The Field Model of Energy Transfer in Actual Circuits

[Gerry Rzeppa]

In this paper (http://science.uniserve.edu.au/school/curric/stage6/phys/stw2002/sefton.pdf) the author describes the transfer of energy in an electrical circuit as follows:

"To explain energy transfer we need to look at what is happening outside the wires. As a consequence of the surface charges on the wires, there is an electric field in the space outside the wires (as well as inside). Also, as a consequence of having a current in the wires, there is a magnetic field in the space around the wires. It is this combination of electric field and magnetic field in the space outside the wires that carries the energy from [source] to [load]. Once the fields are set up, the energy travels through space, perpendicular to both the electric field and the magnetic field, at the speed of light."

I've read similar descriptions in a number of places, often accompanied by a diagram showing the various fields that are participating in this energy transfer, like this:

Now here are four photos of alternative layouts for the filament wiring in vacuum-tube amplifiers:

(The bottom right amplifier uses the chassis as one of the conductors in the filament circuit.) The total current carried by these wires is typically 2 to 3 amps, and the voltage (most often AC, but sometimes DC) is usually 6.3 volts. A similar variety of layouts can be found in the high-voltage portions of such circuits as well.

Now based on the diagram above, I would think the electric and magnetic fields in these various configurations would be significantly different. As different, perhaps, as the diagram above and, say, this one:

And yet I've found (in spite of the claims of various aficionados) no significant subjective difference in the performance of such widely different layouts; and certainly no objective difference in the voltage and amperage readings at the corresponding spots. Sure, we might get a little more or less hum one way or another (though, in practice, that's not as easy to predict as one might think); but we never see a significant change in the voltage or current based on the layout.

In other words, it seems to me that while these various fields may be truly present as described, it's hard for me to imagine that they are the primary conduit for the transfer of energy in such circuits. Seems to me that the energy is being transferred through the actual conductors (wires or chassis or both), not via the space surrounding those conductors.

Comments?

 

[mfb]

If you calculate the fields in the last layout in detail, you'll get the same result as for the first diagram. The calculation is just much more complicated - a good argument why circuit analyses are always done with voltages and current flows in the conductors unless the circuit is designed for really high frequencies.

 

[Dale]

In other words, it seems to me that while these various fields may be truly present as described, it's hard for me to imagine that they are the primary conduit for the transfer of energy in such circuits.

Why is that hard for you to imagine? Also, why does it matter?

Seems to me that the energy is being transferred through the actual conductors (wires or chassis or both), not via the space surrounding those conductors.

How would you make an experiment to determine that? For example, have you even calculated how quickly the energy flux falls off outside of the wire to give you an idea of the spatial scales you have to investigate? Do you have a method for measuring energy flux that you could use inside and outside?

 

[Gerry Rzeppa]

If you calculate the fields in the last layout in detail, you'll get the same result as for the first diagram. The calculation is just much more complicated...

I don't know about that because I don't know how to do the calculation (and haven't seen it done by anyone else). But I find it hard to believe that the size and shape of the fields are identical when in one case the wires are narrow and twisted around each other, and in another the entire chassis is employed as one of the conductors.

I do know that the theoretical intent behind twisting the wires (top left layout) is to neutralize those fields in the interest of minimizing hum -- and yet these supposedly neutralized fields somehow manage to transport the same amount of energy to the filaments!

Why is that hard for you to imagine? Also, why does it matter?

It matters because I'm trying to understand what the author is saying. If, as he suggests, the energy transfer is via the fields (and not the wires), and if the fields are by definition sensitive to the relative sizes, lengths, and positions of the wires, then the performance of the circuit should be equally sensitive to changes in those sizes, lengths, and positions. This has not been the case in my (admittedly limited but apparently relevant) experience: thus, it is "hard for me to imagine."

How would you make an experiment to determine that?

The "experiment" -- crude though it may be -- is as I've described it. If the primary conduit for energy transfer is the fields and not the wires, then the energy transfer (ie, the power, the volts and amps that reach the filaments) should be more sensitive to the circuit layout than they appear to be. Surely the size and shape of the fields around a twisted pair of wires is different than the size and shape of the net field around a single wire plus a whole chassis? But I could, of course, be quite wrong: it's admittedly a "smell test" and not a rigorous examination at microscopic scales.

Here's another experiment we can perform easily enough. If you could roughly sketch out in bright colors -- or even describe in words -- the size and shape of the electric and magnetic fields in the four cases shown in black-and-white below, I think that would go a long way toward showing me how the difference in layout does not greatly affect the size and shape of those fields and thus does not greatly affect the transfer of energy via those fields.

Perhaps this will help. When I read practical engineering texts -- whether about filament windings or transmission cables -- I get the distinct impression that these fields around the wires are essentially unwanted side effects that we should strive to minimize (unless, of course, we're building an antenna); on the other hand, the physicist (like the above author) appears to be telling me that these fields are the essential and primary means of energy transfer. Can you see how this might confuse a beginner like me?

 

[Jeff Rosenbury]

If you consider wires to be wave guides, this might make more sense. Use the Poynting Vector: S=ExH.

BTW, there is a measurable difference between the twisted path and the straight path. When you examine it with the complex poynting vector you will find the twisted path has significantly higher losses.

This all comes back to Maxwell's equations. Ohm's laws is a form of Maxwell's equations where some values are assumed and held constant, as are nearly all the formulas we use as electrical engineers. Thus we don't have to solve the field equations each time. But the field equations are still there, hidden by our simplified math.

So knowing this we can take a short cut to find the extra losses in twisted field path. That short cut is to measure the additional resistance that the longer wire provides and find the I²R loss. Suppose we made the path straight and used the same length of wire? We can't but we could add a loop at the end. Then the losses in the loop would match the losses in the twisting (neglecting inductance/capacitance which changes slightly with geometry).

 

[Dale]

The "experiment" -- crude though it may be -- is as I've described it. If the primary conduit for energy transfer is the fields and not the wires, then the energy transfer (ie, the power, the volts and amps that reach the filaments) should be more sensitive to the circuit layout than they appear to be.

What is the length scale involved? What makes you think that the layouts that you have shown are any different than a straight wire for the energy transfer? What sensitivity to the layout is predicted and not observed? What specific measurements were taken that differ from what should have been measured?

You are simply assuming that the layout should change something without calculating anything and without even specifying what specific variable you think should change and how.

If, as he suggests, the energy transfer is via the fields (and not the wires), and if the fields are by definition sensitive to the relative sizes, lengths, and positions of the wires, then the performance of the circuit should be equally sensitive to changes in those sizes, lengths, and positions.

Two very different field configurations may still have similar Poynting fluxes at the load.

 

[Gerry Rzeppa]

You are simply assuming that the layout should change something without calculating anything...

Yes, that's exactly what I'm doing, because I've been told for years that the layout can and does change things. As I said earlier (some clarifications in bold):

"When I read practical engineering texts -- whether about filament windings or transmission cables -- I get the distinct impression that these fields around the wires are essentially unwanted side effects that we should strive to minimize by changing the layout (unless, of course, we're designing an antenna or a transformer or an inductive charger and thus want to maximize those side effects by changing the layout). On the other hand, the physicists appear to be telling me that these fields are not side-effects, but are the essential and primary means of energy transfer even in simple, hard-wired circuits."

Two very different perspectives, I think. I'm trying to figure out the best way to describe electricity and electrical circuits (like tube guitar amps) to a ten-year-old. So I'm qualitatively (as oppsed to quantitatively) evaluating different conceptual models with that ten-year-old in mind.

 

[Gerry Rzeppa]

If you consider wires to be wave guides, this might make more sense.

Yes, that thought did cross my mind. But there still appears to be a difference in perspective: are the fields the effect and the arrangement and movement of charges the cause? or it is the other way around? In another thread on this site from some time ago a poster was asking similar questions about electric and magnetic fields. I thought mentor jtbell gave a helpful response, the cricical statement in bold below:

In what sense is energy "real?" In what sense are electric and magnetic fields "real?" ... You're venturing (probably without realizing it) into very deep philosophical matters here. Electric fields, magnetic fields, energy, and many other things, are all concepts that we use to explain the observed relationships involving physical objects and their motions. But "energy" is not a substance in and of itself; it exists only in the context of two or more objects. We cannot observe energy in isolation, or measure it directly. We can only infer the amount of energy involved in any process by measuring the positions and locations of objects before, during, and/or after that process, and performing some calculations. Similarly for electric and magnetic fields. After all, we define the presence of electric and magnetic fields by the observation that certain objects placed in proximity to each other move in ways that cannot be explained by simple contact-type pushes or pulls. I'm not trying to denigrate or belittle these concepts, or suggest that there's any useful replacement for them. They're amazingly useful, and I would not want to try to do physics without them. But we need to keep in mind the dangers of excessive reification of the concepts that we invent to explain what we actually observe. That is, we need to be careful not to associate "too much reality" to them.

Makes me lean toward the former perspective (where the fields are effects and the arrangement and movement of charges are the causes).

