Why don't electrons leave a negatively charged metal in air?

[ZapperZ]

> How likely do you think it is that you can depend on an electron to tunnel out of a metallic surface without any applied potential? Calculate this yourself if you wish.

Without any applied potential? The one due to the extra electrons remaining in the metal, not good enough? Did we put any limits to how many these electrons were?

Again, another example of things being clouded for no apparent reason. Why do you need to put extra electrons on anything in your understanding of electrical discharge/breakdown? Whose model are you using that require you to put extra electrons on anything? ALL model of breakdown, be it in vacuum systems and in air, requires no such starting point. Having high field-emission regions, yes, but extra charges? Nope!

Until you show me a model that requires such extra electrons placed on the metal to account for such discharge/breakdown effects, I would consider this as nothing more than a confusing and unnecessary distraction.

Zz.

 

[Ulysees]

> You were asking about what a "work function" is, didn't you?

No I wasn't. You provided this idea, and I understood it immediately.

> I explained it.

No, you just insisted that it's right when no one had said it was wrong. Just lack of intuitiveness, remember?

> Have you figured out how the values of the work function are obtained?

It doesn't matter, we're talking about corona discharge in this topic.

> Do you think space-charge effects are included in this value?

No, did I say otherwise?

> If not, then why did you bring it up" in your understanding of what a work function is?

I wasn't talking about work function, I was talking about corona discharge.

> This has nothing to do with being "real". It has everything to do with narrowing down the the exact topic of discussion.

But you're not talking about the exact topic (corona discharge), you're talking about work function, only part of the topic.

So I will

 

[Defennder]

I think he was asking was if the repulsive electrostatic force from the other electrons on the charged surface would increase the probability that a particular electron on the charge surface would be able to tunnel out of the metallic lattice.

In other words, let us consider 2 cases, a uncharged metallic conductor and a exact same metallic conductor but with charges in it. Is the probability that an electron would tunnel out of the conducting surface higher in the 2nd case than in the first?

 

[ZapperZ]

I give up.

Zz.

 

[Ulysees]

> Why do you need to put extra electrons on anything in your understanding of electrical discharge/breakdown?

Because the topic is about corona discharge.

 

[ZapperZ]

I think he was asking was if the repulsive electrostatic force from the other electrons on the charged surface would increase the probability that a particular electron on the charge surface would be able to tunnel out of the metallic lattice.

In other words, let us consider 2 cases, a uncharged metallic conductor and a exact same metallic conductor but with charges in it. Is the probability that an electron would tunnel out of the conducting surface higher in the 2nd case than in the first?

But this is not the case that resulted in his "corona discharge". No model of such effects have been included in any of the models that I've looked at. And considering that I do extensive research work on this topic, I'd say that I've seen almost every single model of breakdown effects that is available in the literature.

That is why I am puzzled why this is even brought up, IF the whole point here is in trying to understand this phenomenon. I've outlined basically the most accepted mechanism that we know of. If people won't want to accept my word on it, then they can do their own leg work. I suggest starting with:

F.R. Schwirzke, "Vacuum breakdown on metal surfaces", IEEE Transaction on Plasma. Science v.19, p.690 (1991).

I thnk I'm done going around in circles with this thread.

Zz.

 

[Defennder]

Well I think this thread is quite interesting and informative, but Ulysees needs to be better able to rephrase and articulate his questions in a more formal manner if he wants to elicit proper replies from the experts here. There are genuine questions which are raised here, but it's a shame that the way they are presented is rather incoherent and confusing.

It happens a lot of time to myself when I'm asking my prof questions. Sometimes the conceptual questions leap to my mind and then to my mouth faster than I can think properly of how to articulate and present them in a more coherent manner. So what I suggest is that Ulysees sits back a little, think of a better way to present his questions in a lengthy coherent post rather than rapid shooting of 1-line posts which are cluttered and difficult to follow.

 

[Ulysees]

You're just stuck, the question is what I asked in the first post.

