Does the pointing vector theory hold true for DC current?

[Antiphon]

What? I wasn't referring to photons, I was talking about the emf through a conductor. Can you point to anything incorrect in my post or not?

Not referring to photons? What do you think the power plant is sending into your house? It's photons.

This doesn't make any sense to me. The force doesn't just magically get into the air and wire, it has to have charges carry it.

This is incorrect. See the sunlight.

If you apply a potential to the conductor on one end, it influences the electrons so that the average movement is in net direction. Each electron only has to move a very very small distance, but the voltage or emf or whatever travels down the conductor very quickly.

This is correct but it's missing the true mechanism of power flowing. The flow of power down a pair of wires is not even a little bit analogous to sound or pressure in a pipe.

Simplest example: perfectly conducting wires. There's no component of the electric field aligned along the wire. It's totally perpendicular to the wire. The power flow is given by the integral of \vec{E} X \vec{H} over a closed surface. If you integrate this in the wires it's zero. If you integrate in the air around the wires it's the power flowing down the line.

Repeat: the energy flowing down a pair of wires travels in the space between the wires as photons and not in the wires as charge x field x distance.

 

[berkeman]

Thread closed overnight...

Okay, re-opening the thread for now. Please keep the discussion civil and technically accurate.

For AC power transmission where the dimensions of the transmission line are comparable to a wavelength, then yes, the power is largely transmitted as an EM wave.

But for DC power transmission, or low-frequency AC transmission (where the distribution system's size is much less than a wavelength), then the power is not being transmitted by "photons".

 

[Per Oni]

How can the electrons not be pushing other electrons? How else would the emf or voltage or whatever move down the line?

Hey folks, those of you who think electrical power is transported by electrons through a wire analogous to water in a water hose, please consider the following:
We know the drift speed of conducting electrons is very low. Say Vd=10^-3 (in fact it’s even a little below this value). Say I want to drive a motor of a modest 1KW dc with 1Amp. Please work out first of all the force required in the connecting cable to be able to develop such a power with this velocity. (A touch high?)
Next look up what size cable is needed for a 1 amp current . In this case, for copper wire the current density can be ~ 10^7 /M^2. From calculated force and wire surface area, a pressure can be calculated. ( A bit steep?)
This example is by no means extreme. Please use instead real existing values for a 100W light bulb in your own home. DO IT!

 

[Evil Bunny]

Where does the current go then, when I run electrical devices? If it is not used up when I turn on a light bulb or use the TV, where does it go?

It goes back to the source. It leaves one side of the source, and the same amount returns on the other side of the source. This never changes...

It's called a ground for a reason.

Incorrect. It does not flow into the ground. It only flows back to the source.

So then what do electric companies charge you for?

You're paying for the energy it took to move the electrons from one side to the other. Back at the generator, some sort of prime mover did work to move the electrons through all your appliances. You pay for that. EDIT: And you pay for the infrastructure to get it from the generator to your house, of course...

 

[russ_watters]

Hey folks, those of you who think electrical power is transported by electrons through a wire analogous to water in a water hose, please consider the following:
We know the drift speed of conducting electrons is very low. Say Vd=10^-3 (in fact it’s even a little below this value). Say I want to drive a motor of a modest 1KW dc with 1Amp. Please work out first of all the force required in the connecting cable to be able to develop such a power with this velocity. (A touch high?)
Next look up what size cable is needed for a 1 amp current . In this case, for copper wire the current density can be ~ 10^7 /M^2. From calculated force and wire surface area, a pressure can be calculated. ( A bit steep?)
This example is by no means extreme. Please use instead real existing values for a 100W light bulb in your own home. DO IT!

What are you trying to say? Are you suggesting that electrical power is really mechanical power like water? That's not at all intended to be implied by the analogy. It's an analogy.

 

[Drakkith]

Hey folks, those of you who think electrical power is transported by electrons through a wire analogous to water in a water hose, please consider the following:

As Russ said, it isn't the same. The electrical force is pushing electrons down the entire conductor. And in reality, the force is pushing the electrons, but it is attracting the protons in the conductor. Since the protons are not mobile they cannot move, and the electrons move by themselves. The key here is that it is the FORCE that causes the electrons to move. Everything about "energy" or "power" flowing here and there and all that is irrelevant and misleading, and a look at the definitions of both will immediately tell you that. This whole thread is about current, the movement of charges. How much work has been performed, or where the energy came from doesn't matter.

 

[Antiphon]

The Poynitng vector definition of power flow is the only one well-defined here. Berkeman, it applies to the DC case as well.

See paragraphs 1-8, speficially 8 for a description of how to carry out the integration.
http://personal.ee.surrey.ac.uk/Personal/D.Jefferies/poynting.html

Yes, the thread was about current. But the topic of power came up because of the false notion that (in a DC case even) current flow happens by the electrons pushing on one another. But this is not correct.

