Current Flow

[mivey]

you are not offering any improved version charlie analoy either.

My version was in post #10.

My offer to the OP in #11 was a reference that discussed these matters in detail or an offer to answer questions. Without hearing from the OP I have no idea of his knowledge level and if he needs more simplification or more detail.

 

[ggunn]

mivey:You foiled all my attempts on reconciliation.

Pistols at dawn!

 

[mivey]

Pistols at dawn!

Super soakers at five paces!

I've noticed sahib takes controversial positions at times and gets a lot of heat now and then. Not that there is anything wrong with that. My quip was a humorous observation, nothing more.

 

[Sahib]

The fact that you actually supports the idea of electrons carting the energy arQqttound should be motivation enough.

True enough electromagnetic field travels at the speed of light but electroms motion in the cpnductor negligibly slow. Yet the field along the conductor is linked to those electrons in the conductor and so its energy.

 

[charlie b]

mivey, I have to dispute your statements to the effect that my simplification is fundamentally flawed. In particular, the notion of electrons picking up energy in the source and dumping it into the load is not so far from the truth that it would require some difficult unlearning later in the student?s studies. I think you can tell the students that individual electrons do not travel far, and that the nature of their true motion will be presented later. At that point, I would also mention that from the viewpoint of a person watching the circuit do its thing, you cannot discern the difference between one electron and another, so it really does not matter which electron moves how far in any span of time. In the meantime, you can say that you will treat current as though it consisted of electrons racing around the circuit, knowing that that is a simplification that will be dealt with at a later date, and then go on to give the students an understanding of ?complete circuits,? and AC versus DC, and how a capacitor works despite not including a metal-to-metal connection internal to the capacitor, and the concept of series versus parallel connections, and a host of other basic concepts. We don?t ever need to apply the concept of drift velocity when we calculate service load, or size motor feeders, or demonstrate selective coordination. Once having been mentioned in a five minute classroom interval, it can just rest in the deep background of our minds, never again to come to the forefront.

But let me get back to the filling and dumping of the coal cars. Please note that I did not introduce that analogy, and allow me to say I don?t like it that much. But two things are true, and are not simplifications: (1) Current is constant in a series circuit, and (2) Voltage drops along the wires heading to the load, drops a whole bunch through the load, and drops a bit more in the wires heading back to the source, and rises again within the source. You will not disagree, I am sure, that energy that was once in the source is now in the load, and that therefore a transfer has taken place. It is equally clear that the motion of charge was deeply involved in causing that transfer to take place, and that the charge that was in motion is the negative charge possessed by electrons. Finally, it is an unsimplified truth that the product of voltage (that changes throughout the circuit) and current (that does not change) represents the generation, transfer, or consumption of power, and that power is the rate at which energy is supplied by the source or used by the load. So I am willing to stich with my simplification that describes current as though it were a race of electrons around a circle.

 

[charlie b]

This is akin to saying when I speak I shoot air molecules out of my mouth at some 700+ mph and they crash into your ear drum for you to hear.
. . . .
Same vein as saying water molecules travel at hundreds of mph and crash into the shore then return to the ocean to get more energy for the next wave.

I agree that those two analogies would not be good or effective simplifications of the way electrical current works. But I disagree that either one is anything like the simplification that I had described. Current requires a complete path. Neither sound nor ocean waves requires a complete path. The question that sometimes arises is, if the utility sends current into a house and then the same current goes back to the utility, why does the homeowner pay for something that the utility gets to keep? The answer to that question is that the utility is depleted of an asset, and that the homeowner gains that asset, and that the asset is energy. The next step is to explain how the energy that the utility once possessed has somehow made its way into the homeowner's possession.

 

[mivey]

The best non-electrical analogy is the pump and pipes.
The source of pressure (analogous to voltage) pushes water down the pipes. The flow is current.
More pressure from the pump results in more flow, all other things being equal.

One weakness of this analogy is that I can take water from a lake and push in into a tank, and the water never has to return to the lake. Electrical current cannot flow from one place to another, without a return path.