You have imagined something and we all have to accept that this is what was asked in this topic. Here's what was asked:

> I'm familiar with lightning rods taking advantage of the mutual repulsion of charges to shoot off a corona discharge off the sharp end and start a thunder, but why doesn't corona discharge happen to all charged metals? What makes air such a good insulator, when it's just gases, relatively few molecules moving all over the place bouncing on each other, how can this be a good insulator?

 

[Ulysees]

In fact what you have imagined even ignores the title of the topic! It says "negatively charged"! Mercy!

 

[ZapperZ]

You're just stuck, the question is what I asked in the first post.

You have imagined something and we all have to accept that this is what was asked in this topic. Here's what was asked:

> I'm familiar with lightning rods taking advantage of the mutual repulsion of charges to shoot off a corona discharge off the sharp end and start a thunder, but why doesn't corona discharge happen to all charged metals? What makes air such a good insulator, when it's just gases, relatively few molecules moving all over the place bouncing on each other, how can this be a good insulator?

1. the "corona discharge" has nothing to do with "mutual repulsion of charges". Again, this has been answered already when I presented to you the scenario of breakdown mechanism. Look at the list I gave. Where does it say "mutual repulsion"?

2. Where is the mechanism that involves putting in extra charges onto the metal? The mechanism that I outlined as no such extra charges being added. It would work even if the metal is isolated from ground, meaning it does not have to have any extra charges.

3. ALL metals can cause a discharge if I have a sharp-enough tip. If there are metals that don't cause a breakdown, we would have used it already in accelerating structures and achive a gazillion volts/meter of accelerating gradient. So asking why it doesn't happen in all "charged metals" is confusing. You're asking for an answer to a scenario that doesn't occur. What kind of an answer were you expecting?

4. The implication of "charge metals" as the requirement for a "corona discharge" is what I've been asking for you to produce. Show me a model in which a "charged metal" is required for such a discharge. This is the 3rd time I've mentioned this already and asked you to produce such evidence or model. Show me a model in which a corona discharge is caused by the addition of addtional charges to the metal. In the model that I had listed, no such addition is necessary. All that was required was regions of high field enhancement. No addition of extra charges at all! Until you can show me such a model for me to study, this "charge metal" scenario doesn't exist.

Zz.

 

[ZapperZ]

In fact what you have imagined even ignores the title of the topic! It says "negatively charged"! Mercy!

People have used misleading topics all the time! I tend to read the content, not "sound bites" that people put in the topic. And yes, I have read your first post very carefully, which is why I found it VERY confusing and didn't respond to it till much later, which I am now regretting.

Not that if all I read were the topic (which had nothing to do with "corona discharge"), then simply telling you "work function" would have been sufficient to answer the topic. I would not have to list the breakdown mechanism at all!

Zz.

 

[ZapperZ]

Addendum: and did people also missed this paper from last year?

http://focus.aps.org/story/v19/st4

Compare it to the model that I listed.

Zz.

 

[Defennder]

Okay here are the questions what I can discern from the original post. Most of them have already been answered, maybe even all, but the thread is a little difficult to follow:

1. The OP's supposition that corona discharge depends on mutual Coulumbic repulsion of electrons in the negatively charged metal in air.

The reply was that corona discharge has nothing to due with mutual charge repulsion.

2. The OP's suggestion that corona discharge doesn't occur in all metals, or why it happens to a differing extent based on the type of metal.

The first one was answered, that it does occur in all metals. I'm not sure if the OP meant to ask the 2nd part of the question.

3. What makes air such a good insulator that corona discharge does not occur readily and all the time.

This one has probably been answered in the first few pages, but there were some inter-discussions which distracted me from the answer.

That's all I can figure at the moment here. It's a half hour past 1 am where I live, so I might have been confused enough about what I wrote, but still...

