The Field Model of Energy Transfer in Actual Circuits

[Dale]

I think even worries about how energy is conveyed in a macroscopic circuit is counter-productive and confusing when the point is to understand what the circuits are doing.

I agree. For understanding most circuits "how" is not necessary, only "how much". Using KVL and KCL I can easily analyze very complicated circuits that I have no hope of analyzing using Maxwells equations.

Also, neither KVL nor KCL make any claim whether the energy is transmitted inside or outside the wire. So there is no contradiction with Maxwell's equations.

 

[nsaspook]

While a simplistic version of the field model is closer to what really is going on, I still think you don't have to "unlearn" hydraulic patterns. They can exist together (without any cognitive dissonance). I can break out the field model when necessary and understand how a transformer operates in the reactive field, but in my day-to-day work of designing and using circuits I go to a hydraulic model almost invariably to understand what is going on. A model can be "wrong" but also useful.
...
Honestly, if when I was 15 people started breaking out terms like Poynting vector or skin effect on me I would have given up and gone into banking or something.

It's not a big deal IF they know about and understand both and know how to integrate both into a framework that's logical and solid. My main objection to hydraulic patterns in teaching is that most people in the early stages of education don't understand closed-loop hydraulic systems or electrical systems to the point where they can see when the analogy fails flat on its face. They see facets, buckets and tubs but not pumps, piping, regulator valves, check valves, flow rates and heat exchangers daily so they tend to see circuits as a water fall or reservoir that's being emptied to the ground. The time spend teaching a rudimentary understanding of closed-loop fluid power systems would be better spent IMO teaching the basics of charge, potentials and energy flows if the training is primarily electrical. To me understanding even simple circuits with a energy/work/power driven mental picture is fundamental to the task to understanding real problems and the root cause of problems in systems efficiently.
A simple example: A vacuum system pump trips the breaker while running and the technician goes down to find the problem. The root cause is not too much current in wires being used by the pumps motor, the root cause is a excessive demand of electrical energy because there is a leak in the vacuum system far from the pump. The technician instead of thinking about the details of volts and current at this point should be looking at what causes energy usage above normal levels and what needs to be done to reduce those energy levels back to normal.

 

[Gerry Rzeppa]

Okay, folks, how about this (for explaining to the kid what happens at the pickup end of the guitar amp system):

1. We tell the kid that the pinkish dots represent globs of electrons flowing first one way, and then the other, when a note is struck on the guitar. And that these electrons change direction more frequently when the pitch of the note is higher, less frequently when the note is lower. And that we call this movement current.

2. Then we tell the kid that the curvy line he sees on the scope is tracking the voltage in the circuit. That the peak values at times t1 and t2 will be larger if the string is struck with more force, smaller if struck with less force. And when the kid asks what the blue dots represent, we look around to see who is listening in before replying:

(a) If a fan of the hydraulic analogy is around, we tell the kid that the blue dots are like greater water pressure; like a taller water tank feeding the bottom and top pipes at times t1 and t2, respectively.

(b) If a fan of the pneumatic analogy is around, we tell the kid that the blue dots are like air molecules which exert greater pressure when there are more of them per square inch of available area;

(c) If a fan of the Lamin's Loaf Lorrie Model is around, we tell the kid that the blue dots represent additional loaves that are being carried by the pink lorries for drop-off at the store labeled 1 MEG;

(d) If a fan of motional EMF is around, we tell the kid that the blue dots represent a excess electrons per proton; and

(e) If a fan of field theory is around, we tell the kid that the blue dots (which any idiot knows should have been drawn outside the conduits) represent waxing and waning electric and magnetic fields around the wires.

3. Then we sum up by saying that however he might picture it, the difference in the voltage across the resistor at any point in time will be equal to the opposition to the flow of current presented by the resistor (measured in ohms) times the amount of current that actually makes it through the resistor (measured in amps). And that the current will be very tiny in this particular circuit because the voltage is small, the resistance is very big, and the pink dots keep changing direction before they have much of a chance to get anywhere. And then we whisper in his ear: "The pink dots never really move very fast at all; but the blue dots almost always move at something pretty freakin' close to the speed of light."

