Capacitor+Grounded Plate+AC Current?

[pickle123]

The title describes my guesses as to what is happening in this problem, but I don't really understand it.

Moderators, please do not delete this... I would give a non-dangerous example if I understood the problem well enough to actually know what the physics is.

In class today, my physics professor, for a demo (please do not try this at home: it is dangerous!) plugged a pickle into an electrical outlet. It is a demo he does every year in his E&M class. The pickle starts on fire. Again, I did not do this, and will not do this, and you should not either. But it is what the professionally done demo was.

What I do not understand is the following: he said that the end connected to the "hot" terminal of the outlet is the side on which the fire always starts. I do not understand why that should be, given that this is an alternating current source. Both sides are effectively exactly the same thing. My thought, then, is that this must have something to do with the fact that neutral is held at approximately ground potential, but I can't quite figure out how this works.

Thanks!

 

[meBigGuy]

I can't see why it always starts on that end. Was the pickle wired to the plug and he inserted the plug, or did he plug the hot side into the grounded pickle? Maybe his connection methodology is such that it favors one side (different probes). Maybe he always connects the ground to the fat side.

 

[anorlunda]

I just sent a private conversation with pickle123 with my answer to his question. I don't want to post it publicly.

pickle123 seems to think that a thread like this is a conversation between him, the monitors, and the experts who might post answers. It is not. It is part of the public record. Even children can find it and read it. In fact, immature young people could even google the phrase " please do not try this at home: it is dangerous!" seeking fun things to do. That is why we don't like subjects like this on public forums. Use closed forums, or private conversations with the mentors or other PF experts to discuss dangerous topics.

 

[DrClaude]

I just sent a private conversation with pickle123 with my answer to his question. I don't want to post it publicly.

pickle123 seems to think that a thread like this is a conversation between him, the monitors, and the experts who might post answers. It is not. It is part of the public record. Even children can find it and read it. In fact, immature young people could even google the phrase " please do not try this at home: it is dangerous!" seeking fun things to do. That is why we don't like subjects like this on public forums. Use closed forums, or private conversations with the mentors or other PF experts to discuss dangerous topics.

I'm sorry, but this is ridiculous. There is nothing inherently dangerous in discussing the phenomenon.

 

[pickle123]

I can't see why it always starts on that end. Was the pickle wired to the plug and he inserted the plug, or did he plug the hot side into the grounded pickle? Maybe his connection methodology is such that it favors one side (different probes). Maybe he always connects the ground to the fat side.

Unfortunately, I am quite sure that it is nothing like this. This professor loves things with simple physical explanations, and this is a question that he has stated that he likes to ask physics graduate students during their oral exam. I asked one of his graduate students about it, and they seemed quite convinced that you could understand this problem by looking at it as a huge resistor in parallel with a capacitor, where one end is grounded. He suggested that this is also why people get zapped by outlets and feel pain at the point of contact, not through the entire path to ground (i.e. there is not much of any actual current, just charging, most of which occurs on the "hot" side--this is what I don't understand).

 

[NascentOxygen]

While being cooked, was the pickle mounted so that its longitudinal axis was vertical or horizontal?

 

[mfb]

How and where did he connect the electrodes?

You feel pain at the point of contact because the current density is higher due to the small area where current can flow. If one of the electrodes has a better contact, then heating there will be lower than at the other side.

 

[pickle123]

He sets the pickle down horizontally on the table, then plugs the electrodes directly into each end (effectively everything is co-linear)

 

[mfb]

Then it should not depend on the connection of the electrodes, but only on their depth in the material and their precise position.

 

[Salvador]

first of all reading this otherwise interesting thread I just want to say that we should really stop this paranoia of saying something that someone might sometime in someplace find worthy of repeating and killing himself in the process, people have done stupid things all the time and also died from them ,
much like Sweden's IKEA store has now banned kitchen knifes in it's stores simply because a muslim used one to stab a woman, the world we now live in is absurd.as for the pickle, why would the pickle be modeled as a capacitor with a resistor in series? the electrodes are conducting and so is the juice in a pickle , or am i getting something wrong here ?

 

[NascentOxygen]

He sets the pickle down horizontally on the table, then plugs the electrodes directly into each end (effectively everything is co-linear)

So why does your subject line mention "grounded plate"?

 

[Salvador]

I find that weird too, the only explanation that comes in mind is that there is a " hot" electrode in the pickle and then the other electrode is attached to a series capacitor which is then connected to the neutral of the mains.

 

[meBigGuy]

The reason you feel pain at the point of shock is because it gets hot due to skin resistance at a single point. If you grabbed with both hands at the same time and exactly symmetrically there is no difference.

The key is that he connects the electrodes individually. Which does he connect first, which last. I would predict that the last connected would be where the burning starts since it would be, while inserting the electrode, higher resistance.

The statement that implied that the problem was not symmetrical because of one side being ground is wholly without substance. A ground node is no different than a power node to the load. It is a linear system.

If identical pickle electrodes were securely wired to a mains plug, and then the professor randomly inserted the plug one way or the other, then I expect you would see random ends ignite.

I also agree with Salvador that the suggested capacitance model is not valid, especially at 50/60Hz. Besides that, I can't see why it would be non-symmetrical.

 

[meBigGuy]

BTW, I found this.
Characterization of Organic Illumination Systems
http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-TN-13.pdf

There doesn't seem to be any explanation why it lights on the one side, but that may be where it flames.
I expect it is the fat end.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/pickle.html
But this one isn't the fat end


So, who knows?

 

[NascentOxygen]

In one of those demonstrations the glow started off at one end, then after a bit the glow shifted to the other end.

I'm surprised by the glow, I didn't expect it to glow near-white. I anticipated the glow would emerge slowly, spreading as a dull red attributable to heating.

 

[mfb]

The glow is not a (purely) thermal effect.

 

[PeterDonis]

he said that the end connected to the "hot" terminal of the outlet is the side on which the fire always starts. I do not understand why that should be, given that this is an alternating current source.

It's an AC source, so the direction of current flow changes, but that does not necessarily mean the direction of energy flow changes. For a purely resistive load, voltage and current switch polarity at exactly the same time, and power is the product of voltage and current, so its sign never changes; in this idealized case, energy always flows from "hot" to load, never from ground to load, so the "hot" side of the load will heat up first. The pickle is probably not a purely resistive load, but if it is fairly close to being so, the direction of energy flow should be fairly close to being constant in the direction from "hot" to load, rather than from ground to load.

 

[meBigGuy]

I have a real problem with that (I'm not saying you are wrong, but that I have a problem with that). The energy dissipation in a resistive element will be distributed evenly based on the ohmic resistance. The electrons move at drift velocity bumping into things as they go. They start moving at all points in the element when the potential is applied.

I do not see how the arbitrary assignment of hot and neutral can affect the result. For example, drive the load from a balanced isolation transformer. The load will see no difference in the driving potential if you arbitrarily ground one or the other of the isolated lines.

 

[PeterDonis]

The energy dissipation in a resistive element will be distributed evenly based on the ohmic resistance.

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.

The electrons move at drift velocity bumping into things as they go.

The motion of electrons is not the same as the flow of energy. Energy is contained in the EM field as well as the electron motion, and it is dissipated in the resistor by thermal vibrations of the atoms, not just drift motion of electrons.

They start moving at all points in the element when the potential is applied.

No, they don't. The potential is not applied instantaneously everywhere; that would violate causality.

 

[Salvador]

but practically speaking, once you connect the pickle , resistor to the load the electrons start moving from one side to the other with whatever the speed of the EM field (speed of light) is through that medium , and if I'm correct it's about 2/3 to that of c in vacuum , so even though as Peter said the electrons don't start moving everywhere at once , for simple questions like does a resistor heats up evenly regardless of it's connection or which end of a pickle lights up first , this shouldn't matter I think because the difference would be unnoticable.

 

[sophiecentaur]

the electrons start moving from one side to the other with whatever the speed of the EM field (speed of light) is through that medium

That is just plain wrong. The speed of the charge carriers is very slow and nothing like c, in a solid. What can happen in a vacuum is quite different because there is nothing to slow them down.
Just check out this link.

 

[meBigGuy]

I think you are reading it wrong. I read it to say that the POTENTIAL, moving at the speed of light, causes electrons all along the pickle to start to move at their drift velocity. The wave of initial movements moves at C in that medium. The "power dissipation front" (my term) moves at C.

The words "start to move at C" can also mean that the starting moved at C.

Regardless of whether that is what Salvador meant, that is the way it works. The beginning of dissipation moves at C in that medium which is pretty quick relative to pickles heating. (I previously said instantaneously - my bad)

Can we focus on the below unless there are substantial disagreements with how fast the beginning of the power dissipation effects propagate through the pickle (at C in the medium).
No one has commented on my example of an isolation transformer with 1 side/other side/no side grounded and whether the pickle could tell the difference regarding which terminal is grounded.

I maintain it could not. In fact you could connect one side to 1 million volts and it could not tell.

 

[NascentOxygen]

I'd need to see the setup and know how many times the professor has repeated the demonstration with always the same results before putting any store in the claim. Of course, were there significant capacitance between the pickle and the electricity ground then the hot electrode would deliver more current than does the neutral wire, but such is not going to be the case here.

I would hope the professor sets a good example for electrical safety by always using an isolation transformer, whether he chooses to announce it or not. And it goes without saying that he has ensured ready access to a fire blanket...responsibly following all the normal safety measures and precautions.

 

[sophiecentaur]

The "power dissipation front" (my term) moves at C.

That's a fair description. The nearest 'offoicial term' I can think of to that would be the Poynting vector, which describes the energy flow in the wave. That would advance with the wave front at switch on.
As you say, an isolation transformer is de riguer in any experiments like that. I remember my very flash Physics master giving us a demo of boiling water in a beaker with two nails, connected directly to the Mains socket on the bench. In those days they didn't have RCDs either. (He did tell us not to try it at home but it was only good luck on my part that I didn't.)

 

[sophiecentaur]

I'd need to see the setup and know how many times the professor has repeated the demonstration with always the same results before putting any store in the claim. Of course, were there significant capacitance between the pickle and the electricity ground then the hot electrode would deliver more current than does the neutral wire, but such is not going to be the case here.

I would hope the professor sets a good example for electrical safety by always using an isolation transformer, whether he chooses to announce it or not. And it goes without saying that he has ensured ready access to a fire blanket...responsibly following all the normal safety measures and precautions.

If the professor is a man of habit, he may be holding the pickles the same way round every time and connecting one particular lead to the stalk end of the pickles. That could have an effect which would need to be eliminated by doing it both ways round an equal number of times.

 

[mfb]

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.

 

[anorlunda]

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.

 

[sophiecentaur]

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.

 

[Salvador]

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.

 

[meBigGuy]

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.

 

[NascentOxygen]

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 consensus among writers is that the light is the sodium spectrum, and arises from the high salt content in the pickled cucumber.

 

[meBigGuy]

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".

 

[Tom Rauji]

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.