Motional EMF from an Electron Flow Perspective

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Gerry Rzeppa

Joined Jun 17, 2015
170
I'm not fully clear on what you are asking here. You say the voltage readings depend on the relative positions of those electrons, but what about the current from those moving electrons?
I'm talking about a reading at a point in time. A point in time is a snapshot; one frame of a movie. Play the preceding frames successively, and you'll see the current that brought about that particular arrangement; play the succeeding frames successively, and you'll see the currents that follow; but at any point in time, there's no movement -- just the current state of the electrons.

So I'm asking for two snapshots of the circuit, one at time t1 and one at time t2. I want to know (a) how are the electrons arranged at those two points in time, and (b) how those arrangements correspond with the "voltage snapshots" that we see on the scope at those points (ie, +1 and -1 volt). You know what my snapshots look like, and you know I think they're accurate; but give me clearer pictures and I'll throw mine away in a heartbeat.
 

Lool

Joined May 8, 2013
116
Well t1 and t2 correspond to the maximum and minimum voltage. These points must correspond to the maximum velocity of the guitar strings. The time t1 might be the guitar string moving upwards, in which case the time t2 is the time the guitar string is moving downwards.

The time between t1 and t2 where the voltage is zero, is the points where the string has zero velocity and is changing direction.

Now, since this is mostly a resistive circuit made from a length of wire in the coil, and a 1 M resistor, the current is probably in phase with the voltage. Let's at least assume that. Of course the coil has inductance, which could modify that, but it might be negligible, particularly for low frequencies like 60 Hz. Let's assume the coil resistance is 1 kOhm.

Now the coil and resistor form a series circuit, so whatever EMF is generated in the coil, the current, in Amps, will be EMF/1,001,000. If 1 V of peak EMF is generated by the flux change, then 0.999 microAmp of peak current will flow at those times t1 and t2. So, let's say that t1 has +0.999 uA and t2 has -0.999 uA. These are the currents that flows in the entire loop at those times. The voltage drop around the loop must equal the EMF generated in the loop. This means that the resistor drops 0.999 V and the coil drops 0.001 V.

So far there has been no need to invoke any notion of charge distribution on the conductor. It turns out that in reality there must be some charge distribution, but the exact amount and where it is would depend on the shape of the wires and geometry, and this is not something that anyone is interested in. The distribution is not important, but certainly there is not going to be 1 Coulomb on the top of the resistor and -1 Coulomb on the bottom of the resistor.

I suppose you can try to figure out exactly what the charge distribution is, but why do you think nobody does this? First, because it is hard to do and it is really a field problem to solve this. Second, because it is not very useful to know it. The charges are small enough that we would not notice the forces involved. But, imagine if you really had 1 C of charge separated by 1 centimeter. What's the force? It would be found approximately with Coulomb's law to be about 90 trillion Newton's. This can't be the correct charge and the actual charges would need to be in the nanoCoulombs in order that we not notice forces and stresses in materials.

To understand circuit operations, it is much easier to look at voltage and current, resistance, capacitance, inductance, etc. Under the assumptions of circuit theory, there can be no buildup of charge on any device. Internally, yes it can happen with a capacitor, but from the terminal behavior of the capacitor, you really have no way to know that. The current going in one terminal always equals the current going out the other terminal. Of course you could monitor the voltage and temperature too, and you would know that energy went in and did not come out as heat.
 
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Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
...It turns out that in reality there must be some charge distribution, but the exact amount and where it is would depend on the shape of the wires and geometry, and this is not something that anyone is interested in. The distribution is not important...
I beg to differ. When the free electrons in a battery are equally distributed, it is dead; but when they're not equally distributed, we can get work out of it. When the free electrons in a big capacitor are equally distributed, we can handle it with impunity; but when those electrons are not equally distributed, we can get hurt. When all the electrons in my guitar amp are equally distributed, nothing happens; but when a bunch of them move in concert with one another -- changing the distribution in the circuit moment to moment -- we get music. How then can we say that the distribution is not important? Seems to me it's the heart and soul of the whole affair. Electrons equally distributed -- nothing. Electrons unequally distributed -- work and sparks and music.

Perhaps we should start with a simpler circuit or two:

electron snapshot 1.jpg

Let's consider the electron distribution in these circuits at a point in time about 10 seconds after they are set up (and are thus relatively stable).

1. I think the circuit on the left will have its free electrons evenly spread throughout at that point in time. Do you agree?

2. I think the circuit on the right will have significantly more free electrons on the bottom than the top at that point in time. Do you agree?

3. I think the reading on the right meter, over time, will decrease in direct proportion to reductions in this imbalance, and that the meter will finally read zero when the imbalance has been removed (ie, the free electrons are equally distributed). Do you agree?
 
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Lool

Joined May 8, 2013
116
When all the electrons in my guitar amp are equally distributed, nothing happens; but when a bunch of them move in concert with one another -- changing the distribution in the circuit moment to moment -- we get music. How then can we say that the distribution is not important?
Where in my description did you see me describe a change in distribution of electrons from moment to moment? I talked about current flow. The simple circuit theory description describes all electrons moving with the same current in a series circuit. All electrons move in concert. There is no distribution change for the AC current. That's the key point I try to make, but you won't accept it. I can't force you to accept it if you don't want to see it.
 

Lool

Joined May 8, 2013
116
Let's consider the electron distribution in these circuits at a point in time about 10 seconds after they are set up (and are thus relatively stable).

1. I think the circuit on the left will have its free electrons evenly spread throughout at that point in time. Do you agree?

2. I think the circuit on the right will have significantly more free electrons on the bottom than the top at that point in time. Do you agree?

3. I think the reading on the right meter, over time, will decrease in direct proportion to reductions in this imbalance, and that the meter will finally read zero when the imbalance has been removed (ie, the free electrons are equally distributed). Do you agree?
1. Yes.

2. Depends on how you define significantly. From a simple circuit theory calculation I will say no i don't agree. If you say there are more electrons at the bottom, how come you can't calculate the value? How come no one else in this forum knows how to calculate it without a computer and a lot of time on their hands? How come nobody every bothers to calculate it? Why do we mostly use circuit theory rules?

3. Isn't this what we are talking about as a misconception. Obviously I don't agree. The battery is able to drive current through the meter because it has EMF, not because there are charges. The charges may be there to some degree, but they are not sufficient to drive the meter for long. Without the supporting EMF, even the meter resistance would drain the charge away quickly and then there would be no voltage reading. I understand that you don't want to accept it.
 

Lool

Joined May 8, 2013
116
Let me try an analogy with flowing water. Imagine you have a tube or hose and you form it into a circuit or circle and fill it completely with water. after you seal it, you take a blow torch and you narrow down or neck-down a section of the tube so that it has a much narrower diameter and it is harder for water to flow through that section. Imagine you have a magical Faraday water circulator that can circulate the water without a pump and you start the water flowing. Do you expect a significant density change in the water and sections of the inner tube to have a buildup of water and other areas to have less water or less water density? To some small degree it might happen but water is not very compressible and so the pressures required are not in the range you can expect for a typical hose.

Now the flowing water has trouble flowing because of the narrow section of tube and that narrow section limits the rate of flow for a given Faraday pressure your magic is applying to pump the water. But, the flow rate is the same in the full circuit and there is no distribution change of the water even though it is flowing.
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
So far, so good.

2. Depends on how you define significantly. From a simple circuit theory calculation I will say no i don't agree. If you say there are more electrons at the bottom, how come you can't calculate the value?
I think I can calculate the value. For nine volts, I'd put the difference between the number of free electrons on the top and bottom at about 9 x (6.28x10^18).

How come no one else in this forum knows how to calculate it without a computer and a lot of time on their hands? How come nobody every bothers to calculate it? Why do we mostly use circuit theory rules?
Tradition. Momentum. Inertia. Don't see the need. Don't know how. Don't care. How should I know?

3. Isn't this what we are talking about as a misconception. Obviously I don't agree. The battery is able to drive current through the meter because it has EMF, not because there are charges.
I agree that the charges have to be forced into the configuration I'm describing; electrons want to be equally distributed. But it seems to me that we can think of the 9 volts as a measure of the "dissatisfaction" of the electrons with the present configuration; the 9 volts represents the sum of all the repulsive forces between the electrons that are piled up where they don't want to be.

The charges may be there to some degree, but they are not sufficient to drive the meter for long.
Not without additional electrons being forced into a similar configuration as the present electrons make their way through the resistor. The chemical reaction in the battery supplies this force -- but the 9 volts is the result of the imbalance of free electrons top vs bottom.

Without the supporting EMF, even the meter resistance would drain the charge away quickly and then there would be no voltage reading. I understand that you don't want to accept it.
See immediately above. I don't have a problem with the idea that we need outside forces to push electrons where they don't want to be. Chemical, inductive, whatever. But when I talk about an arrangement of electrons at a point in time -- and the relative surpluses/deficiencies that arrangement represents -- how that configuration arose (and/or how long it can be maintained) is not the issue.

Let me try an analogy with flowing water.
The analogy with water pressure is flawed because (a) we typically think of water as a continuous, not a discrete substance; (b) water molecules under such pressures do not have a different atomic structure (ie, more or fewer electrons); and most importantly (c) water pressure is not caused by the mutual repulsion of water molecules. But electrical pressure (voltage) is caused by like charges repelling -- at least in my current theory! Force those electrons into an uncomfortable (ie, non-uniform) configuration, and you get electrical potential: voltage.
 

Lool

Joined May 8, 2013
116
I think I can calculate the value. For nine volts, I'd put the difference between the number of free electrons on the top and bottom at about 9 x (6.28x10^18).
OK, so based on this response I really need to drop out of participation in this thread. I don't mind trying to help you out, but you just are not willing to take serious consideration of things I've said. I already explained that this answer, which is the same you gave before, represents 9 Coulombs of charge. I've already explained the ridiculously large force that would result if your answer were correct. The circuit would literally explode if you could hold those electrons on the conductors.

I've mentioned several areas where you revealed very deep misunderstanding of physics. All I can do now is suggest that you go back and reconsider what I've said. Baring some small mistakes that I'm sure I've made in miss-wordings and slips of the pen, basically what I told you is correct and certainly a more reasonable way to teach a kid basics of electricity, once he is old enough. Of course there are better sources of information out there than my words, and they are readily available to those who wish to make use of them.

You are locked in to our own ideas and are convinced they are right. I don't have the ability to change your mind on this. Good luck in your search.
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
I really need to drop out of participation in this thread...
I'm sorry to hear that. But thank you for all the time and effort you have put into this. I assure you it was time well spent: even if I haven't (yet) fully bought into the conventional model, I do know more now than I did before, and I have you to thank for that.
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
If anyone is still reading this thread, I think I've found something helpful, here:

http://www.matterandinteractions.org/Content/Articles/circuit.pdf

It's a paper entitled "A Unified Treatment of Electrostatics and Circuits" by Sherwood and Chabay and it describes a different approach to teaching electronics that helps with common beginner questions like the ones I've been raising in this thread.

One example is the intuitive idea (not at all unique to me) that electrons must pile up at the entrance of a resistor in order to provide a potential difference relative to the other end. They describe (on the very bottom of page 9 and page 10) a classroom experiment that demonstrates that "surface charge" actually does pile up at the entrance of resistors. Here's their graphic:

surface charge 1.jpg

Now I don't know if they'd be willing to equate their "variation in surface charge" with my "distribution of electrons" (though I don't see what else it could be) -- but it's clear that we're both on the same conceptual and intuitive track.