How about that? Say "yea" and we can put this baby to bed. I'll work out the transformers and capacitors and the rest on my own.

 

[Jeff Rosenbury]

Perhaps using several analogies, while pointing out that they are analogies. When the child starts asking "why", tell her that's why math is so important.

You might add a slinky to help with understanding waves.

 

[nsaspook]

Okay, folks, how about this (for explaining to the kid what happens at the pickup end of the guitar amp system):

Thank you for that post, it makes my decision to use a scientifically sound theory with my child even better for me. It also shows that fairy tales are easy to write but are hard to make consistent with reality.

 

[Dale]

Okay, folks, how about this (for explaining to the kid what happens at the pickup end of the guitar amp system):

Is the kid more interested in the music or the physics? If the kid is more interested in the music or even equally interested, then I would probably focus on the circuits rather than on the pickup. The signal from the pickup is the music, and it is interesting to learn what the circuit does to the music in terms of filtering and amplifying. That will probably be more useful at this point anyway.

(c) If a fan of the Lamin's Loaf Lorrie Model is around, we tell the kid that the blue dots represent additional loaves that are being carried by the pink lorries for drop-off at the store labeled 1 MEG;

If that happens then you just ignore them since that is such a flawed analogy. For example, ...

"The pink dots never really move very fast at all; but the blue dots almost always move at something pretty freakin' close to the speed of light."

What kind of sense does it make to say it is like lorries carrying bread when the bread moves millions of times faster than the lorries? Maybe the lorries should use bread instead of petrol.

 

[Gerry Rzeppa]

Is the kid more interested in the music or the physics? If the kid is more interested in the music or even equally interested, then I would probably focus on the circuits rather than on the pickup. The signal from the pickup is the music, and it is interesting to learn what the circuit does to the music in terms of filtering and amplifying. That will probably be more useful at this point anyway.

I realize you may not agree with me on this point, but having successfully taught both children and adults a wide variety of subjects for decades, I have become convinced that effective education involves (a) pictures, (b) words, and (c) formulas, in that order. After all, words have no real meaning unless there is a physical experience or at least a mental image to attach them to; and formulas, which define relationships between previously understood images and words, obviously cannot logically precede the definition of the things being related.

I was not focusing on the pickup in my previous post; I was simply starting a one end of the system to illustrate how we might give the kid (a) a picture, (b) some associated words, and (c) a formula that (a) illustrate, (b) describe, and (c) mathematically relate current, voltage and resistance. We could have as easily started at the other end of the system and talked about the generator in the power plant (using the same terms). Or we could have started with the speaker. The goal is to give the kid (a) a picture, and (b) some words, and (c) a handful of formulas that he can use, intuitively and correctly, throughout.

The missing ingredient in the literature appears to be the proper picture and the associated words. Sure, various individual concepts and components are illustrated and described in many different ways -- but I've yet to find a single coherent visual/verbal description that can be used throughout.

Now those of the "accountant" school of thought (as Ian M. Sefton calls it in the paper referenced in my initial post) think the formulas themselves constitute the picture, and that any attempt to get beyond that is a mistake. And Ian puts it:

"The accountants view energy solely as a mathematical attribute of physical systems, and it does not need any kind of conceptual model beyond that. They will tell you that it is a mistake to think of energy as a kind of substance. It is just an abstract quantity that, when you calculate it properly, always tallies. In this conceptual model energy is nothing like matter, it is just a mathematical abstraction that expresses some aspects of the behaviour of the natural world. In the case of our circuit the accountants will answer: “Who says that anything has to carry the energy? That’s the wrong question, because all we have to do is account for changes in energy in a kind of balance sheet – the precise location of the energy is not important and may be unknowable.” The accountant’s attitude is in the same tradition of thinking as the idea of action at a distance in which, for example, we don’t seek to understand how Earth’s gravity can reach out and grab the Moon (and vice versa). We just accept as a fact that it does. Similarly we don’t ask where the potential energy of the Earth-Moon system might be; it just belongs to the system. Taking the same conceptual view, it is grossly incorrect to talk of the PE of an electron. Science Teachers’ Workshop 2002 Understanding Circuits 3 If we apply this whole-system, action-at-a distance, model to our simple circuit the question of precisely how energy gets out of the battery and into the globe is not answerable. End of story."

Ian disagrees with this view, as I do (though it seems that some on this forum lean in that direction). And Ian attempts to fill the void with his version of the field model, but he fails (in my mind) because his resulting model is complicated, anti-intuitive, and -- in most practical cases -- incalculable.

So it seems to me the place where we could all make a lasting contribution to the field (no pun intended) is in the development of a concise, coherent, simple model of the various concepts and components involved. I have been working on that very idea for nearly a year now, on and off, and have been met -- to my surprise -- with resistance from almost every quarter. (As here. I'm making some progress in another thread and a couple of cranks jump in with some nasty remarks and my thread is terminated -- not because of what I said, but because of what they said. Ditto right here: I wanted to revisit a former topic a couple of posts up in the light of new information that had been gathered and new participants in the discussion, and my post is deleted with a "general warning" about my behavior. I really don't understand where such resistance to making something "as simple as possible, but no simpler" comes from. But I'm pretty sure it can't be a good place.)

Constructive criticism, which leads step by step to a useful solution is, of course, a good thing. Such criticism would invariably say not only what was wrong at each step, but what was right and should be retained in the next iteration. But non-constructive criticism -- and especially a disbelief in the value or possibility of the desired result -- nips the future flower in the bud.

I believe there exists a simple and comprehensive way of picturing and describing electricity that is both easy enough for a kid and consistent enough with established formulas for him to retain and use for a lifetime. If anyone here would like to help me find it, great. Sign up below and I'll start a fresh thread so we can get off on the right foot. Otherwise, I'm outta here.

 

[Dale]

I realize you may not agree with me on this point, but having successfully taught both children and adults a wide variety of subjects for decades, I have become convinced that effective education involves (a) pictures, (b) words, and (c) formulas, in that order.

I think that sounds like a reasonable pedagogical approach, but even before (a) you first have to decide what concepts you want to teach. Once you know what you are going to teach (and why) it becomes easier to pick the appropriate pictures, words, and formulas.

For EM there are three possible theories that you could choose to teach from:
1) Circuit theory
2) Classical electrodynamics
3) Quantum electrodynamics

I think that QED is not on the table, but there remains a decision about whether you want to teach circuit theory or classical EM. If you want to describe the way that the pickup functions then you probably need EM, but if you want to describe how the music is made then circuit theory is probably sufficient. In my opinion, circuit theory will be more useful to know, so that is what I would teach unless there is a compelling reason to teach EM.

If you go with circuit theory, then you have to accept some of its limitations. Specifically, circuit theory does not even address the question of where the energy flows. So anything that you teach about the location of the energy flux will be out of scope. It doesn't matter what pictures, words, or formulas you use, the location of the energy flux is simply not something that is answered by circuit theory.

If you have such a strong desire to teach about the location of energy flux that you cannot restrain yourself, then you must teach EM because that is the minimal theory which addresses the question. If you absolutely must teach that concept then you cannot avoid the theory which describes it. The classical EM stance on the topic is clear: the energy is transferred outside of the conductor.

There is therefore no scenario, IMO, where it is appropriate to teach that energy is transferred inside the conductor. Circuit theory is silent on the topic, and classical EM states the opposite.

 

[jim hardy]

i read that article linked in post #1 and didn't like it. I found his objections half baked...

If nothing goes on with free charges inside the wire, how does he explain Seebeck effect ?

Not denying that fields can describe energy transfer with superb elegance
over the entire spectrum from DC to light
but that's no reason to abandon imminently practical hydraulic analogies.

One must be aware of and explain the limits of hydraulic analogy, though.
Mr nsaspook hit one of my favorites, charge unlike water has no affinity for ground.