 

[peter0302]

I'm slightly confused about the claim that vacuum would be insulator. After all, a free electron is pretty free if it is in vacuum, so vacuum is not really insulator in the sense that electrons could not move in it. Perhaps this sentence simply means that vacuum behaves as an insulator, because it is difficult for electrons to leave the metal into the vacuum due to the mirror charge effect?

But then they say that the vacuum is better insulator than air! Is this claim based on experimental fact? If the claim is true, and on the other hand vacuum is not really insulator, doesn't it mean that the air is not insulator at all, but instead the air actually only helps current conduction outside solids?

You don't seem to know what conduction and insulation mean. Here's a hint:

V=IR

What do you think the value of R is for a vacuum vs. air vs. rubber vs. copper? Hint, think high to low.

I give up.

Zz.

Same here. Every time a question is answered it's rephrased in a way that has nothing to do with the previous question.

jostpour, your personal attacks are unnecessary and unfounded. Just because I'm giving a classical explanation to a question that _has_ a classical explanation (i.e. conduction v. insulation) does not make it wrong, and inserting QM concepts into a problem just to make yourself sound more informed does not make it so.

 

[ZapperZ]

Okay here are the questions what I can discern from the original post. Most of them have already been answered, maybe even all, but the thread is a little difficult to follow:

1. The OP's supposition that corona discharge depends on mutual Coulumbic repulsion of electrons in the negatively charged metal in air.

The reply was that corona discharge has nothing to due with mutual charge repulsion.

2. The OP's suggestion that corona discharge doesn't occur in all metals, or why it happens to a differing extent based on the type of metal.

The first one was answered, that it does occur in all metals. I'm not sure if the OP meant to ask the 2nd part of the question.

3. What makes air such a good insulator that corona discharge does not occur readily and all the time.

This one has probably been answered in the first few pages, but there were some inter-discussions which distracted me from the answer.

That's all I can figure at the moment here. It's a half hour past 1 am where I live, so I might have been confused enough about what I wrote, but still...

I'm surprised you didn't go blind after figuring all of that out! :)

The issue on "air such a good insulator" really has nothing to do with the discharge process. This is because air (or any neutrals) is really the "innocent bystander". The electrons emitted from the field-emitter are the ones that's causing the damage by ionizing these neutrals. The neutrals can't help but be ionized when the conditions are right. This is the plasma discharge that we observe, and what was observed in the link that I just gave.

Thanks for highlighting what have been answered already.

Zz.

 

[Ulysees]

> 1. the "corona discharge" has nothing to do with "mutual repulsion of charges". Again, this has been answered already when I presented to you the scenario of breakdown mechanism. Look at the list I gave. Where does it say "mutual repulsion"?

nothing? So the tunneling is not caused by other electrons being present in the negatively charged metal and pushing it out? If you say so...

2. Where is the mechanism that involves putting in extra charges onto the metal?

Should I provide a big battery or something otherwise the title confuses you?

Your mechanism was an explanation of work function, which is only a tiny part of this topic: corona discharge.

> asking why it doesn't happen in all "charged metals" is confusing. You're asking for an answer to a scenario that doesn't occur. What kind of an answer were you expecting?

This scenario does occur in ALL charged metals. It just takes a large enough voltage. It also happens for all shapes.

So the question was why not at all voltages. I'm not asking it any more.

> 4. The implication of "charge metals" as the requirement for a "corona discharge" is what I've been asking for you to produce. Show me a model in which a "charged metal" is required for such a discharge.

In other words, you want proof that a metal without charge, will never produce a corona. Actually I think if it's hot enough, vibrations will cause some electrons to exceed the work function, ie go far enough to escape. So we can talk about uncharged metals too if you want.

> Show me a model in which a corona discharge is caused by the addition of addtional charges to the metal.

A Tesla coil connected to a sphere. Here it is:

As the ac current goes up in the increasing part of the sinewave, the concentrations of electrons in the spheroid go up too. When they're high enough, electrons jump off more, and the process of corona discharge begins.

> Until you can show me such a model for me to study, this "charge metal" scenario doesn't exist.

Alright now?

