Kelvin water dropper and Walter Lewin

[gnurf]

I recently came across MIT prof. Walter Lewin's fascinating demonstration of a Kelvin water dropper, which can be seen here:

https://www.youtube.com/watch?v=http://www.youtube.com/watch?v=oY1eyLEo8_A
(EDIT: If your browser is like mine that didn't work, try this: )

As shown in the diagram below, the setup consists of a bucket of water and some empty paint cans that act together in a positive feedback loop where the charge builds up to create a battery with a potential difference of several kilovolts. In the MIT demo, there are also two closely placed conductors from the each of the opposite charged buckets so that the buckets are discharged (resulting in a spark in the air between them) when the potential exceeds some threshold.

From wikipedia's http://en.wikipedia.org/wiki/Kelvin_water_dropper#Principle_of_operation" section:

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 charge 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.

Am I right to think that this wouldn't work with pure H₂O (i.e an insulator with no free electrons)? Also, the prof. says that "the way it works is actually quite subtle", so I'm wondering the if the wikipedia entry is the whole story, or if there's more to it?

 

[Fish4Fun]

One of the links at the bottom of the wikipedia page you referenced leads here:

http://amasci.com/emotor/kelvin.html

A great read (if a bit rambling).

I vaguely remember a discussion of this experiment in some mostly forgotten physics class. Thank you for this post as I have thoroughly enjoyed revisiting the topic! I do wonder if there are any practical applications for this phenomena wrt power generation, though I suspect the yield in most cases would be far too small.

Fish

 

[KelvinDude]

Am I right to think that this wouldn't work with pure H₂O (i.e an insulator with no free electrons)?

No. Water self-ionizes. Pure water has a pH of 7, which means, at equilibrium, it has approximately 10^-7 hydronium and hydroxide ions in solution at any given time. Granted, it may have no "free electrons" in the sense you may be thinking, but it does have charged particles, which is sufficient for this experiment.