And, one must avoid both snobbery and reverse snobbery.

old jim

 

[Dale]

Not denying that fields can describe energy transfer with superb elegance
over the entire spectrum from DC to light
but that's no reason to abandon imminently practical hydraulic analogies.

There is no need to abandon the analogy, but all analogies break down somewhere (otherwise it wouldn't be an analogy). It is important to know where that breakdown is. In this case, one place where the hydraulic analogy differs from EM is the location of the energy flux.

Honestly, the equations of circuit theory are as easy or easier than hydraulics, so the analogy is of limited value anyway.

 

[Gerry Rzeppa]

I think that sounds like a reasonable pedagogical approach, but even before (a) you first have to decide what concepts you want to teach. Once you know what you are going to teach (and why) it becomes easier to pick the appropriate pictures, words, and formulas.

I want to teach the kid how and why this circuit does what it does:

And I've developed a no-solder-all-banana-jack modular version of it so we can experiment as we go along:

Of course we'll be including in the study some prerequisites (playing with magnets, etc) and the things at the two ends (the guitar and the power company). But this is all a means to a greater end: I'd like to come out of this with a conceptual model of electricity that, as I have said, will serve the kid for the rest of his life.

For EM there are three possible theories that you could choose to teach from: 1) Circuit theory; 2) Classical electrodynamics; 3) Quantum electrodynamics.

I'd rather not begin with that kind of compartmentalized thinking. Seems to me we may need a little of each to get the job done.

If you want to describe the way that the pickup functions then you probably need EM, but if you want to describe how the music is made then circuit theory is probably sufficient. In my opinion, circuit theory will be more useful to know, so that is what I would teach unless there is a compelling reason to teach EM.

The compelling reason to include electric and magnetic fields in the discussion is that such fields play critical roles in several places: the pickup, the power company, the transformers (power and output), and the speaker come immediately to mind.

If you go with circuit theory, then you have to accept some of its limitations. Specifically, circuit theory does not even address the question of where the energy flows.

I believe it is impossible to form a decent mental image of voltage without an understanding of how potential differences "get around". A problem that I think plagues a lot of beginners is that they're told there is a simple and very fundamental relationship is between current (which is typically defined as the flow of electrons in a wire) and resistance (which is a property of materials, conductors and insulators alike), and voltage which is vaguely or obscurely defined -- and the student thus naturally assumes that voltage can be fully accounted using only the other two concepts (current and resistance). It's clear (at least to me) that the problem is with the missing picture and description of voltage. Get that in focus, I believe, and the rest will be easy.

If you have such a strong desire to teach about the location of energy flux that you cannot restrain yourself, then you must teach EM because that is the minimal theory which addresses the question.

Whatever it takes. But there are some very big and black intuitive holes in everything I've read about "energy being transferred via the fields" and those holes must be filled up with reasonable explanations in the form of pictures and words. Here, for example, is William Beaty's attempt at clarifying the matter: http://amasci.com/elect/poynt/poynt.html . Unfortunately, he raises as many questions as he answers. For example, part of his description says:

"...whenever a battery powers a light bulb, the battery spews electrical energy into space. That EM field energy is then grabbed firmly by the wires and guided by them. The field energy flows parallel to the wires, and eventually it dives into the lightbulb filament."

Some questions that immediately come to my mind are, How much energy is lost before the wires grab the fields? Exactly how (and why) do the wires do this grabbing? I thought it was the current in the wire that generated the field; what gives? What makes that energy "dive into" the lightbulb filament? Etc. I suspect it will take a good deal of time and energy to get questions like those answered since there is so little non-mathematical literature on the subject available. Are you up to that?

If you absolutely must teach that concept then you cannot avoid the theory which describes it. The classical EM stance on the topic is clear: the energy is transferred outside of the conductor. There is therefore no scenario, IMO, where it is appropriate to teach that energy is transferred inside the conductor. Circuit theory is silent on the topic, and classical EM states the opposite.

The question, again, is, Are you (and the other folks here) willing to take the time and energy necessary to explain how and why "energy transfer outside of the conductors" is a real, reasonable, and useful concept? So we can come up with pictures and words that are better than Beaty's "black gob of hairs" (note his own remark under this last of his diagrams):

Clearly, if we're going to apply this theory to the whole guitar amp circuit above, significant simplification will be necessary!