A little later they say, "The analysis of circuits in terms of surface charge provides answers to some profound questions students sometimes raise, questions which may have no satisfactory answer within the context of traditional analysis... [for example] 'What is the difference in the conditions at the two ends of a resistor?' It is quite unsatisfying to say merely that the potential is different, and quite satisfying for the student to see that a difference in surface-charge density at the two ends of the resistor leads to a large electric field inside the resistor, which drives the current." Here again I find empathy for my intuitions: turns out it is a difference in charge density that accounts for the potential difference across a resistor.

Other related articles that I've found helpful are:

http://www.astrophysik.uni-kiel.de/~hhaertel/Circuit/electric_circuit.pdf

http://www.astrophysik.uni-kiel.de/~hhaertel/PUB/voltage_IRL.pdf

https://www.tu-braunschweig.de/Medien-DB/ifdn-physik/ajp000782.pdf
 

Lool

Joined May 8, 2013
116
For the record, these references are consistent with several things I mentioned, and not consistent with your theory. Consider the following quotes i took from those references just to give you and idea.

"For such a quantitative determination it is not possible to measure directly the sur- face charges or the accompanying Coulomb forces. The density of the additional electrons on the surface of current carrying conductors are in general rather small
and are depending on different geometrical factors. The same holds for the gradi- ents of their density."

"Order of Magnitude - lf a current of 1 A is flowing through a wire, there are about 10^19 electrons passing through each cross section within one second. Because of the enormous strength of the Coulomb interaction and the very high mobility of electrons in metals, it takes only a few electrons at the surface of the wire to push 1019 electrons around in a circle and to overcome the resistance of a metallic wire. "


Compare this to the following comments I said in this thread.

"It turns out that in reality there must be some charge distribution, but the exact amount and where it is would depend on the shape of the wires and geometry,"

"When current flows, there can be certain charge distributions created on conductors, but any point of the circuit does not build up an overall excess or deficit of electrons. The charge distribution is secondary and one never thinks of it. All it does is allow charges to flow around bends, but again nobody ever thinks of it that way." (note this is an example of me not choosing the best wording for my description)

"Now based on the above you can note that I'm not saying that the principle you describe is wrong, but I say that you can neglect it"

"I suppose you can try to figure out exactly what the charge distribution is, but why do you think nobody does this? First, because it is hard to do and it is really a field problem to solve this. Second, because it is not very useful to know it."


Are you really suggesting to teach these references to beginners or your kid? I would not suggest that. These are concepts that should be brought up later in teaching, or to answer specific questions from students (similar to yourself) that grasps that there must be charges that are typically ignored. At that point it can be explained that although it is true that there is, the amount of charge is small and not the primary consideration to understand circuit operation.
 

Wendy

Joined Mar 24, 2008
23,415
Many cases we teach beginners concepts that are wrong or incomplete so they can work with them, then fill in the gaps later.
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
For the record, these references are consistent with several things I mentioned, and not consistent with your theory.
Two things regarding that. Please bear with me.

My wife spends a lot of time teaching our little guys the fundamentals -- reading, writing, arithmetic, etc. Sometimes she'll spend days working on a particular concept (say, something about long division), repeating the same thing over and over, yet he just doesn't get it. Then I'll be walking by and I'll say essentially the same thing she's been saying and the light will go on in the little guy's head. It takes time to form correct neural patterns in a head, and the last repetition of a thing just before comprehension is going to be the one that "is remembered" or "strikes home" or "gets the credit." In other words, I didn't miss or ignore what you were saying, I just wasn't ready to completely assimilate those things. (Still am not, regarding some parts.) You're doing great on that front. Thanks.

