General > General Technical Chat

"Veritasium" (YT) - "The Big Misconception About Electricity" ?

<< < (174/396) > >>

SandyCox:

--- Quote ---How do you expect to put or extract energy from an EM system if you do not allow for charges to change position (and moving when doing so)?

--- End quote ---
Exactly the point I am trying to make. How do we do it? How does the lightbulb harvest energy from the static electric and magnetic fields around it?

--- Quote ---The circuit with a current flowing is not a static system. It has charges moving. It's quasi-static.

--- End quote ---
The electric and magnetic fields are constant over time. So it's a static system. Charge may move at a constant rate in a static system.


And we cannot add any solenoidal field to the Poynting vector. It violates the conservation of angular momentum.

SandyCox:
If anyone knows of a way to harvest energy from the electric and magnetic fields near a High Voltage DC (HVDC) powerline, then get to your local patent office as fast as you can. You will become very rich.

adx:

--- Quote from: Sredni on January 05, 2022, 09:11:12 pm ---... still, the phi J representation appears to be weaker than the ExH representation. ... this alternative method is not a general method that takes into consideration the whole physical system. In fact, it does not take into account what happens in the space around the conductors and elements. ...

--- End quote ---

It shouldn't - that space serves no purpose and is provably irrelevant to the (quasi)static case.


--- Quote ---4)  ... one conductor is bringing power to the resistor and the other one doesn't? Does it reverse if we reverse the polarity?

--- End quote ---

Obvious perhaps, but the Poynting vector evades that inconvenience by having power flow though an area it can't possibly get to - literally avoiding getting caught out by cutting through the fields and avoiding roads. Phi j makes sense if one considers that the flux of power represents the potential energy with respect to a known reference. So the fact that the 'power field' contains a constant flux at the extremes of voltage isn't any surprise when an integral of a closed surface needs to be used to arrive at the differential (or difference, change of power flux is where the energy goes).

Weak argument, but at least it has one.


--- Quote ---5)... What exactly makes the energy hide into the conductors?

--- End quote ---

Quite possibly the fact it isn't all in the conductors - what's left goes in the conductors. The energy in flowing current is mostly outside the wire, the energy due to the electric field between the wires is there. But the energy transferring by the current and externally applied electric field has little to do with the internal energy (in the sense of 1/2LI^2 and 1/2CV^2). So non-internal energy could be considered to be hidden in the wires, with radiation coming as an external term or from the DC energy. Which is where my argument above comes in - a DC system cannot transfer power unless it is charged with current and voltage energy. No one doubts that is in the field(s). All doubt is over the energy conducted through an area. Even the concept of "energy flux" is odd; a flux of charge is stationary, a flux of charges physically moves. Is it possible to have energy transfer over such an area without something physical moving?


--- Quote ---Let's start with an EM beam at very high frequency, such as a laser beam. Is the energy in the space occupied by the beam? I guess it is. Let's lower the frequency and consider an RF antenna beam: is the energy in the space? I guess it still is.

--- End quote ---

Not necessarily. In the feed, there is current in the walls. Without this it couldn't be constrained not to flow in random directions.

As the frequency lowers, there is undoubtedly potential energy in the voltage antinodes. That is where the current is the highest. So that is where the energy resides. Each cell is like a battery zipping along; E-field moving along with flow of charge (or opposite charge moving in a return path). (I am not suggesting the electrons are moving at the speed of light however, just the current.)

So the "error" my head created must not be real and the answer correct: DC is a travelling wave propagating at the speed of light locked at phase 0. The fields don't "freeze", there is nothing in the progression of lowering frequency that suggests the speed is in any way reducing - even when the size of a cell (half wave) runs well over the edge of the screen. There is always something just out of view that could potentially zip through, and that will be at the speed of light. You just can't see it until it happens, providing an illusion of steady state.

It therefore follows that the energy continues to flow at the speed of light, exactly as the RF behaviours suggest (yes, Maxwell and Poynting). There we have exactly the same problem with current, whether it be in wires or walls of the waveguide (or that copper block I cut through with the rubidium laser): It exists if the wave is not to be free. Charges move longitudinally, and are worked on and do work, and move in chains which are absolutely required to conduct the field to a distant source as the frequency gets too low to focus effectively. If that needs to be thought of in terms of magnetic field (technically correct because the effect travels at the speed of light and is a force) then so be it, but ultimately it is a string of balls being pushed along by a force. It depends on what your personal view of current is.

Sredni:

--- Quote from: SandyCox on January 08, 2022, 08:19:02 am ---Exactly the point I am trying to make. How do we do it? How does the lightbulb harvest energy from the static electric and magnetic fields around it?

--- End quote ---

Those fields are what cause the surface charge distribution that will have current flow into the resistor.


--- Quote ---The electric and magnetic fields are constant over time. So it's a static system. Charge may move at a constant rate in a static system.

--- End quote ---

I agree that I shouldn't have used 'quasi-static' for this steady-state electrodynamic system. I have already written a longer answer to explain what I meant and how you can get energy in and out of the system at constant velocity. I need to fetch a quotation from a book, but I don't remember which book it was so I will post it later.


--- Quote ---And we cannot add any solenoidal field to the Poynting vector. It violates the conservation of angular momentum.

--- End quote ---

Here I was answering to the post you deleted. The one about subtracting ExH to himself to prove there is zero energy flow. Was it someone else, or I just imagined/misconstrue what you wrote in it?


--- Quote ---If anyone knows of a way to harvest energy from the electric and magnetic fields near a High Voltage DC (HVDC) powerline, then get to your local patent office as fast as you can. You will become very rich.

--- End quote ---

Or very jailed. I will not mention current transformers, too easy.
Here's another method: attach a capacitor to the line and draw your power.
Oh, energy flows in the conductor joining the powerline to the capacitor? Let's remove that part. Upper armature is the cable, lower armature a metallic plate placed under the cable, let's say half a meter (less? Do you like sparks?) and attached to a metal pole. If no power will flow, then you can do this experiment safely keeping the metallic pole in your hands with your naked feet on the moist ground below.

Make sure you bring someone with you when you do the experiment.
Possibly with a broom and an urn.

Of course, DC not AC - silly me.
Ok, I can take a bit of energy out from that as well. It's in the answer I have written but I am using the charge near the resistor to make a charge in space move.

rfeecs:

--- Quote from: SandyCox on January 07, 2022, 12:27:36 pm ---Will a fluorescent lightbulb glow in a static electric field?

--- End quote ---

Yes:



But would it work in a vacuum?  Probably not.

The way to get the bulb to light up is simple.  Connect it with wires.  The Poynting theorem still holds.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod