How Water Drops Generate Electricity: Explained

[VertexOperator]

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 :)

 

[OmCheeto]

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 :)

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.

The mechanism by which charge separation happens is still the subject of research.

Weird... I always assumed I knew how lightning formed. It struck me as obvious. I guess I'll have to go back to school.

 

[OmCheeto]

Ah ha! I found Lewin's video.

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

I like professor Lewin.

 

[VertexOperator]

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?

 

[VertexOperator]

Doesn't lightning form when two oppositely charges clouds collide with each other? Clouds get charged when... never mind lol

 

[OmCheeto]

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?

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.

A Review of Thunderstorm Electrification Processes
Field and laboratory evidence points to the importance of interactions between particles of the ice phase, in the presence of liquid water droplets, in separating electric charge in thunderstorms.

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
or
D. A freakishly smart PF member will steer us to the correct answer



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This reminds me, that I no longer understand how batteries work and need to post a question in the chemistry section.

 

[VertexOperator]

Thank you!
You are very nice :)

 

[Bobbywhy]

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

Cheers, Bobbywhy

 

[VertexOperator]

Thank you :)

 

[OmCheeto]

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

Cheers, Bobbywhy

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

THE BASIC THEORY

Even though water has no overall electric charge, it is full of movable electric charges (called ions). Half of the water's charges are positive and half are negative.

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!

I originally didn't understand simple electrical physics at all, but I didn't know it (this after getting a BS in Electrical Engineering!) Later I finally figured out what was wrong, fixed my problems, and now I understand it pretty well.

 

[nsaspook]

I know water molecules are polarized, but that statement says they are ions.

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

 

[willem2]

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

Even pure water has a concentration of 10^(-7) OH^- and H⁺

 

[OmCheeto]

... that generate high voltage by induction.
...]

Cheers, Bobbywhy

Induction?

Electromagnetic induction is the production of a potential difference (voltage) across a conductor when it is exposed to a varying magnetic field.

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".

the two materials only need to come into contact and then separate for electrons to be exchanged. After coming into contact, a chemical bond is formed between some parts of the two surfaces, called adhesion, and charges move from one material to the other to equalize their electrochemical potential. This is what creates the net charge imbalance between the objects. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away

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.

 

[nsaspook]

Even pure water has a concentration of 10^(-7) OH^- and H⁺

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.ncbi.nlm.nih.gov/pmc/articles/PMC2845823/

http://www.lsbu.ac.uk/water/mol-easy.html
http://www.lsbu.ac.uk/water/ionis.html

 

[Bobbywhy]

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

 

[rcgldr]

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.

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.

 

[OmCheeto]

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

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

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

I know that it's real now, because it has a wiki page...

 

[Menaus]

Induction?

"Induction" isn't restricted to the electromagnetic form, there is also electrostatic induction. In this case the voltage is induced through the electrostatic variant.

http://en.wikipedia.org/wiki/Electrostatic_induction

 

[sophiecentaur]

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.

 

[OmCheeto]

...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

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.

 

[sophiecentaur]

The drops in Millikan are actually charged, though, afaik.

 

[NascentOxygen]

Ah ha! I found Lewin's video.

An interesting point in Prof Lewin's video is that his set-up departs from the usual description of how this needs to be arranged. His set-up seems to differ from his own diagram, too.

Usually, we are told to carefully adjust the flow rate so that the stream breaks up into discrete drops just about where it enters the hollow tubes, this causing the drops to become insulated from the stream above. But his video (and here I acknowledge the camera work could do with considerable improvement) appears to show water streaming into and out of the hollow tubes in an unbroken stream. The stream does spread chaotically horizontally as potential builds up, but it is nothing like a procession of discrete water drops.

 

[Bobbywhy]

Here is a tutorial and an excellent description of the detailed theory of operation of the Kelvin Water Dropper at the website of the National High Magnetic Field Laboratory. It provides a clearly written and definitive explanation of how the electrostatic generator works. See:
http://www.magnet.fsu.edu/education/tutorials/java/kelvinwaterdropper/index.html

Cheers, Bobbywhy

 

[OmCheeto]

Here is a tutorial and an excellent description of the detailed theory of operation of the Kelvin Water Dropper at the website of the National High Magnetic Field Laboratory. It provides a clearly written and definitive explanation of how the electrostatic generator works. See:
http://www.magnet.fsu.edu/education/tutorials/java/kelvinwaterdropper/index.html

Cheers, Bobbywhy

Thank you. While I was waiting for Java to download, I checked out the link to the Leyden jar referenced by your link. They said a Leyden Jar could be used to store the energy from the Kelvin Water Dropper. It reminded me of an experiment my university professor once did for the class. It was of a dissectible capacitor. He said; "If you don't understand what is going on here, you will never understand electricity". Well, needless to say, I've never understood his experiment. But the Leyden jar link explained it. So now, after 23 years, I finally understand what was going on.

If anyone is curious as to what I'm talking about, skip to 1:30 in the following video:



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