But secondly, there's a difference between the way you've been saying those things and the way these other guys have said them. You've said, in essence, "You may be right but the effect is negligible so you're better off forgetting all that and just doing what everyone else does." While these guys have said, "The way this is traditionally taught doesn't work well because it's counter-intuitive and therefore we need to find some way to make it intuitive without distorting the facts." I just did a search on Google and found scores of students asking, "Do electrons pile up at the entrance to a resistor?" And none of the responses (aside from the articles referenced above) said, "Great question. I can see how you might think that. And in fact, surface charges do pile up, just as you suspect. And they do play an essential role in the operation of the circuit." Instead those struggling students got the standard, quantitative (rather than qualitative) hard-to-picture formula-centered replies that either ignore or pooh-pooh their intuitions. When one's intuitions are dealt with in this way it has an learning-hindering ring that sounds suspiciously like "just do as you're told" or "get a decent haircut." Not good for kids.

Are you really suggesting to teach these references to beginners or your kid?
I'm suggesting that we can't begin by describing electricity in a way that naturally leads a kid to intuitively conceive of electrons piling up at one end of a resistor and then, when that intuition surfaces, ignore or pooh-pooh it. We need a model that leads to correct intuitive insights.

Allow me to rephrase an earlier question:

ohm\'s law 1.jpg
Clearly (at least "clearly in the minds of all kinds and sorts of beginning students") something different is going on at the bottom of that circuit versus the top. What it is? and How should we picture it? and thirdly -- but not until those first two questions have been answered -- How does the picture we've just described jive with Ohm's Law?
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
Many cases we teach beginners concepts that are wrong or incomplete so they can work with them, then fill in the gaps later.
Ah, Wendy. Based on other of your replies on this site, I was kind of hoping you'd be looking over our shoulders on this one.
 

Wendy

Joined Mar 24, 2008
23,415
I've been following this thread. When I started learning electricity at 7 years old I started with the water model. It is not very good, but it worked for a young kid.

Of course, you can't launch model rockets without electricity.

Then, at 14 or so, my parents got my brother and I a 250 in 1 Radio Shack projects kit. There was no going back after that.

Start with simple concepts. Most kids instinctively know they won't understand everything in fullness, but small steps is the ticket.
 

BR-549

Joined Sep 22, 2013
4,928
Gerry,

The more you compress the hairnet, the more negative voltage there is, the more you expand the hairnet, the more positive the voltage there is.
 

Lool

Joined May 8, 2013
116
Two things regarding that. Please bear with me.

My wife spends a lot of time teaching our little guys the fundamentals -- reading, writing, arithmetic, etc. Sometimes she'll spend days working on a particular concept (say, something about long division), repeating the same thing over and over, yet he just doesn't get it. Then I'll be walking by and I'll say essentially the same thing she's been saying and the light will go on in the little guy's head. It takes time to form correct neural patterns in a head, and the last repetition of a thing just before comprehension is going to be the one that "is remembered" or "strikes home" or "gets the credit." In other words, I didn't miss or ignore what you were saying, I just wasn't ready to completely assimilate those things. (Still am not, regarding some parts.) You're doing great on that front. Thanks.

But secondly, there's a difference between the way you've been saying those things and the way these other guys have said them. You've said, in essence, "You may be right but the effect is negligible so you're better off forgetting all that and just doing what everyone else does." While these guys have said, "The way this is traditionally taught doesn't work well because it's counter-intuitive and therefore we need to find some way to make it intuitive without distorting the facts." I just did a search on Google and found scores of students asking, "Do electrons pile up at the entrance to a resistor?" And none of the responses (aside from the articles referenced above) said, "Great question. I can see how you might think that. And in fact, surface charges do pile up, just as you suspect. And they do play an essential role in the operation of the circuit." Instead those struggling students got the standard, quantitative (rather than qualitative) hard-to-picture formula-centered replies that either ignore or pooh-pooh their intuitions. When one's intuitions are dealt with in this way it has an learning-hindering ring that sounds suspiciously like "just do as you're told" or "get a decent haircut." Not good for kids.