BTW, there is a measurable difference between the twisted path and the straight path. When you examine it with the complex poynting vector you will find the twisted path has significantly higher losses. This all comes back to Maxwell's equations. Ohm's laws is a form of Maxwell's equations where some values are assumed and held constant, as are nearly all the formulas we use as electrical engineers. Thus we don't have to solve the field equations each time. But the field equations are still there, hidden by our simplified math. So knowing this we can take a short cut to find the extra losses in twisted field path. That short cut is to measure the additional resistance that the longer wire provides and find the I²R loss. Suppose we made the path straight and used the same length of wire? We can't but we could add a loop at the end. Then the losses in the loop would match the losses in the twisting (neglecting inductance/capacitance which changes slightly with geometry).

I like it. But it does make me think:

1. The "shortcut" method seems to attribute the difference in performance to the greater resistance of a longer wire, not to a difference in the size and shape and interaction of the various fields. The latter, I presume, would be included in the "inductance/capacitance which changes slightly with geometry" that you mention at the end, and which you imply is negligible (relative to the greater resistance of the longer wire).

2. The "shortcut" method, with its appeal to Ohm's Law (and thus E, I and R), implies that the important stuff -- the non-negligible stuff, the "action," so to speak -- is in the wires and not outside of them. Current is so-many electrons per second past a given point, and resistance is a physical property of the conductor. And it's easy to picture instantaneous energy transfer through a physical medium simply by referring the kid to a "Newton's Cradle" device without reference to "fields" of any kind.

So again I find myself, at least for now, leaning away from the field model of energy transfer.

 

[Dale]

I've been told for years that the layout can and does change things

What SPECIFIC things can and does the layout change?

I am challenging you to be specific because you have not thought clearly through your own objection. The thing that you are asking about is a correct description of energy transfer (The Poynting vector) in EM. Your objection to the correct description is focused on the shape of the fields. You have made no connection between the shape of the fields and the energy transferred by the field. You assert that the shape of the conductor should have some effect on something, but you have not thought it through enough to say what effect it should have nor how that relates to energy transfer.

By the way, whether wanted or unwanted, the fact that energy transfer between disconnected conductors can occur at all is clear disproof of the idea that the energy transfer occurs only within the conductor. That much should be qualitatively understandable by a 10 year old.

Which dominates is a quantitative rather than a qualitative question, but that question must acknowledge the fact that qualitatively there is at least some energy transported outside the wire.

 

[nsaspook]

Makes me lean toward the former perspective (where the fields are effects and the arrangement and movement of charges are the causes).

1. The "shortcut" method seems to attribute the difference in performance to the greater resistance of a longer wire, not to a difference in the size and shape and interaction of the various fields. The latter, I presume, would be included in the "inductance/capacitance which changes slightly with geometry" that you mention at the end, and which you imply is negligible (relative to the greater resistance of the longer wire).

2. The "shortcut" method, with its appeal to Ohm's Law (and thus E, I and R), implies that the important stuff -- the non-negligible stuff, the "action," so to speak -- is in the wires and not outside of them. Current is so-many electrons per second past a given point, and resistance is a physical property of the conductor. And it's easy to picture instantaneous energy transfer through a physical medium simply by referring the kid to a "Newton's Cradle" device without reference to "fields" of any kind.

So again I find myself, at least for now, leaning away from the field model of energy transfer.

Welcome. We've had this discussion elsewhere about how to explain electrical energy in a physics compatible way to children. The 'shortcut' method is not an alternative to fields being the energy carrier. Ohms Law is just a simplification when the geometry of conductors including their physical lengths is small when compared to the electrical lengths so energy transfer is from reactive fields around those conductors not EM radiation into space between conductors. There are many ways to use analogy without forming misconceptions but I'm dubious that you will listen to the good advice here. Please prove me wrong.

How do novices and experts model electric current? Stocklmayer and Treagust (1996) in their study concluded that children coming into high school have a vague and often fearful image of electricity such as they see „a fire‟ or „a dangerous animal‟ as similes for electricity. Once they encounter formal circuitry they acquire a mechanistic model of electron movement through a wire which, by analogy and metaphor, is closely allied to fluid transfer. Teachers of electricity have a strong belief that the particulate model is both useful and „correct‟. They themselves hold these images, and few have a strong field concept. Clearly, however, their students have difficulty with the model and hold misconceptions and alternative conceptions about circuit behavior which lead to difficulties with problem solving. The electron-transfer model is ubiquitous: it appears in all texts in Australia whether they are designed for physics or engineering students or for electricians. Experts such as electrical engineers and lecturers in engineering and physics, however, do not retain this particulate imagery. They regard electricity as a field-like phenomenon, formed of endless loops. Their understanding is inextricably tied to the concept of electricity as useful energy. Their vocabulary deals with the task, the load, the job, and the field. They find the micro-view of electron transfer irrelevant and, at times, confusing.

http://versys.uitm.edu.my/prisma/view/viewPdf.php?pid=17159

To say "But there still appears to be a difference in perspective: are the fields the effect and the arrangement and movement of charges the cause? or it is the other way around?" is irrelevant as there is a synergistic effect in circuits where both are important to explain how the energy transfer system works as a whole in detail for source -> wiring -> load. The guiding and manipulation of electrical energy in fields is the reason we have circuits. A wire without EM energy is just a lump of metal, EM energy moving in space is just a wave. It's when we combine the two that cool things happen. To me that what you need to teach to young children. Focusing on one without the other is confusing when you look at it closely because it's a incomplete picture like explaining human reproduction with just the male or female half of the equation.

