Conceptual Understanding for Voltage in a Circuit

[rumborak]

I was asking mostly from the viewpoint that the EM field is after all the "provider" of the energy to the resistor, and thus there seems to be a flow from the battery to the resistor, through the air. Even when not visible necessarily because it doesn't change, I guess that energy flow is still going at c.

 

[sophiecentaur]

I was asking mostly from the viewpoint that the EM field is after all the "provider" of the energy to the resistor, and thus there seems to be a flow from the battery to the resistor, through the air. Even when not visible necessarily because it doesn't change, I guess that energy flow is still going at c.

I haven't considered this very deeply but I question how relevant the speed of light is to this particular question. Certainly, if you turn off the Power switch, the interruption would only be noticed after a time d/c. But it is not valid to talk in terms of 'individual photons' making their way around the circuit.
I could ask whether you would consider the gravitational field that is providing your weight force, is traveling at c towards the centre of the earth??

 

[rumborak]

Well, it *is* a question worth asking I feel, especially when there is an energy flow associated with it like in a circuit. I mean, from a pure formula description of it, essentially nothing is happening once the EM field has been established to its steady-state value. But at the same time, we know there is energy being transported that is draining the battery and heating the resistor. *Something* seems to be traveling, likely at c, from the battery to the resistor.

 

[sophiecentaur]

I guess you could consider a wave of Zero Hz, still travels.

 

[Biker]

Well, it *is* a question worth asking I feel, especially when there is an energy flow associated with it like in a circuit. I mean, from a pure formula description of it, essentially nothing is happening once the EM field has been established to its steady-state value. But at the same time, we know there is energy being transported that is draining the battery and heating the resistor. *Something* seems to be traveling, likely at c, from the battery to the resistor.

Why does anything have to travel? Once you establish a ""Static field" (Force doesn't change over time, The field doesn't "evolve"). The field is everywhere, No propagation of anything happening. Energy doesn't propagate.

In a static field ( no radiation, no accelerated charges) , Then that means that the forces have to be instantaneous for momentum conservation to hold ( Isn't that right? )

 

[rumborak]

No propagation of anything happening. Energy doesn't propagate.

What do you mean? The energy of the battery is ending up on the other end of the circuit, heating the resistor. Energy doesn't just magically appear in places, it has to travel there.

If there is no net radiation, Then that means that the forces have to be instantaneous for momentum conservation to hold ( Isn't that right?)

If the forces were instantaneous, it would obviously violate relativity. So that's a no-go right there.

*Something* is traveling from the battery to the resistor, even in steady state.

 

[Biker]

What do you mean? The energy of the battery is ending up on the other end of the circuit, heating the resistor. Energy doesn't just magically appear in places, it has to travel there.

It is already there because the field is there. Energy is not an object. Wherever there is a field, energy is there too.(Limited knowledge of relativity just basics) First, we wouldn't care if it takes time or not, The field inside the conductor is constant. So how time can affect this?
Second, If you somehow have a charge moving with a constant velocity, no information can be obtained so it doesn't violate relativity?

 

[rumborak]

It is already there because the field is there. Energy is not an object. Wherever there is a field, energy is there too.

If that was the case, and no energy would flow since "the energy is already there", the battery would never drain. Clearly, there is a flow from the battery to the resistor (I'm starting to repeat myself here).

And yes, energy *is* an object in the sense that has to abide by the same rules of transport as regular mass does. It can't travel faster than c, it has to go from A to B like any ol' other object.

 

[davenn]

How does that behavior change for a static EM field like in a circuit?

I don't think you can have a static EM field, by nature it is always moving at the speed of light

don't confuse that with a static electric or magnetic field

If that was the case, and no energy would flow since "the energy is already there", the battery would never drain. Clearly, there is a flow from the battery to the resistor (I'm starting to repeat myself here).

At circuit switch on ( a DC circuit) there is a pulse of EM around the circuit. This pulse provides/impart6s the initial motion to the electrons aka charges
hence the light globe lights almost instantaneously. Once the electrons/charges are on the move, they are providing the ongoing energy to the light globe

This of course is different for an AC circuit where the EM field is always active and cycling.

 

[rumborak]

I don't think you can have a static EM field, by nature it is always moving at the speed of light
don't confuse that with a static electric or magnetic field

I was using "EM field" as shorthand for electric and magnetic. I wasn't implying traveling waves.
But it's an odd comment to make of yours. There is no conecptual difference between a static electric/magnetic field and an EM wave other than the rate of change, right? They are just different incarnations of the same electromagnetic field.

At circuit switch on ( a DC circuit) there is a pulse of EM around the circuit. This pulse provides/impart6s the initial motion to the electrons aka charges
hence the light globe lights almost instantaneously. Once the electrons/charges are on the move, they are providing the ongoing energy to the light globe

Errrrr ... I can only presume you have not been reading the previous posts here. The very point of this discussion is that it is NOT the electrons providing the energy, but instead the Poynting vector, I.e. ExB traveling through space. That's what was mysterious to Feynman, and to me.

 

[davenn]

I was using "EM field" as shorthand for electric and magnetic. I wasn't implying traveling waves.

then you are confusing the issue ... be clear

Errrrr ... I can only presume you have not been reading the previous posts here.

yeah I have and all I have seen is your constant argument against what everyone else is saying

The very point of this discussion is that it is NOT the electrons providing the energy, but instead the Poynting vector, I.e. ExB traveling through space.

well that's pretty vague

 

[rumborak]

Well, I can say I'm in good company with Richard Feynman, and the guys at that Israeli university who wrote the paper I linked to. All I see here is a ton of hand-waving, or plain promotion of wrong notions such as that it's the electrons transferring the energy from the battery in a "pushing down the wire" way.

To quote Feynman:

" So our “crazy” theory says that the electrons are getting their energy to generate heat because of the energy flowing into the wire from the field outside. Intuition would seem to tell us that the electrons get their energy from being pushed along the wire, so the energy should be flowing down (or up) along the wire. But the theory says that the electrons are really being pushed by an electric field, which has come from some charges very far away, and that the electrons get their energy for generating heat from these fields. The energy somehow flows from the distant charges into a wide area of space and then inward to the wire."

And my additional "wrinkle" here is that since the Poynting vector involves the static magnetic field, clearly that must be part of the mechanism of energy transfer too.

 

[sophiecentaur]

That's what was mysterious

There are many situations in Science where we have this problem. In this situation, what is actually worrisome? The 'speed' involved with the transfer of Power in a steady state situation is, I would say, not relevant because speed /velocity involves identifying how the 'location' of something is changing in time. In a DC situation, there is nothing that can be identifiable; everything is unchanging. We have already discarded the description of an EM wave as a stream of photons / little bullets and we can't approach it that way, as with water molecules in a constant flowing river. At DC, of course, the photon energy is Zero! (=hf=h0=0) so they have no meaning, in any case. The only way to approach this impasse is, I think, to consider an AC signal and see what happens as f→0. The velocity, of the wave in that case, would be c and, as there are no continually flowing DC circuits (f≠0, ever), in real life that could be good enough for us.
Perhaps it would be better to ignore the concept of any Power flowing at a 'speed' and say that the only speed involved with Power Flow from A to B is the speed of propagation of a change in Power. It may be irksome to accept that there is no 'motion' of the Power but you have to examine why we want an answer to that question. We all feel the need to leave things 'tidy' in our minds. QM is another example where this self indulgence cannot be allowed and we just have to 'Get Over It', perhaps.