 

[Ulysees]

>> In fact what you have imagined even ignores the title of the topic! It says "negatively charged"! Mercy!

> People have used misleading topics all the time!

Can't you just say sorry?

 

[Defennder]

Ulysees this is getting tiresome. I'm surprised that Zapper still bothered to reply to your posts after your repeated dismissive and ill-informed 1-line posts scattered throughout the entire thread. You're here to learn by asking questions, not confound a PF mentor by going around in circles parading the same questions in different forms which have already been answered. The dripping sarcasm inherent in your reply above doesn't help either.

 

[Ulysees]

> The reply was that corona discharge has nothing to due with mutual charge repulsion.

That's actually wrong, corona discharge has everything to do with mutual charge repulsion! That's why it occurs at pins more easily.

 

[Ulysees]

Ulysees this is getting tiresome. I'm surprised that Zapper still bothered to reply to your posts after your repeated dismissive and ill-informed 1-line posts scattered throughout the entire thread. You're here to learn by asking questions, not confound a PF mentor by going around in circles parading the same questions in different forms which have already been answered. The dripping sarcasm inherent in your reply above doesn't help either.

Defender, don't assume I have read everything when a post appears, I'm replying to your posts one by one. Will get to yours in a bit.

 

[Defennder]

The reason why it occurs for pins, for eg. more easily is because pins have a small radius of curvature and the potential induced in air is inversely proportional to the radius. What has this got to do with mutual charge repulsion?

 

[jostpuur]

In post #22 peter0302 explains that in an insulator electrons are bound to individual molecules.

In a metal the electrons roam freely throughout the metal. Current begins to flow because electrons are being "stolen" from one end of the conductor and so electrons from the other end move toward the newly created charge imbalance. This doesn't happen easily in insulators because in those substances the electrons are much more tightly bound to the individual moleucles and therefore it requires a much higher potential to liberate them. You don't need quantum mechanics to understand this.

In post #39 he explains that because valence bands are not full, electrons are allowed to move from atom to atom.

The reaosn metals are good conductors is because the valence electrons generally roam freely from atom to atom, because the valence shells are usually NOT full, and so when there is a charge imbalance, to which the electrons are attracted, the resistance is low. Insulators have some freedom of electron movement as well, of course - but much less. Hence - resistence.

Again, you really don't need QM to understand this!

and also that the reason why electrons cannot get onto air molecules is the same reason why electrons don't move in solid insulators. (although they actually move in solid insulator pretty much)

The question was why don't electrons jump from a positively charged metal to an air molecule. The reason is the same as why electrons don't jump from a positively charged metal to a rubber insulator. The valence shells of the insulators are generally full, and so a higher potential would be required to put an electron in the next higher shell than is available.

In posts #43 and #74 he interprets the critique of his posts as personal attacks against him

Looks like cargo cult science.

What does that mean? I'm a liar?

I'll try to keep this attack as an attack against your explanation, and not against you.

jostpour, your personal attacks are unnecessary and unfounded.

and attempts to keep the authoritative position

I give up.

Zz.

Same here.

peter0302, I am not an aggressive person normally, but the incorrectness of your explanations and your style of appearing as a scientifical authority draws my attention too much.

 

[Ulysees]

The reason why it occurs for pins, for eg. more easily is because pins have a small radius of curvature and the potential induced in air is inversely proportional to the radius. What has this got to do with mutual charge repulsion?

Extra charge goes to the surface because of mutual repulsion. Of all the parts of an arbitrary surface, which ones are further from the "middle"? The bulges and the pins, and that's where extra charge goes more. But when there's too much in a bulge, it stops more electrons from coming. It's just equilibrium.

 

[Ulysees]

Defender:
> 3. What makes air such a good insulator that corona discharge does not occur readily and all the time.
> This one has probably been answered in the first few pages

Actually this one has not been answered fully. Nobody said how far apart air molecules have to be in order for electrons to stick to them at a large enough scale for corona discharge, a factor that seems to me to be a quantum issue, that's why it was posted here. I was looking for detailed explanations in terms of forces and probability density functions (or wavefunctions of the electron).