The weakness is helped by considering a closed pump system rather than an open system, like a hydraulic system transferring energy from the pump to the load.

 

[mivey]

True enough electromagnetic field travels at the speed of light but electroms motion in the cpnductor negligibly slow. Yet the field along the conductor is linked to those electrons in the conductor and so its energy.

Why the need to equate charge flow and energy flow? Why the need to equate the wave with the medium? Why equate energy flowing in one direction with particles flowing in multiple directions?

Trying to equate charge flow with energy flow contradicts fundamental concepts. There is no good reason to do it. They are different things, just like sound and air are different things.

The energy travels in the EM wave and is coupled with the charges but the charges do not carry the energy around. The kinetic energy of the charges is fractional compared to the energy traveling down the circuit. The circuit acts like a waveguide for the EM wave. A guide, not a bucket.

 

[mivey]

mivey, I have to dispute your statements to the effect that my simplification is fundamentally flawed. In particular, the notion of electrons picking up energy in the source and dumping it into the load is not so far from the truth that it would require some difficult unlearning later in the student?s studies.

Educators disagree with you and so do I. No hard feelings though: to each his own.

...In the meantime, you can say that you will treat current as though it consisted of electrons racing around the circuit, knowing that that is a simplification that will be dealt with at a later date..
We don?t ever need to apply the concept of drift velocity when we calculate service load, or size motor feeders, or demonstrate selective coordination...

I see no good reason to plant that false seed of electrons racing around a circuit that will have to be unlearned later. I agree there is no reason to discuss drift velocity at an early stage.a

But let me get back to the filling and dumping of the coal cars. Please note that I did not introduce that analogy, and allow me to say I don?t like it that much.

Electrons picking up energy and dumping at the load is the same thing.

But two things are true, and are not simplifications: (1) Current is constant in a series circuit

Yes. So the concept of electrons losing energy around the loop as they travel is fundamentally flawed. The ones leaving the battery have the same energy as the ones returning.

Voltage drops along the wires heading to the load, drops a whole bunch through the load, and drops a bit more in the wires heading back to the source, and rises again within the source. You will not disagree, I am sure, that energy that was once in the source is now in the load, and that therefore a transfer has taken place. It is equally clear that the motion of charge was deeply involved in causing that transfer to take place, and that the charge that was in motion is the negative charge possessed by electrons.

The electric and magnetic waves carry the energy. They are created by the separation of charges and the movement of charges. Electron's negative charges not the only charges involved and are not the energy carriers. You have to distinguish between the propagation medium and the energy wave that travels through the medium.

Finally, it is an unsimplified truth that the product of voltage (that changes throughout the circuit) and current (that does not change) represents the generation, transfer, or consumption of power, and that power is the rate at which energy is supplied by the source or used by the load.

They are indeed a means to measure what is happening with the wave propagating through the system. That does not support "the notion of electrons picking up energy in the source and dumping it into the load" as being close to the truth.

So I am willing to stick with my simplification that describes current as though it were a race of electrons around a circle.

You should know better now. I will leave you with this quote to consider:

I know that most men - not only those considered clever, but even those who are very clever and capable of understanding most difficult scientific, mathematical, or philosophic, problems - can seldom discern even the simplest and most obvious truth if it be such as obliges them to admit the falsity of conclusions they have formed, perhaps with much difficulty - conclusions of which they are proud, which they have taught to others, and on which they have built their lives.

"What Is Art" - Leo Tolstoy, opening line of Chapter 14, Aylmer Maude translation, 1899

 

[Sahib]

Any acceleration of a charge carrier, whether it changes the speed or not, will produce radiation.
Once you get to a high enough energy range you have to use linear accelerators instead of circular ones for electrons. Otherwise it would radiate as much energy on each lap as you put into it on each lap.

You mean Newton first law of motion invalid for electrons motion?

 

[mivey]

I agree that those two analogies would not be good or effective simplifications of the way electrical current works. But I disagree that either one is anything like the simplification that I had described. Current requires a complete path. Neither sound nor ocean waves requires a complete path.

The notion of electrons picking up energy and dropping it off is the one that is like the sound and ocean analogy.