In the steady state DC case of zero resistance there is zero electric field vector in along the wire inside and out. Drak, there simply is no electric force pushing current down the wire. The current, once established, moves under it's own momentum (contained NOT in the electron mass but in the magnetic field.)

The ONLY way to mathematically compute the power flow is via the fields in the space around the wire. It is extremely suspect to claim that the power is confined to the wires where there is only a static magnetic field at best and zero electric field.

 

[Per Oni]

What are you trying to say? Are you suggesting that electrical power is really mechanical power like water? That's not at all intended to be implied by the analogy. It's an analogy.

Ok we both now that electrical power is not like water through a hose.

My point is however that you and others (using this analogy) want to transport power down a wire. According to you, this power is transported via electrons flowing through a fairly thin conductor in such a way that one electron pushes the proceeding one and gets pushed by the one behind.

I just want to stop here and see whether this is your point of view.

(I am sorry that I cannot respond quickly but mostly have a short time in the UK evening for posts).

 

[Drakkith]

Drak, there simply is no electric force pushing current down the wire. The current, once established, moves under it's own momentum (contained NOT in the electron mass but in the magnetic field.)

I have been looking up this phenomena due to this thread and another. I'll have to get back to you.

 

[Per Oni]

In conclusion:

Just some points to explain why I’m so opposed to the fairly innocent notion of the “water hose / bicycle chain” analogy. My reason is that it only perpetuates the idea that energy/power to a load flows through a wire. That really is a ridiculous notion as I have shown. Mind however that some of the very biggest names have slipped up on this one.

There really is no alternative to this problem then using the theory as set out by the likes of Antiphon. To me this theory makes sense in an actual physical way but also mathematically. Now, there are not many good web sites or books I can refer to, just because strangely enough it’s still a controversial idea. I’m also not 100% convinced of all the points that Antiphon has raised, such as the photons and momentum but no doubt science will crack those points eventually.

 

[Drakkith]

Let me ask of you this. Can you explain how it works WITHOUT using either of the terms energy or power? Neither of them really explain anything to me. (And I don't think they either of them can "flow" since they aren't physical objects)

 

[sophiecentaur]

Let me ask of you this. Can you explain how it works WITHOUT using either of the terms energy or power? Neither of them really explain anything to me. (And I don't think they either of them can "flow" since they aren't physical objects)

Firstly, is anything, a truly 'physical object' except in as far as you can sort of identify it as having a localised region of influence? Is the wave function of an electron when it's in a bound state 'really' a physical object?

If someone 'generates' some Power in a Power Station and someone uses that Power in their home then something can be said to have 'flowed' from A to B.

Surely this thread can't get us anywhere because all we are doing is comparing various models, some of which are better than others.

 

[Drakkith]

Firstly, is anything, a truly 'physical object' except in as far as you can sort of identify it as having a localised region of influence? Is the wave function of an electron when it's in a bound state 'really' a physical object?

If someone 'generates' some Power in a Power Station and someone uses that Power in their home then something can be said to have 'flowed' from A to B.

Surely this thread can't get us anywhere because all we are doing is comparing various models, some of which are better than others.

We can start by using terms that actually describe something useful. Saying power flows through the line says absolutely nothing about what is occurring unless you already know how it works. And please, spare the "is anything truly physical" BS.

Surely this thread can't get us anywhere because all we are doing is comparing various models, some of which are better than others.

How else would you get to the answer? Comparing models and asking questions about them seems like a very good way to learn.

 

[sophiecentaur]

"The answer"?
I think this thread is not about answers as much as 'use mine'. But I suppose that goes for many of our threads.
The question of whether or not Power flows 'through' or 'along' or 'because the line is there' is one of those 'really' questions. Without some current flowing, there is no power transfer and so it seems to be reasonable to associate the two. I did wonder about the comment on the Power being due to magnetic fields - that doesn't take the transmission voltage into account or the fact that DC through a Transformer winding (high B field) would, somehow, be different from power along a straight wire of the same resistance.

I do agree that the 'pushing electrons through' / quasi hydraulic idea has to be dodgy.

 

[Drakkith]

Without some current flowing, there is no power transfer and so it seems to be reasonable to associate the two.

Which is why I am asking to avoid talking about energy or power. Neither of those terms refer to anything specific, but only to the overall effect. I KNOW power flows from here to there or whatever, but I don't know how. And I agree that there are no absolute answers, but only answers of varying accuracy.

 

[Per Oni]

"The answer"?
I did wonder about the comment on the Power being due to magnetic fields - that doesn't take the transmission voltage into account or the fact that DC through a Transformer winding (high B field) would, somehow, be different from power along a straight wire of the same resistance.

That comment led me to think about a simple experiment which can hopefully test the validity of the pointing vector theory for a dc current.
However I will start a new thread because it’s drifting to far from the op. Hope to see you and Drak back in the general section.