 

[Jeff Rosenbury]

In AC circuits, the energy is in the fields. What the copper does is provide a place where currents can travel to make the fields.

Since the copper isn't a perfect conductor it "slows" the fields causing losses. It does this by slightly changing the direction of the resulting field so it is not completely parallel to the wire (in the math this is a complex/imaginary shift).

Some of the energy is directed to the wire rather than around the wire. That energy is lost as I²R heat. The amount lost depends on the amount of slowing which depends on how well the conductor can support the "eddy" current. (The words "eddy current" is usually used in magnetic type circuits and have some implications that this current doesn't, but the concept is similar.)

 

[Gerry Rzeppa]

What SPECIFIC things can and does the layout change?

Here are three that I've come across.

Quoting from Merlin Blencowe's Designing Tube Preamps for Guitar and Bass: "Pairs of noisy AC wires (eg., mains and heater feeds) should be neatly twisted so the opposing magnetic fields around each wire are forced to occupy the same space, causing them to cancel each other out." So one thing that a particular layout can apparently accomplish is the neutralization of the magnetic fields around two wires.

And here's a typical diagram explaining differences in transmission layouts:

So it seems a particular layout can also "cancel field induction" and "balance capacitances to ground."

...you have not thought it through enough to say... how that relates to energy transfer.

Seems to me that magnetic fields that have canceled each other out will transfer less energy than magnetic fields that have not canceled each other out. I would expect "cancelled field induction" and "balanced capacitances" to also affect energy transfer in some degree.

By the way, whether wanted or unwanted, the fact that energy transfer between disconnected conductors can occur at all is clear disproof of the idea that the energy transfer occurs only within the conductor.

Everyone with a radio knows that energy can be transmitted over long distances by electromagnetic fields; I'm not arguing that point. I'm saying that it's hard for me to image that the energy transferred by the fields outside the wires in, say, a guitar amp, is significant when compared with the energy that is transferred by the electrons inside the wires (transformers, of course, excepted).

That much should be qualitatively understandable by a 10 year old.

Sure. The part that's a hard sell is the idea that the energy that powers our electric drill out by the tree-house is being transferred from the AC outlet, not through the 50-foot extension cord, but via invisible fields around the cord. Why? Because he knows everything around the cord is safe (it's those wires inside the cord we need to be careful with).

And I don't think it will become easier to sell after the kid has been introduced to diagrams of fields in various configurations like this:

Mainly because he's going to remember that drill by the treehouse and how it behaved exactly the same whether the extension cord was straight, messy, or wrapped up in a coil for storage.

Which dominates is a quantitative rather than a qualitative question, but that question must acknowledge the fact that qualitatively there is at least some energy transported outside the wire.

Agreed. But the author of the paper referenced in my initial post doesn't say that. His statement is unqualified, and his example is a battery, some wire, and a light bulb. Yet he claims:

"It is this combination of electric field and magnetic field in the space outside the wires that carries the energy from [source] to [load]. Once the fields are set up, the energy travels through space, perpendicular to both the electric field and the magnetic field, at the speed of light."

If he had said, "In this example, the electric field and magnetic field in the space outside the wires carries a relatively insignificant amount of energy from the source to the load -- the significant portion is transmitted through the wire itself," we wouldn't be having this conversation.

 

[Gerry Rzeppa]

http://versys.uitm.edu.my/prisma/view/viewPdf.php?pid=17159

I'm not sure what your point was is referencing that paper. All I got out of it was what I already knew: the way the subject is and has been taught to novices doesn't work anywhere near as well as it should. It may be true that some "experts such as electrical engineers and lecturers in engineering and physics... regard electricity as a field-like phenomenon, formed of endless loops." But those are clearly not the experts who have been writing the books for kids.

To say "But there still appears to be a difference in perspective: are the fields the effect and the arrangement and movement of charges the cause? or it is the other way around?" is irrelevant as there is a synergistic effect in circuits where both are important to explain how the energy transfer system works as a whole in detail for source -> wiring -> load.

I understand that. That's why I think the end-to-end guitar amp system is such a good circuit for beginner study. It's got both currents and fields all over the place: in the pickups, in the transformers, in the speakers, in the capacitors. But while it's true that the currents can affect the fields, and the fields can affect the currents, I think it's helpful to avoid philosophical chicken-and-egg questions with beginners by adopting one perspective or the other from the start. Right now I'm leaning toward the "certain arrangements and movements of matter create various electric and magnetic fields" perspective rather than the converse "certain electric and magnetic fields create various arrangements of matter" approach. We understand that the fields can affect the matter -- but only because the matter generated the fields in the first place. But I'm flexy, believe it or not. Show me a decent field-oriented electronics text for ten-year-olds and I'll see how it works when we try to describe a guitar amp with it.

The guiding and manipulation of electrical energy in fields is the reason we have circuits.

That's one way of looking at it. The problem is that two of the three important nouns in that sentence -- energy and fields -- are abstract and unfamiliar. And not just to kids.

A wire without EM energy is just a lump of metal, EM energy moving in space is just a wave. It's when we combine the two that cool things happen.

But we can't "combine the two" until we have the two in hand. And the usual starting point is matter, not field; we start with a wire and magnet (a hunk of matter with the atoms arranged in a particular way); or we start with a couple of chemicals; etc. I don't think anybody starts with an electric field; where, without matter, would they get one?

Focusing on one without the other is confusing when you look at it closely because it's a incomplete picture like explaining human reproduction with just the male or female half of the equation.

We can tell the kid, in terms of your analogy, either (a) the male plants his seed in the female's garden; or (b) the female attracts seed for her garden via invisible and nearly irresistible forces. Both are true; but the former is more prosaic, more physical, and doesn't involve mystical forces and is thus an easier place to start. After all, every adult understands the mechanics of reproduction; but who can say he (or even she) fully understands the womanly wiles that surround it?

Again, I'm not suggesting that we should focus on one to the exclusion of the other -- that's impossible in the context of a guitar pickup or a power transformer or a speaker. I'm saying (particularly in this thread) that I don't see how anybody can say that the primary means of electrical energy transport between a battery and a bulb is a field and not a wire.

 

[Gerry Rzeppa]

In AC circuits, the energy is in the fields.

Are you saying the energy is somewhere else in DC circuits?

What the copper does is provide a place where currents can travel to make the fields.

That's where you lose me. Where is the energy that makes the current that in turn makes the fields? It's obviously not in the fields, since they haven't been made yet.

 

[Gerry Rzeppa]

The other day I stumbled on the "Lamin's Loaf Lorries Model" (https://www.iop.org/publications/iop/2013/file_61204.pdf) developed by the students at the College of Richard Collyer and published by the Institute of Physics. Here's the gist:

https://www.physicsforums.com/attachments/lorries-1-jpg.87013/
The thing that struck me is that this model, unlike most other beginner models, makes a clear distinction between electrons and the energy that is being carried by those electrons. It is thus easy for the student to see, as he invariably and intuitively suspects, that while the current is the same on both sides of the shop (load), there really is more of something to the right of the shop than there is to the left. Not more electrons, but more energy being carried by those electrons (and dropped off, as heat or light, in the load).

An added advantage is that volts -- and in particular, the unfamiliar units of measure associated with volts (joules per coulomb) -- can now be easily pictured: one volt is one loaf per lorry.

What do you folks think of this model for a beginner?

 

[Dale]

Quoting from Merlin Blencowe's Designing Tube Preamps for Guitar and Bass: "Pairs of noisy AC wires (eg., mains and heater feeds) should be neatly twisted so the opposing magnetic fields around each wire are forced to occupy the same space, causing them to cancel each other out." So one thing that a particular layout can apparently accomplish is the neutralization of the magnetic fields around two wires.

Probably a book on guitars is not the best source for learning physics. This particular quote is simply wrong. You cannot neutralize the magnetic field by twisting two conductors around each other.

What you can do is to concentrate the magnetic field. It becomes less intense away from the twisted pair, but more intense between the pair. This concentration of the magnetic field, in turn concentrates the Poynting vector, meaning that a bigger fraction of the energy transferred flows through the fields between the pair rather than the fields around the twisted pair.

The conductor configuration with the most concentrated possible magnetic field is called a coaxial cable. It completely eliminates the magnetic field outside the conductors with no stray magnetic field at all. But there is still a very strong magnetic field between the two conductors. Thus there is still a place for energy to be conducted by the fields. Again, in even the most extreme conductor configuration, the magnetic field is concentrated, not canceled.

The observed behavior of a twisted pair or a coaxial cable is entirely consistent with the proposed idea that the energy is transferred through the fields around the wire. If it were transferred only through the conductor then the conductor geometry would make no difference.

I'm not arguing that point. I'm saying that it's hard for me to image that the energy transferred by the fields outside the wires in, say, a guitar amp, is significant when compared with the energy that is transferred by the electrons inside the wires (transformers, of course, excepted).

