8
    I've been looking at making one of these with my family for our annual science project when we all get together with our expensive degrees. And, I stumbled across this feud between Veritasium and Thunderf00t on YouTube. Based on Thunderf00t's recent breakthrough on the Coulombic explosion, I'm intrigued.
     
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