Meaning of "Static Electricity" and Physical Interpretation

[Enjolras1789]

TL;DR Summary

I am confused on what "static electricity" means. It seems more related to electric potential (voltage). I struggle to understand what is occurring in classic examples of "static electricity" given that actual electron charge transfer is very high-energy.

If a balloon and a sweater are rubbed together, high-school science teachers like to say "the electrons transferred to the balloon in the form of static electricity." Then, it is often charming to show that two such balloons repel one another "because they have more electrons." Can we unpack what is actually going on here? Obviously this description is incorrect.

Actual charge-transfer requires a lot of energy, especially for something like rubber. We know that an entire charge of e cannot be moving from one object to the other.

What interactions specifically occurred between the balloon and sweater? I find it hard to answer that any charge density meaningfully occurred between the objects (these are not metals). I imagine that the essential property is that an electric polarizability of the rubber allowed for a partial negative charge density fluctuation along the surface? But if so, did we generate a partial positive charge density along the sweater? If this is correct, the polarizations taking place must be REALLY small, given the enormous amount of energy needed relative to electron transfer. Is thinking of these in terms of the D displacement vector (think like chapter 4 in Griffith's Electrodynamics book) correct?

I am assuming that the dry-ness of all this merely tells us that water's presence would make the electrical polarization lesser, because excess charge density would distribute itself into the water?

Is "static electricity" akin to something like "an electric potential difference in two objects which manifests by repulsive forces?"

 

[Merlin3189]

... given that actual electron charge transfer is very high-energy.

Then, it is often charming to show that two such balloons repel one another "because they have more electrons." ... Obviously this description is incorrect.Actual charge-transfer requires a lot of energy, .... We know that an entire charge of e cannot be moving from one object to the other.

I don't understand why you say this.
An electron-volt is 1 e-18 J (ish) I don't know what the voltage on a rubbed balloon is, so I'll guess around 33 kV. So transferring an electron requires 5 e-15 J.
To get 0.1 g force off attraction (1 mN, but milliNewtons are not in my experience) needs about 33 nCb
Which means about 2 e11 electrons and I end up with about 5 e -3 J
That does sound on the high side for rubbing a balloon for a few seconds, but not enough to say "cannot be moving from one object to another"
(Do please check my very rough and ready arithmetic - last night I spent an hour solving 3 simultaneous equations for a simple circuit analysis question and getting several impossible answers before giving up and using Excel! So I know my brain is losing it!)

But getting back to the point, I'm not sure why you think shifting electrons around requires much energy? They're titchy little things, hardly any mass nor charge. The only reason they can do anything much, is that they go round in gangs - really big gangs. If a few drop out or switch sides, it's surprising the rest even notice.

 

[Enjolras1789]

I am confused by your reply.
"But getting back to the point, I'm not sure why you think shifting electrons around requires much energy? They're titchy little things, hardly any mass nor charge."

The point of metals vs. insulators is that electrons only flow in a net directionality under very specific conditions, when a conduction band is possible. The ionization energy of n-hexane is ~10eV (and the material in a sweater likely make that a much bigger number). Ten eV is a HUGE number. The accessible energy thermally is ~0.5eV.

If what you say is true, wouldn't you predict everything in the world is a metal?

 

[Vanadium 50]

10 eV is not a lot of energy.

You might estimate the number of electrons needed to zap yourself on a doorknob.

 

[tech99]

Static electricity arises when different materials are placed in contact and then separated, resulting in tribo- electricity. Rubbing the materials increases the effect by increasing contact. There is a Wiki article mentioning the various theories as to how electrons become transferred, but it does not seem to be a high energy effect (as stated by Merlin3189). For a dielectric I suggest it is the same as an ordinary capacitor , where we are distorting the electron positions slightly, rather than removing them from the atom.

 

[Drakkith]

If a balloon and a sweater are rubbed together, high-school science teachers like to say "the electrons transferred to the balloon in the form of static electricity." Then, it is often charming to show that two such balloons repel one another "because they have more electrons." Can we unpack what is actually going on here? Obviously this description is incorrect.

It's not incorrect. That's exactly what happens.

Actual charge-transfer requires a lot of energy, especially for something like rubber. We know that an entire charge of e cannot be moving from one object to the other.

Assuming e is the charge of a single electron, then it absolutely can be moved from one object to another. Charge transfer requires a miniscule amount of energy unless you're transferring huge amounts of charge. In fact, charge transfer can actually release energy in many cases. I wouldn't be surprised if the charge transfer between, say, rubber and wool, actually releases energy instead of consuming it.