That is a quantitative question, not a qualitative question. Are you interested in a quantitative answer? I don't have one for a twisted pair, but I do have one for a coaxial cable which is even more concentrated than a twisted pair.

Sure. The part that's a hard sell is the idea that the energy that powers our electric drill out by the tree-house is being transferred from the AC outlet, not through the 50-foot extension cord, but via invisible fields around the cord. Why? Because he knows everything around the cord is safe (it's those wires inside the cord we need to be careful with).

That is because it is not the EM energy flux that is dangerous, it is the current. The energy transferred by the field is given by the Poynting vector. But for safety what is important is the work done on matter. Energy that simply transfers in and out of the body without doing any work on the tissues causes no harm. You cannot feel energy flux, only energy deposition.

The work done on matter is given (microscopically) by $E\cdot J$, and because tissues are Ohmic the $E$ is proportional to $J$. So the dangerous part of the electricity, the current, is indeed contained within the wires.

If he had said, "In this example, the electric field and magnetic field in the space outside the wires carries a relatively insignificant amount of energy from the source to the load -- the significant portion is transmitted through the wire itself," we wouldn't be having this conversation.

Why do you assume it is an insignificant fraction when you have not done any quantitative analysis? You have no justification for thinking that your proposed statement is correct.

You clearly understand that some fraction is outside the wire, but you have no reason to claim that it is insignificant. You are going from a qualitative statement "some energy is transmitted by the fields" to a quantitative statement "it is an insignificant amount". The latter is completely unjustified unless you have actually worked the problem.

 

[Dale]

What do you folks think of this model for a beginner?

It is untenable.

First, which side carries the loaves? Since the negative sign on the electron charge is just a convention, the loaves (if they are actual physical things) cannot depend on that convention. And since assigning one side to be 0 volts is also a convention it cannot depend on that either.

Second, the velocity of the charge carriers is incredibly slow, on the order of mm/hr. So if you attach a 5 V source and fill up those charge conductors in the wire with 5 V loaves of energy then if you switch to a 10 V source it should take several hours before the new high-energy loaves reach the load.

Third, for AC circuits the charge carriers never move more than a millimeter (usually far less), so you would never be able to transport loaves attached to electrons from the power plant to your house using AC.

 

[Jeff Rosenbury]

You seem to have a misunderstanding of energy. That's not surprising since few do (if anyone does). Energy is a form of stability (or perhaps anti-stability). It causes objects to assume more stable formations. Thus objects with a high field potential seek lower potential.

Think of a steel ball on to of a cliff. The rock has energy because it is higher in the gravity field. Does that make the energy in the field? Or does the ball have the energy? That seems a philosophical question. To me the energy seems to be in the relationships.

Now suppose we attach the ball with some others to a two string pendulum. You have likely seen such a toy. If you drop one ball, the energy transfers to another. Again. is the energy in the balls, or the field? To me it seems to take both.

But unlike in gravity, in electricity there are two fields. One is electric and one magnetic. Energy can exist in the charges being higher in the electric field like the ball on a cliff. But it can also exist in a relationship between the electric and magnetic field without any steel balls (charges). We call these self supporting packets of energy photons. Since the energy can exist solely in the fields, but cannot exist solely in the charges, we tend to model the energy as being in the fields. (And remember electrons have little mass and move slowly, thus carry little energy under normal circumstances.)

Please try to understand Maxwell's equations. Understanding how a time varying magnetic field causes an electric field and a time varying electric field causes a magnetic field is essential for understanding electricity on the level you are trying to grasp.

The math for a DC circuit is different since the Poynting vector is designed for use with AC (RF) circuits. Still, there is an electric field fringing from the wire and a magnetic field around the wire. The energy transfer is the cross product of the fields. But with DC there is also an electric field along the wire. Its cross product leads into the wire and causes I²R heating.

On a meta level, we are describing part of something called the "standard model" of physics. While the standard model has some holes in it, it is remarkably consistent with our observations of the universe. More importantly though this website is dedicated to teaching the standard model. While there are millions of other models, we only discuss the standard model here.

 

[nsaspook]

Again, I'm not suggesting that we should focus on one to the exclusion of the other -- that's impossible in the context of a guitar pickup or a power transformer or a speaker. I'm saying (particularly in this thread) that I don't see how anybody can say that the primary means of electrical energy transport between a battery and a bulb is a field and not a wire.

We say it because it explains what we measure when we build circuits. When we design circuits on paper with these rules, run the math and finally measure a real circuit built from those rules the measurements match the theory of energy flow in fields. In a vacuum tube what happens to the KE of the accelerated electrons inside the tube? Does it go to the speaker as electrical energy to create sound or is it dissipated as heat on the tubes plate? The vast majority of anode heat comes from the kinetic energy of electrons as they strike the anode warming it instead of transporting electrical energy past that point in the circuit.