The question that sometimes arises is, if the utility sends current into a house and then the same current goes back to the utility, why does the homeowner pay for something that the utility gets to keep?

Thus arises the problem with teaching that electrons pick up energy and drop it off. Had we never started them down that path they would not have to unlearn it.

The answer to that question is that the utility is depleted of an asset, and that the homeowner gains that asset, and that the asset is energy. The next step is to explain how the energy that the utility once possessed has somehow made its way into the homeowner's possession.

Yes. But why not introduce the concept of energy from the very beginning? Any child in a pool of water (or trampoline, etc.) can easily see a demonstration of waves and energy. It is not too difficult to approach at the elementary level. There is no need to go down the path of an electron carting energy around as it only leads to confusion later.

Stating that there is a sea of electrons in metals is a good enough start for discussing electric wave mediums. Drift velocity and what not do not even need to be mentioned. The fact that the water does not race across the pool but the energy does is a good start for fundamental concepts.

A simple belt and pulley system can demonstrate a circuit loop. The belt fibers (free electron surrogates) do not have traverse the loop for energy to reach the other side. There is no reason to have them thinking the electrons have to race around the loop at energy delivery speeds. It just leads to confusion later and we have plenty of examples that are not as misleading.

 

[Sahib]

I've noticed sahib gets a lot of heat now and then.

Here is cold now. Would like to be near hot sources.

 

[mivey]

Here is cold now. Would like to be near hot sources.

What? The 80's is cold? Normal in India must be like an oven.

 

[Besoeker]

Seems to me we have drifted seriously off course.
This from the OP:

electrical wiring diagrams

Note wiring diagrams.

 

[Sahib]

What? The 80's is cold? Normal in India must be like an oven.

I am sitting in an air-conditioned room.

 

[mivey]

Seems to me we have drifted seriously off course.
This from the OP:


Note wiring diagrams.

See OP post #8.

 

[mivey]

I am sitting in an air-conditioned room.

Well then there's that.

 

[Sahib]

Why the need to equate charge flow and energy flow? Why the need to equate the wave with the medium? Why equate energy flowing in one direction with particles flowing in multiple directions?

Trying to equate charge flow with energy flow contradicts fundamental concepts. There is no good reason to do it. They are different things, just like sound and air are different things.

The energy travels in the EM wave and is coupled with the charges but the charges do not carry the energy around. The kinetic energy of the charges is fractional compared to the energy traveling down the circuit. The circuit acts like a waveguide for the EM wave. A guide, not a bucket.

I did not equate charge flow and energy flow; just stated that energy flow corresponds to change in electrons mass as the field moves along the conductor and in that sense the electrons in the conductor 'carry the field and so its energy'.

 

[mivey]

I did not equate charge flow and energy flow; just stated that energy flow corresponds to change in electrons mass as the field moves along the conductor and in that sense the electrons in the conductor 'carry the field and so its energy'.

Not enough mass to get it done.

Even if for some strange reason we would group the energy stored in the magnetic field with the moving electron QM mass (and ignoring that for other type conductors we have charge carriers that display less QM type behavior than electrons), we have left out the energy stored in the electric field.

 

[mivey]

I did not equate charge flow and energy flow; just stated that energy flow corresponds to change in electrons mass as the field moves along the conductor and in that sense the electrons in the conductor 'carry the field and so its energy'.

The energy propagates through the field connected to the electrons. It does not move along with the electron and thus does not move along with the current.

Think of these:

Think as if the source is trying to accelerate all of the charge carriers in the loop at the same time and the load is trying to slow them all down. The net result of the opposing actions is the constant current throughout the loop. That is the magnetic field energy portion.

Think of the source separating positive and negative charges and creating an electric field that acts like a capacitor where we can store and remove energy. The load drains energy from this capacitor and the source keeps it charged. This is the electric field energy portion.

You can have voltage and no current, or current and no voltage but current and voltage separately will not result in an energy exchange. To have a net energy exchange from the source to load you have to have both voltage and current (both the electric field and the magnetic field).

The energy flows in the EM fields, not along with the current.