 

[davenn]

Unfortunately, he raises as many questions as he answers. For example, part of his description says:

"...whenever a battery powers a light bulb, the battery spews electrical energy into space. That EM field energy is then grabbed firmly by the wires and guided by them. The field energy flows parallel to the wires, and eventually it dives into the lightbulb filament."

that is an absolutely shocking description ! ... surely you can recognise that, particularly after what has been said in this thread

"the battery spews electrical energy into space"

cant get anything more incorrect than that
The only things I have seen spewing out of batteries is acid from a wet battery and flames from a lithium battery

"... the EM field isn't "grabbed" by anything"

the EM field is radiated out from the moving electrons

... and eventually it dives into the light bulb filament."

that's seriously atrocious
"eventually" ? is he implying it takes a significantly long time ?
you have already stated that you know the EM field travels at near the speed of light

"it dives into the light bulb filament"

give me a break

toss these dreadful pieces of text ( so called science) into the garbage bin where they belong
and stop referring to them as has been suggested by others earlier in the thread

Dave

 

[nsaspook]

There is no need to abandon the analogy, but all analogies break down somewhere (otherwise it wouldn't be an analogy). It is important to know where that breakdown is. In this case, one place where the hydraulic analogy differs from EM is the location of the energy flux.

Honestly, the equations of circuit theory are as easy or easier than hydraulics, so the analogy is of limited value anyway.

Exactly. I base my response to this thread on what I believe is a very successful introduction to electricity that the military uses for new students. While the details are too advanced for a 10 year old the sequence of what's learned is not. Because the military uses 'electron' flow as the standard they first teach elementary physics so students have a basic understanding of Matter, Charge, Energy and Electricity as the foundation of latter analogies with water and problems in circuit theory as they mentally picture how electrons move in response to fields. My objective is not to teach my child EM at 10. The objective is to provide the proper foundation to understand circuit theory in a similar way so when she gets to the point of studying EM it will be Second Nature to her.

http://www.phy.davidson.edu/instrumentation/NEETS.htm

 

[Dale]

I want to teach the kid how and why this circuit does what it does:
...
I'd like to come out of this with a conceptual model of electricity that, as I have said, will serve the kid for the rest of his life.

Then circuit theory will be of more value for both the short term and long term goals.

Seems to me we may need a little of each to get the job done.

I would be very careful introducing an incomplete theory. I have yet to see even world renowned experts who could do so without causing more confusion than they resolved.

The compelling reason to include electric and magnetic fields in the discussion is that such fields play critical roles in several places: the pickup, the power company, the transformers (power and output), and the speaker come immediately to mind.

You should probably also cover thermodynamics so that you can explain heat engines from the power company. And of course you couldn't do that without discussing the formation of coal. And of course geology plays a critical role as does paleontology and biology. And who could discuss biology without photosynthesis and solar energy and thermonuclear reactions. And of course you would need general relativity too to explain the formation of the sun. Then he will have a good understanding of how that guitar circuit works.

You need to focus your instruction, much more than you need to worry about whether you have a compelling picture or not. Your stated goal can be accomplished with circuit theory, so anything you introduce beyond that is probably a distraction.

Are you up to that?

No. I think it is a bad idea. Not every concept needs to or should be introduced to a beginning student. I wouldn't teach concepts that they are not prepared for.

Clearly, if we're going to apply this theory to the whole guitar amp circuit above, significant simplification will be necessary!

Yes. Circuit theory.

 

[Gerry Rzeppa]

Well, clearly this isn't going to work. Bye all.

 

[Dale]

Clearly not. You are trying to get advice on how to do something and refusing to even consider the resulting advice that it is not a good thing to do.

I hope you at least recognize that if you teach that energy is transferred inside the wire you are teaching a falsehood that is not claimed by circuit theory and is refuted by classical EM. You may come up with perfect pictures and words to teach it, but the concept itself remains false.