 

[Gerry Rzeppa]

Probably a book on guitars is not the best source for learning physics.

Ah, but Merlin's book is no ordinary book on guitar amps. It's chock-full of all those formulas you guys like to talk about.

You cannot neutralize the magnetic field by twisting two conductors around each other. What you can do is to concentrate the magnetic field.

Seems to me (and apparently to Merlin) that it depends on exactly how the conductors are twisted. If the conductors are twisted in such a way that the fields are "in phase" with each other then yes, we would expect concentration; on the other hand, if they're twisted in such a way that they are "out of phase" with each other, we'd expect cancellation. In fact, this transmission line illustration specifically speaks of cancelled, not concentrated, induction:

I'm sure you can see the problem a beginner like me faces. Whom should I believe? Why?

 

[Gerry Rzeppa]

It [the Lamin's Loaf Lorries Model] is untenable.

Let's see if we can't save some time. Exactly what is the ideal conceptual model of electricity for a ten-year-old trying to understand a vacuum-tube guitar amp?

(Jeff, nsaspook, and anyone else please feel free to answer as well. In fact, a concurrence on this matter would be most helpful.)

 

[analogdesign]

Let's see if we can't save some time. Exactly what is the ideal conceptual model of electricity for a ten-year-old trying to understand a vacuum-tube guitar amp?

(Jeff, nsaspook, and anyone else please feel free to answer as well. In fact, a concurrence on this matter would be most helpful.)

A few of the people on here disagree with me, but I think the hydraulic model of electricity is highly pedagogical and useful. https://en.wikipedia.org/wiki/Hydraulic_analogy

It was developed largely by Oliver Heaviside himself. It's important to keep in mind that it has limitations. Physicists tend not to like it (and I understand their complaints) but I think it highlights in some ways the difference between physics and engineering. While it is Maxwell's equations that govern what happens in electrical circuits the very essence of Electrical Engineering is to simplify physics with models to the maximum extent that still allows us to build functional apparatus. That's why we use things like Ohm's law, the square law approximation for vacuum tubes and MOSFETs, the hybrid-pi model and so on.

 

[analogdesign]

a ten-year-old trying to understand a vacuum-tube guitar amp?

I don't know if this will be helpful but when I was 15 I learned about vacuum-tube guitar amps (and solid-state amps) and this is the model I learned which was more than sufficient to understand largely what was going on.

I was told to think of the tube as a kind of valve on a dam that was holding back a large resevoir of electricity (the power supply). The signal from the guitar that impinged on the plate caused the valve to open and close letting out small amounts of water from the dam. It was easy enough for me to imagine a small signal (my hand opening and closing a valve) turning into a much large signal (water rushing over the spillway in time with my manipulations of the valve).

Anything "deeper" or more correct than this will probably be overkill and will cloud the issue. Ask yourself what are you trying to achieve explaining a tube amp to a 10 year old and then select your model based on the principle of "fitness to purpose".

 

[Dale]

Seems to me (and apparently to Merlin) that it depends on exactly how the conductors are twisted. If the conductors are twisted in such a way that the fields are "in phase" with each other then yes, we would expect concentration; on the other hand, if they're twisted in such a way that they are "out of phase" with each other, we'd expect cancellation.

OK, so show me the calculations. How exactly can one twist a conductor so as to cancel the field everywhere rather than concentrate the field somewhere? I don't know of any possible configuration of conductors that will accomplish this, but if you know "such a way" then by all means please present it.

I'm sure you can see the problem a beginner like me faces. Whom should I believe? Why?

Who you choose to believe is up to you. I typically don't choose to believe anyone. I prefer to learn the math so that I can either work it out for myself or follow someone else's math. Here is an example of the problem worked out quantitatively for a coaxial line: http://depa.fquim.unam.mx/amyd/arch...ia_a_otros_elementos_de_un_circuito_20867.pdf
Here is a good page on Poynting's theorem:
http://web.mit.edu/6.013_book/www/chapter11/11.2.html

 

[Dale]

Exactly what is the ideal conceptual model of electricity for a ten-year-old trying to understand a vacuum-tube guitar amp?

That I don't know. I have never tried to teach EM to a 10-year-old. All I can tell you is that the conductor-only model is fundamentally incompatible with basic experience, as we agreed earlier. There must be at least some energy transmission which occurs via the fields outside a conductor.