If anyone can provide such a description in simple terms, it will be greatly appreciated.

 

[peter0302]

peter0302, I am not an aggressive person normally, but the incorrectness of your explanations and your style of appearing as a scientifical authority draws my attention too much.

Except you still haven't told anyone what was incorrect. You just don't like my "hueristic" approach. And yes your posts are personal despite your purported attempts not to be.

peter0302, I am not an aggressive person normally, but the incorrectness of your explanations and your style of appearing as a scientifical authority draws my attention too much.

Right.

 

[peter0302]

You might want to take a look at this also:

http://en.wikipedia.org/wiki/Insulator_(Electrical)
http://en.wikipedia.org/wiki/Metallic_bond

Maybe you should edit it!

[Edit]
I'll make one correction to the above, which is that the moving electrons in a conductor are not necessarily bound to _specific_ atoms but they are nonetheless bound to the lattice. They are not "free" electrons liberated by a work function which was my orignal point.

 

[Ulysees]

You're here to learn by asking questions, not confound a PF mentor by going around in circles parading the same questions in different forms which have already been answered.

When a mentor distorts your words in such an obvious way (maybe because he forgot about Tesla coils), what are you supposed to do?

If you hear something that you know to be untrue (eg that mutual repulsion has nothing to do with pins), are you supposed to pretend it's true because you're the OP who's asking?

 

[jostpuur]

ZapperZ, I want your opinion on this reasoning:


The resistance of vacuum is zero, because if a particle has some momentum and is traveling in vacuum, it keeps the same momentum. Right?


If you have two separate metal objects, and vacuum in between, and try to get current carried out from one object to another, you will measure very high resistance. However, this does not mean that the resistance of the vacuum would be great, but that the resistance of the

metal + boundary of metal and vacuum + vacuum + boundary of metal and vacuum + metal

is great, and the resistance arises in the boundaries due to the mirror charge effect. Right?


It is an experimental fact, that vacuum is better insulator than air. That means, that the current will break through more easily, if there is air in between? Right?


All this together implies, that the air alone is not an insulator at all. Right?


If the question is "What makes air such a good insulator, when it's just gases, relatively few molecules moving all over the place bouncing on each other, how can this be a good insulator?", the answer is, that actually the air is not an insulator?

 

[jostpuur]

Or hmhmhm... argh. Was this in contradiction with the vacuum tubes now? I'm not sure...

 

[ZapperZ]

Defender:
> 3. What makes air such a good insulator that corona discharge does not occur readily and all the time.
> This one has probably been answered in the first few pages

Actually this one has not been answered fully. Nobody said how far apart air molecules have to be in order for electrons to stick to them at a large enough scale for corona discharge, a factor that seems to me to be a quantum issue, that's why it was posted here. I was looking for detailed explanations in terms of forces and probability density functions (or wavefunctions of the electron).

If anyone can provide such a description in simple terms, it will be greatly appreciated.

You never asked that. In fact, I've given a references to that Schwirzke paper that describes this in detail!

"This neutral density produced by just one monolayer is almost
atmospheric density. The electron mean-free-path length for
ionizing neutrals lambda = l/(n*sigma) depends on the ionization
cross section sigma, which in turn is a function of the electron
energy. For many gases the ionization cross section has a
broad maximum value of about sigma ~ 10^-16 cm^2 for electrons,
with an energy between 50 eV to 150 eV. In an electric field of
10^6 V/cm, a field-emitted electron has gained 100 eV at the
distance of 10^-4 cm (Fig. 1). Thus lambda ~ 5 x 10^-5 cm and about
20% of the emitted electrons have a chance for an ionizing
collision within d~ 10^-4 cm."

There's no "quantum issue" with such a thing at all other than the ionization potential of each of the atoms. Everything above is treated classicallly.

Zz.