The Big Misconception About Electricity

[beecee]

The misconception is that electrons carry potential energy around a complete conducting loop, transferring their energy to the load.

What say you?

I found the following video on u tube that discusses the scenario re the transmission of electricity.

It also involves a giant circuit, a battery and a light bulb. The circuit stretches 300,000 kms long returning to the light bulb 1 metre away, and the question being ( disregarding resitiance) how long before the light bulb lights up....5 seconds, 1 second, 2 seconds, or 1/c seconds, or none of these....

Anyway, the video explains it better then I......

 

 

Edited November 21, 2021 by beecee

[MigL]

The field is always there.
Changes to the field propagate at c .
It has little to do with the propagation of the particles, which are, themselves, over-threshold excitations of that field.

[Ken Fabian]

Off the top of my head (and without watching the video)... leaving aside resistance across such a distance, ie with superconductivity, and absence of electromagnetic fields (the solar system having them) I would expect the initial wave of current flow to proceed at C.

An electron doesn't have to travel the full length before there is a current. It should be a bit like water in a primed hose (with water already along it's length) where the flow doesn't begin when the water entering at one end reaches the other end, but begins when the pressure change does (in that case, at the speed of sound). When that pressure wave (is that correct terminology?) reaches the end the water nearest that end flows out first.

[TheVat]

It would be 1/c, wouldn't it?   Because the field is there,  and the switch and light are 1 meter apart?   Electrons do not actually flow (just jiggle a tiny bit in an AC circuit), energy is propagated by the EM field around the wire.  I think MigL has it correctly.

I will watch,  and check my work.   

1 hour ago, Ken Fabian said:

Off the top of my head (and without watching the video)... leaving aside resistance across such a distance, ie with superconductivity, and absence of electromagnetic fields (the solar system having them) I would expect the initial wave of current flow to proceed at C.

An electron doesn't have to travel the full length before there is a current. It should be a bit like water in a primed hose (with water already along it's length) where the flow doesn't begin when the water entering at one end reaches the other end, but begins when the pressure change does (in that case, at the speed of sound). When that pressure wave (is that correct terminology?) reaches the end the water nearest that end flows out first.

I don't know if the video will make this point,  but there is no hose.   I am pretty sure energy from a utility goes through a bunch of step-up and step-down transformers, which means there is no hose continuity.   Only field continuity.   

 

Edited November 22, 2021 by TheVat
Cwocntjr

[Ken Fabian]

4 hours ago, TheVat said:

I don't know if the video will make this point,  but there is no hose.   I am pretty sure energy from a utility goes through a bunch of step-up and step-down transformers, which means there is no hose continuity.   Only field continuity.   

 

My knowledge of the underlying theory is surely weak, however I note the original post posited a simple DC circuit, without transformers. Energy crossing between coils isn't the same thing.

If electrons don't shift around then how does energy in a DC circuit flow? Or AC without any moving of electrons back and forth? How do capacitors work if there is no accumulation (or absence) of electrons?

I will have a view of the video - although I'd prefer the OP had a summary of what it says.

[swansont]

One objection I have is the suggestion that the fields are propagating well away from the wires. But those fields drop off with distance, and radiate in directions away from the load; they don’t turn around like they’re homing in on the light bulb.

Also: What if you send the wire into a volume that’s encased by a magnetically shielding conductor? The parallel E field would be shielded as well. 

So yes, the energy is contained in the fields - you could calculate the KE of the electrons and see e.g. that 1 Coul of charge with V = 1V, moving at ~1mm/sec, does not have anywhere close to 1 Joule of KE - but these fields can’t be out in random open space

[studiot]

I felt that the video, whilst strictly correct in one sense, was disingenuously hiding some things, whilst making too much of a meal of others.

In this respect and bearing in mind @Phi for All's recent comment favouring the written word over videos:

References to a transatlantic telegraph cable bears little relation to the working of a power supply cable and could have been omitted.
It was unjustifiable showmanship.

Whilst they did say that the battery and the light were 1 metre apart, they di nothing to bring that out, except at the end in a smart alec, aren't we clever kind of way.
They failed to mention the fact that the disparity in the length of the supply and return conductors was so large  at 1 : 300,000,000.
I can't think of any normal power supply connections with such a ratio. Much closer to 1 : 1 would be the norm.

So an interesting bit of electrophysics would have been to analyse for two cases, the one they showed and one with both conductors at 300,000,000 metres in length.

All in all I prefer the more measured discussion of the topic by Francis Sears (MIT) in his famous book 'Electricity and Magnetism'.
Francis also make very clear the distinction between potential and emf.

 

 

Edited November 22, 2021 by studiot

[Danijel Gorupec]

At first glance, I didn't find any error in the video. Although some things could be stated more precisely (and I agree with Studiot that the reference to telegraph cable only blurred the explanation).

For those who like to think in more common terms, just note that such a wire loop would have a very large inductance (back-of-envelope calculation, something over 700 Henry), so it is sure (as they also said) that the voltage at the lamp would raise slowly (that is, the first effect at the lamp will be at roughly 1/c time, but it will take much longer for the thing to settle).

I would also comment that if wires are arranged in some other way, the result could be different. For example, if both wires (plus and minus) are not separated away from each other, but are closely packed inside a long cable that goes very far away and then returns to the lamp (or especially if a coaxial cable is used), the lamp will only start lighting much later (considering a somewhat idealized case).