This is clear from interference along wires, energy transfer across capacitors, line isolation transformers and power transformers, as well as from other technologies like radio and microwaves. In each of these cases there is clearly energy transfer outside of a conductor. Your 10-year-old is familiar with all of these technologies, so they will be able to understand the concept that energy cannot be limited to the conductor only. Since there must be some energy transmission through the fields outside of the wire they simply cannot avoid the fields if they want to understand the energy transmission.

In contrast, the field model is compatible with experience, calculations, and experimental evidence. It says that energy can be carried from one conductor to another through the fields surrounding each, which agrees with the experience of interference between cables. It says that the fields are larger the more energy is being transferred, which agrees with the experience that you get more interference from 60 Hz power cables than any other cable. It says that placing cables perpendicular will result in less transfer than cables parallel, which also agrees with basic sound-system experience. It says that a coaxial cable will not have any energy transfer outside of the cable, which agrees with experience as well.

Your objections to the field model boil down to "it's hard for me to imagine", and "a guitar book says" as well as a variety of claims about the field model where you simply misapply it and draw erroneous conclusions about what should happen in a given situation. Instead of wasting all of this effort trying to avoid it and mischaracterize it, why don't you learn what it actually says. Then, if you still choose to reject it, at least you will have an informed opinion on the matter, rather than just conjecture and confusion.

 

[Gerry Rzeppa]

That I don't know. I have never tried to teach EM to a 10-year-old. All I can tell you is that the conductor-only model is fundamentally incompatible with basic experience, as we agreed earlier.

Agreed, the conductor-only model of energy transfer is fundamentally incompatible with basic experience (because we all have experienced, for example, radios).

But do we agree that the field-only model energy transfer (as described by the author of the paper cited in the initial post of this thread) is also incompatible with basic experience? I'm pretty sure most people would think that more of the energy that goes from battery to bulb is transferred via the wires than the fields.

There must be at least some energy transmission which occurs via the fields outside a conductor.

Sure. But in the case of, say, the supply lines to the tube filaments in a guitar amp, it seems that the bulk of the energy transfer is taking place in the wires and not around them. The fields, in this instance, really do appear to be nothing more than unwanted side effects of the current flow in the wires. Yes?

 

[nsaspook]

Let's see if we can't save some time. Exactly what is the ideal conceptual model of electricity for a ten-year-old trying to understand a vacuum-tube guitar amp?

(Jeff, nsaspook, and anyone else please feel free to answer as well. In fact, a concurrence on this matter would be most helpful.)

The same model I used with my 10yo. A simplistic version of the the field model was used because I won't have to unlearn mental patterns created with hydraulic or other particlistic models. She accepted it verbatim like most children do with new information at that age and now uses her fingers to make little wave patterns that move across space when we talk about electricity.

 

[Gerry Rzeppa]

Ask yourself what are you trying to achieve explaining a tube amp to a 10 year old and then select your model based on the principle of "fitness to purpose".

What I want to leave the kid with is a simple-as-possible conceptual model of electricity that will let him picture current, voltage, resistance, capacitance, inductance, reactance, and electric and magnetic fields in a unified and useful way.

...I think the hydraulic model of electricity is highly pedagogical and useful.

A simplistic version of the the field model.

That I don't know. I have never tried to teach EM to a 10-year-old.

Okay, that sounds like one in favor of the hydraulic analogy, one for a simplistic version of the field model, and one abstention. Anyone else?

 

[analogdesign]

The same model I used with my 10yo. A simplistic version of the the field model was used because I won't have to unlearn mental patterns created with hydraulic or other particlistic models. She accepted it verbatim like most children do with new information at that age and now uses her fingers to make little wave patterns that move across space when we talk about electricity.

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. For example, the "solar system" model of an atom is not correct, but to me it makes it easier to understand how the Pauli exclusion principle leads to a quantum theory of solids.

I think it boils down to "what are you trying to achieve". 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. The wavelengths of the signals in a guitar amplifier will be smaller than the conductors (and components) country mile and that is what circuit theory is all about, simplifying Maxwell's Equations into Ohm's and Kirchoff's laws and then solving the circuit.

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.

 

[Dale]

But do we agree that the field-only model energy transfer (as described by the author of the paper cited in the initial post of this thread) is also incompatible with basic experience?

No. I have no personal experience nor am I aware of any experiment which is incompatible with the field model. We have discussed several examples already, and so far no incompatibility with experience has been shown.

I'm pretty sure most people would think that more of the energy that goes from battery to bulb is transferred via the wires than the fields.

Most people may think that, but they probably are making the same mistake that you were regarding thinking that it is the energy flow which is dangerous when it is actually the current which is dangerous.

it seems that the bulk of the energy transfer is taking place in the wires and not around them.

That is a quantitative claim which you have not substantiated. Please provide a reference which performs this quantitative calculation and supports your claim. The only reference I have reaches the opposite conclusion.

 

[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.