Consider, for a moment, the differing electronegativity values for the elements. That is, the tendency for neutral atoms to attract electrons in a bond more strongly than atoms of another element do. Fluorine has one of the highest electronegativity values of any element. Despite being neutral, fluorine is just dying to grab onto some electrons when any are nearby. So much so that it will react with almost anything it comes into contact with, releasing more energy per bond than just about any other chemical bond.

But we don't have to stop at single atoms. Molecules and even the outside layer of atoms on bulk substances have their own affinity for electrons. Two different materials coming into contact often results in electrons being transferred from one material to the other because of this different affinity.

 

[Enjolras1789]

10 eV is not a lot of energy.

You might estimate then number of electrons needed to zap yourself on a doorknob.

kT at STP is about 1/40 eV. Ten electron volts is a TON of energy. Do you think everything is a metal?

 

[Enjolras1789]

Static electricity arises when different materials are placed in contact and then separated, resulting in tribo- electricity. Rubbing the materials increases the effect by increasing contact. There is a Wiki article mentioning the various theories as to how electrons become transferred, but it does not seem to be a high energy effect (as stated by Merlin3189). For a dielectric I suggest it is the same as an ordinary capacitor , where we are distorting the electron positions slightly, rather than removing them from the atom.

OK, this is more the kind of analysis I am used to in solid-state physics or electrodynamics.

Your comment about "where we are distorting the electron positions slightly, rather than removing them from the atom" makes perfect sense to me. I thought to regard this as a small D (using Griffiths notation) vector, but I think calling it a charge polarization works also. Should we therefore regard this as an electric potential difference created?

The rubbing of the materials seems the most difficult part to explain, physically. Empirically, the dielectric is higher compared to just air if some charge polarization occurs. Why is the motion helpful, I wonder?

I speculate that the motion is necessary because the polarization "relaxes" on a timescale slower than the motion if its static? But that's still an incomplete picture.

 

[berkeman]

Ten electron volts is a TON of energy.

WITW? Please stop waving your hands and show us some references that say that. Moving 10 electrons through a potential difference of 1V does not take a lot of energy. Lordy.

Edit/Add -- you mentioned in a thread many years ago that your background is Solid State Physics. Is that true?

I am a solid state physicist.

 

[Drakkith]

The rubbing of the materials seems the most difficult part to explain, physically. Empirically, the dielectric is higher compared to just air if some charge polarization occurs. Why is the motion helpful, I wonder?

I don't know about a dielectric, but my understanding for why rubbing is good for charge transfer:

Rubbing mainly just causes more contact between atoms, allowing more charge transfer to take place. Remember that surfaces are virtually never perfectly smooth at the atomic scale. Rubbing squishes surfaces together, helping to get rid of surface irregularities, brings new atoms into contact with each other so that you can have more chances to transfer charge and prevent any local charge saturation that might cause charge transfer to stop, and can scrape off any oxide or other thin layers that might inhibit charge transfer.

The point of metals vs. insulators is that electrons only flow in a net directionality under very specific conditions, when a conduction band is possible.

At room temperature even an insulator will have some number of charges in its conduction band, so current flow is entirely possible for an insulator. It simply takes larger voltages for significant current flow. Apply 100 volts to a material with 100 Mega Ohms of resistance and you'll get a microamp of current, which is still around 10¹² electrons per second, and we only need fractions of a coulomb's worth of charge to see all of the effects of static electricity, perhaps a microcoulomb's worth.

 

[Enjolras1789]

WITW? Please stop waving your hands and show us some references that say that. Moving 10 electrons through a potential difference of 1V does not take a lot of energy. Lordy.

Edit/Add -- you mentioned in a thread many years ago that your background is Solid State Physics. Is that true?

I am going to ignore your very rude remarks questioning me and claiming that I am "waiving my hands...". No one here has cited any references, so do not get uppity. Additionally, "Lordy" is not exactly adding anything constructive to the conversation.

kT at room temperature is roughly 1/40 of an eV. 1/40 eV << 10 eV.
We are talking about a phenomenon which is occurring at room temperature. It does not strike me that the rubbing motion itself is generating electrons in particular, since rubbing most things in life certainly does not give this result. If you want to say the electrons are completely leaving, the explanation has to work for this instance with these materials, and not give an erroneous prediction in general (i.e., everything in the world is a metal and electric currents are flowing everywhere at all times).