I'm suggesting that we can't begin by describing electricity in a way that naturally leads a kid to intuitively conceive of electrons piling up at one end of a resistor and then, when that intuition surfaces, ignore or pooh-pooh it. We need a model that leads to correct intuitive insights.

Allow me to rephrase an earlier question:

View attachment 88693
Clearly (at least "clearly in the minds of all kinds and sorts of beginning students") something different is going on at the bottom of that circuit versus the top. What it is? and How should we picture it? and thirdly -- but not until those first two questions have been answered -- How does the picture we've just described jive with Ohm's Law?
These are all excellent points and I agree with them.

My problem is that you are not dealing enough with quantitative answers. Hence, you have no feel for magnitudes and no feel for dominant effects. Even after me explaining how big 1 Coulomb of charge is and the trillion pounds of force implied, you still didnt get it.

All the things discussed above come into play and are studied in electromagnetics courses after people have the math, science and technical background to make sense of it. You are struggling with this because you put the cart before the horse.

Nonetheless, your comments here are exactly on the mark, and I agree.
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
Gerry, The more you compress the hairnet, the more negative voltage there is, the more you expand the hairnet, the more positive the voltage there is.
I'm trying to picture that. The net, I presume, has electrons at the vertices and the "strings" between them represent the forces of like repelling like; perhaps like this (only wrapped around the outside of a wire):

hairnet 1.jpg

If that's a correct image, then I can see how compressing the net would increase the pressure/voltage (since the electrons want to get as far away from each other as possible); and how expanding the net would have the opposite effect.

But that image also suggests that electrons in a circuit pretty much stay in their own "neighborhood" and don't actually flow around a circuit. Yes?

It's also not clear where the additional electrons come from when the net contracts, or where the extras go when the net expands. For example, this contracted net has nearly four times as many electrons as the less-compressed net above in the same space:

hairnet 2.jpg

Where do these extra electrons come from/go to?
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
These are all excellent points and I agree with them. My problem is that you are not dealing enough with quantitative answers.
I'm sorry, but I believe quite strongly in "Pictures before words, words before numbers." Most people don't realize it, but all language (except that small part that deals with direct sensory experience) is metaphorical in nature; therefore, most words mean nothing unless there's a picture to go with them. And full comprehension of a formula requires both words and pictures.

Hence, you have no feel for magnitudes and no feel for dominant effects. Even after me explaining how big 1 Coulomb of charge is and the trillion pounds of force implied, you still didnt get it.
The reason I have trouble with that particular assertion is that I've read in many places that a coulomb's worth of electrons (6.28x10^18) is about the size of a grain of salt; and that moving that many electrons past a point every second yields just one amp of current; and that a deficiency of that number of electrons in one spot versus another represents an electrical potential of just one volt. I'm having trouble seeing how you get a "trillion pounds of force" out of any of those images.

All the things discussed above come into play and are studied in electromagnetics courses after people have the math, science and technical background to make sense of it. You are struggling with this because you put the cart before the horse.
Actually, I think it's the "math heads" that have put the cart before the horse; or, more precisely, have forgotten about horses altogether. Case in point. We tell a kid that electricity is electrons moving, in concert, in a wire. Then we tell him that a resistor is a bottleneck; like a much thinner wire. So the kid immediately and naturally and intuitively says, "So those electrons pile up at the entrance, right?" And we say, though the part in brackets is usually left out: "NO! [The intuitions we all but begged you to dream up are entirely wrong!] According to this law and that formula..."

Nonetheless, your comments here are exactly on the mark, and I agree.
So let's get back to our horses and put them in front of the cart:

ohm\'s law 1.jpg
Horse 1 (picture): How am I supposed to picture what's going on in this circuit?
Horse 2 (words): What's different about what's going on up top vs down below?
Cart: How do we hook that picture and those words up with the formula E=IR?
 
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