Capacitor+Grounded Plate+AC Current?

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  • #26
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In the idealized case of a constant current flowing through a resistor, yes. But we're talking about starting from zero current, then applying voltage to produce non-zero current. During the transition, energy dissipation in the resistor is not the same everywhere; it starts where energy first flows into the resistor.
That is certainly an effect if you apply GHz AC, but I don't think it matters at 50 or 60 Hz. The pickle has more than enough time to stay extremely close to equilibrium current flow and potential drop.
 
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  • #27
anorlunda
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I can imagine one scenario that seems to fit the description.
  • First, some rectification is happening. The mechanism and polarity, I don't know.
  • Ions migrate toward the cathode, and begin to electroplate the surface with a high resistance material.
  • Resistive heating intensifies the rectification and accelerates the electroplating, casing a runaway condition.
So, my one word answer to the OP would be electroplating.

Since my complaint about safety (#2) has been so stingingly rejected by the community in this very fun thread, let me explain myself better. A quick search on Youtube showed me that "pickle fires" are already an Internet meme, that pre-existed this thread. When I searched that term on Youtube, I not only found a video, but on the right side of the screen was a list of related videos that read (in order).
  1. Lighting A Pickle on Fire
  2. Electric Pickle
  3. Exploding Pickles
  4. Live 50 Caliber Round in a Microwave (1,079,003 views)
In #3, Dr. Claude called my safety complaint ridiculous. I agree. But IMO ridiculous escalation is not only foreseeable it is actually the norm when attractive nuisance topics are put into the hands of teens and pre-teens. It is not just the danger that makes the topic so risky, it is the combination of AC mains power and common household items that make it attractive to teens and pre-teens. Multiply that with the power of search engines to find "fun" things to amaze your friends and the risk is magnified.

If the professor in the OP had simply worked a little harder, I'm sure he could have done the same experiment, starting with a test tube, salt water, and some chemical not common in households. That would have make the topic equally interesting to engineers, but much less attractive to kids and I would not have complained that it was dangerous.

I know that most of us here are PF scholarly types, and not trained to think like a middle-school vice-principal deciding what is appropriate for a student assembly. A better analogy would be the producers of the Mythbusters TV show. That show's whole business model is based on spectacularly dangerous things like explosions, but I wager that the Mythbusters producers would not allow a segment about applying AC power to common kitchen items. Ready accessibility greatly increases the chances if it actually being tried at home. Admonishments like "Don't Try This" merely increase the appeal.. It is my view that since PF is so public and indexed by search engines, that we have an ethical responsibility to think like those Mythbuster Producers.
 
  • #28
sophiecentaur
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Since my complaint about safety (#2) has been so stingingly rejected by the community
Yes. That surprised me greatly, considering the response to many other less dodgy practices.
I agree with your basic message about safety - especially about the "don't try this at home folks" line. I can't think of anything more likely to make people Do it at home when they find they have all the ingredients.
When you think of how PF always stresses that Mains Electricity should be dealt with by Qualified Engineers only, it does seem inconsistent and, well, dangerous to play with mains and pickles. Croc leads connected to L and N on the kitchen table????????? OMG. It gives me the shivers if I find a lead with a plug one end and nothing on the other.
 
  • #29
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yes sophie , you were reading it wrong , mebigguy understood my not so easy to read wording (sorry for that) I was implying that the em wave that moves through the medium travels at near c, in other words the electrons feel the force on them from one side to the other at c but ofcourse they themselves move slowly.
 
  • #30
meBigGuy
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I am surprised that light emission was not the goal of the class demonstration, or even mentioned by the OP. Pickle chandeliers are a new thing in my life.


1. Can we assume that the primary cause of pickle destruction is ohmic heating?

2. Are we agreed that there is no "energy directionality" effect with a 60Hz AC source that would favor ohmic heating of one side over the other?

3. Are we all agreed that there is no basic physics related phenomena pertaining to AC source characteristics (such a neutral or hot) that would cause the pickle to change temperature differently in the vicinity of that electrode.

4. In the youtube videos and hyperphysics videos I see the light phenomena starting on the small side or the large side, so the size of the end is probably not the only criteria.

5. The OP has not made it clear whether the professor plugged an electrically-hot electrode into a pickle (with the other side already connected). That method of applying power could cause that side to get hotter since there might be higher initial resistance for enough time to favor heating on that end.

My personal opinion is that the determination of which end will ignite has more to do with the pickle's pickling uniformity (salt distribution, etc) or electrode characteristics than with any fundamental physics principle.

I hope everyone checked out the ground breaking research done on this phenomena:
Characterization of Organic Illumination Systems
http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-TN-13.pdf

To me this effect is as much fun as grape-plasmas.
 
  • #31
NascentOxygen
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A clue to the operating principle of the pickle light emitter might be found by examining why its emission eventually ceases; the lighted region seems to jump about and emission starts and ends as suddenly as though a switch were thrown, and this is not consistent with the glow emitted due to a sizeable body being heated to incandescence. In fact, a concensus among writers is that the light is the sodium spectrum, and arises from the high salt content in the pickled cucumber.
 
  • #32
meBigGuy
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There is no doubt that the glow is not heat glow. Did you read the paper I linked to? It is proposed there is plasma arcing within the pickle.
I think it is plausible that the side of the pickle to actually burn first would be the same side that first glows. I've seen the small side glow, and the fat side glow. I have no theory founded in electronic or physics theory (such as the proposed idea that the energy flow in an AC circuit has a "direction" because of the difference between neutral and ground) that would support the professors claim that "the same side always burns first".
 
  • #33
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I agree with those who say it is the order of connection, or the resistance right at the point of connection. It cannot be anything else. The big problem is why the resistance would be lower at one connection point. It may be that something is distributing the connection at the grounded neutral or "cold" terminal causing a larger cross sectional area (and less resistance) for that termination, or something is concentrating current in a smaller cross sectional area at the "hot" terminal causing the same current through a smaller area (more I^2 R heating from more resistance around that termination). It cannot be anything else.

This reminds me of a clock. My 120V electric motor ship clock was subjected to a lightning surge that shorted near one end of the winding to the motor frame. This made it sensitive to stray capacitance or leakage to ground. If I installed the plug one way, it ran forward. If I installed the line plug the other way, it ran backwards. I had an unbelievable amount of fun with that clock, until I discovered the clock face was at 90 volts or 30 volts to ground, depending on insertion. The pickle, however, cannot be like the clock because impedance levels are far different. It cannot be sensitive to displacement currents via capacitance like the high impedance clock motor.
 

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