 

[berkeman]

I am going to ignore your very rude remarks questioning me and claiming that I am "waiving my hands...". No one here has cited any references, so do not get uppity. Additionally, "Lordy" is not exactly adding anything constructive to the conversation.

kT at room temperature is roughly 1/40 of an eV. 1/40 eV << 10 eV.
We are talking about a phenomenon which is occurring at room temperature. It does not strike me that the rubbing motion itself is generating electrons in particular, since rubbing most things in life certainly does not give this result. If you want to say the electrons are completely leaving, the explanation has to work for this instance with these materials, and not give an erroneous prediction in general (i.e., everything in the world is a metal and electric currents are flowing everywhere at all times).

You did not address my request that you post references showing that that 10eV is a TON of energy. And yes, you continue to wave your hands. As a Solid State Physicist, you should have plenty of references that you can post.

Last chance. Post the links and show us quantitatively why 10eV is a large amount of energy, or your thread will be closed. Think for a moment how long it will take a single AA battery with 1.5V output voltage to be discharged with a current of 10 electrons per second...

 

[Merlin3189]

The point of metals vs. insulators is that electrons only flow in a net directionality under very specific conditions, when a conduction band is possible. The ionization energy of n-hexane is ~10eV (and the material in a sweater likely make that a much bigger number). Ten eV is a HUGE number. The accessible energy thermally is ~0.5eV.

If what you say is true, wouldn't you predict everything in the world is a metal?

Yes, I had not thought of the work involved in removing the electron from its immediate environment, even for metals. Now I can only rely on my betters to describe the triboelectric effect. Perhaps the few atoms/mols touching are close enough that electrons can, though losing energy on leaving their parent atom, regain a similar amount (or more?) on adopting the new parent, meaning that not much net energy is required. If they can get me over that tricky barrier, pulling it away against a static field seems easy.
My suggestion for the rubbing, if it's not complete misdirection, is that it allows multiple different points of close contact. And I suppose the materials need to be insulators, to stop the excess charge always moving to the points of contact and repelling further transfer.

All materials aren't metals because electrons can't move through the bulk of some materials. Triboelectric transfer seems to be very much a surface effect, between different materials. Surface atoms seem to behave somewhat differently from those inside the bulk of the material.

My comment was really a reaction to your "enormous energy", which is not what I see macroscopically, but I now understand your perspective is different as a SS specialist at the atomic level. Now I'll leave it to the experts who can talk about the quantum ideas and share serious maths with you.

 

[Enjolras1789]

I don't know about a dielectric, but my understanding for why rubbing is good for charge transfer:

Rubbing mainly just causes more contact between atoms, allowing more charge transfer to take place. Remember that surfaces are virtually never perfectly smooth at the atomic scale. Rubbing squishes surfaces together, helping to get rid of surface irregularities, brings new atoms into contact with each other so that you can have more chances to transfer charge and prevent any local charge saturation that might cause charge transfer to stop, and can scrape off any oxide or other thin layers that might inhibit charge transfer.At room temperature even an insulator will have some number of charges in its conduction band, so current flow is entirely possible for an insulator. It simply takes larger voltages for significant current flow. Apply 100 volts to a material with 100 Mega Ohms of resistance and you'll get a microamp of current, which is still around 10¹² electrons per second, and we only need fractions of a coulomb's worth of charge to see all of the effects of static electricity, perhaps a microcoulomb's worth.

I appreciate your response; I find this interesting and helping to get at the heart of the matter.

So the argument is more predicated on surface effects? i.e., the rubbing is "activating" the surface in some sense, making/exploiting defects, such that there is a legit full charge transfer? Your point about the microCoulomb is relevant.
If I am interpreting your argument correctly, I like it. Most people here make arguments predicated on reasoning which would lead one to conclude that this should occur for ALL materials, no matter what. To explain this phenomenon, one cannot just say "because electrons flow easily," or that would predict electrons should transfer when lots of things rub on each other.
But if it is a surface effect, this makes sense: the balloon is a narrow surface with only air inside, such that the charge transfer doesn't "dissipate" into the bulk of the material (since the "bulk" of the balloon is air). That way the macroscopic observation of repulsion makes sense.

Do you agree, or do I misrepresent your reasoning?

