General > General Technical Chat
Veritasium "How Electricity Actually Works"
hamster_nz:
I'm starting to come around to the idea that the electrons carry the energy, and it's not in the fields around.
All it would take to convince me would be if somebody could design a circuit that could extract 1% of the energy travelling in the wires in the diagram attached.
If you could build it inside the inner 'box' of wires that would be perfect, as I suspect an electric field exists between the inner and outer wires... You could even use a GND reference in the box if you want (but note that none of these wires are attached to GND)
Naej:
--- Quote from: ejeffrey on May 11, 2022, 08:09:01 pm ---
--- Quote from: EEVblog on May 11, 2022, 11:03:52 am ---
--- Quote from: dunkemhigh on May 11, 2022, 10:40:49 am ---I think that's missing an important thing. No doubt we are all mostly agreed that there is some fields stuff going on before the wires are connected, but what it's really about is after that, when there is a solid wired connection. Does the energy flow in the wire, on the wire (skin) or is the wire merely a guide and the energy actually flows still in the field? As I see it, and it's sometimes tricky to remember what the argument is about, it's that last option which is the crux of the video and this discussion.
--- End quote ---
For me the question is entirely about DC and energy inside vs outside the wire. Nothing to do with switches, transmission lines, capacitors, inductors, transformer theory, antenna theory etc etc.
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For steady currents you can calculate energy density (ignoring prefactors and constants of nature) as qV + I*Phi [Phi == magnetic flux]. or E^2+B^2 and you will get the same answer. The former describes the electric energy in terms of charges, the latter in terms of fields. You can't really get away from describing the magnetic component in terms of some field in free space because there is no scalar magnetic potential, so I have picked a form where the current plays a role, but I don't need to refer to equivalent circuit elements like L.
[...]
The magnetic component is harder to nail down. For the field centric approach it's no problem: B is unambiguously defined everywhere, so we can just integrate up B^2. But the flux * Phi representation is sort of inherently non-local: its is the current around a loop times the magnetic flux through the loop, so a product of quantities measured at two different locations.
So at DC, you can consistently define the electric component of the energy density to the wire *surface* as an alternative to the fields. When you include the magnetic component or deal with AC or transient behavior you pretty much have to fall back to a field based approach to energy density. There is no reasonable way to quantitatively define the energy to be stored in the volume of the wires.
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Yes there is, you replace whatever phi is by A the vector potential, and you get the potential energy.
TimFox:
Actually there is a scalar magnetic potential, but it is not relevant to general field calculations.
It is used in electromagnet design, including air gaps, and is analogous to scalar electric potential in a circuit, replacing resistance (electrical) by reluctance (magnetic).
Naej:
--- Quote from: hamster_nz on May 11, 2022, 08:30:48 pm ---I'm starting to come around to the idea that the electrons carry the energy, and it's not in the fields around.
All it would take to convince me would be if somebody could design a circuit that could extract 1% of the energy travelling in the wires in the diagram attached.
If you could build it inside the inner 'box' of wires that would be perfect, as I suspect an electric field exists between the inner and outer wires... You could even use a GND reference in the box if you want (but note that none of these wires are attached to GND)
--- End quote ---
First show your solution for a circuit which extract 1% of the energy travelling in the vacuum in the diagram.
vad:
--- Quote from: ejeffrey on May 11, 2022, 08:09:01 pm ---
So at DC, you can consistently define the electric component of the energy density to the wire *surface* as an alternative to the fields. When you include the magnetic component or deal with AC or transient behavior you pretty much have to fall back to a field based approach to energy density. There is no reasonable way to quantitatively define the energy to be stored in the volume of the wires.
--- End quote ---
There are devices that can store energy within conductive media. They call them lasers and masers, including RF masers.
However copper tubes in Veritasium experiment hardly qualify for a maser :)
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