 

[Enjolras1789]

Yes, I had not thought of the work involved in removing the electron from its immediate environment, even for metals. Now I can only rely on my betters to describe the triboelectric effect. Perhaps the few atoms/mols touching are close enough that electrons can, though losing energy on leaving their parent atom, regain a similar amount (or more?) on adopting the new parent, meaning that not much net energy is required. If they can get me over that tricky barrier, pulling it away against a static field seems easy.
My suggestion for the rubbing, if it's not complete misdirection, is that it allows multiple different points of close contact. And I suppose the materials need to be insulators, to stop the excess charge always moving to the points of contact and repelling further transfer.

All materials aren't metals because electrons can't move through the bulk of some materials. Triboelectric transfer seems to be very much a surface effect, between different materials. Surface atoms seem to behave somewhat differently from those inside the bulk of the material.

My comment was really a reaction to your "enormous energy", which is not what I see macroscopically, but I now understand your perspective is different as a SS specialist at the atomic level. Now I'll leave it to the experts who can talk about the quantum ideas and share serious maths with you.

No worries on the confusion; I inherently framed the argument in my head predicated on work functions vs. what we are observing spontaneously in the absence of external forces. I completely missed the boat on the problem in not appreciating that it is a surface-driven phenomenon, not something that can be appreciated from a metal/insulator point of view. I would be curious to hear your thoughts on the comment I made to Drakkith above, to see if you agree or not.

 

[Enjolras1789]

Yes, I had not thought of the work involved in removing the electron from its immediate environment, even for metals. Now I can only rely on my betters to describe the triboelectric effect. Perhaps the few atoms/mols touching are close enough that electrons can, though losing energy on leaving their parent atom, regain a similar amount (or more?) on adopting the new parent, meaning that not much net energy is required. If they can get me over that tricky barrier, pulling it away against a static field seems easy.
My suggestion for the rubbing, if it's not complete misdirection, is that it allows multiple different points of close contact. And I suppose the materials need to be insulators, to stop the excess charge always moving to the points of contact and repelling further transfer.

All materials aren't metals because electrons can't move through the bulk of some materials. Triboelectric transfer seems to be very much a surface effect, between different materials. Surface atoms seem to behave somewhat differently from those inside the bulk of the material.

My comment was really a reaction to your "enormous energy", which is not what I see macroscopically, but I now understand your perspective is different as a SS specialist at the atomic level. Now I'll leave it to the experts who can talk about the quantum ideas and share serious maths with you.

I think that between your answer and Drakkith, you have illustrated a lot of the heart of the matter for why this occurs spontaneously in a way that wouldn't predict this to happen for every material in the world. Thank you.

 

[Enjolras1789]

You did not address my request that you post references showing that that 10eV is a TON of energy. And yes, you continue to wave your hands. As a Solid State Physicist, you should have plenty of references that you can post.

Last chance. Post the links and show us quantitatively why 10eV is a large amount of energy, or your thread will be closed. Think for a moment how long it will take a single AA battery with 1.5V output voltage to be discharged with a current of 10 electrons per second...

I just explained it to you. The accessible energy thermally is in the ballpark of 1/40 of an eV. If you need a reference to compute kT, go grab literally any thermal physics book in the world (I like Reif's). You are claiming a solution predicated on an amount of energy which is orders of magnitude greater than what is available. This is not a rare event, this is a spontaneous event. Any explanation of the phenomenon requires that:
a) you work within the energy of the system (atoms do not spontaneously get 10eV of energy to ionize at room temperature)
b) has to work with the materials in question in the circumstance in question
c) does not predict that all materials would have this effect, because clearly certain materials do this but not others

 

[Vanadium 50]

Do you think everything is a metal?

Don't get snotty with me, boy. I'm trying to help you understand.

Not everything is thermal. When you rub two bodies together the energy to liberate the electrons is not thermal. It comes from the rubbing.

 

[Enjolras1789]

Don't get snotty with me, boy. I'm trying to help you understand.

Not everything is thermal. When you rub two bodies together the energy to liberate the electrons is not thermal. It comes from the rubbing.

Dude. Relax. No one is being snotty. Furthermore, calling me "boy" could be interpreted in some rather "Let's telephone HR" kinds of ways.
I am making the point that your argument would require one to believe everything is capable of performing this effect, i.e., everything is a metal. Clearly, these specific materials do this but NOT others. You cannot pull this trick with an orange and glass. In other words, I found your explanation lacking because I felt it would make a false prediction. Regardless of what the right answer is, just relax. Surely you play devil's advocate to poke hole's in other people's logic yourself?

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

After a Mentor discussion, this thread will remain closed. We recommend that the OP do more reading about the triboelectric effect and how it works.