General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
<< < (178/396) > >>
rfeecs:

--- Quote from: daqq on January 10, 2022, 09:29:34 am ---COMSOL jumps into the fray:

https://www.comsol.com/blogs/how-long-does-it-take-an-engineer-to-turn-on-a-light-bulb/?utm_content=bufferc79f7&utm_medium=Social&utm_source=LinkedIn&utm_campaign=comsol_social_pages

Simulation video included.

--- End quote ---

Interesting that the video shows the fields (and presumably the currents) are imbalanced between the top and bottom wires.  You have a physical transmission line being fed in an imbalanced way.  In a circuit simulator, you would have to model this as two coupled lines with even and odd mode impedances.  A single ideal transmission line would not model this effect.
rfeecs:

--- Quote from: Uttamattamakin on January 08, 2022, 10:05:53 pm ---It's not "at odds" with it.  All QFT predicts is that while energy can flow via the path Pyonting predicts it can also flow via an Infinite number of other paths.  Incuding paths via the wire.  When one carries out the computation, taking account the presence of existence of the wire as a path of charges which are very close toegher for the battery to interact with, the path of highest probability is along the wire.  The path suggested by Pyonting also exist but the probability of conduction via that path is low. 

The classical theory is not "wrong" it is just too limited for this situation.  The very size of it makes the speed of light relevant and so relativity has to be accounted for.


--- End quote ---

I don't understand this argument.  The classical theory (Maxwell's equations) is compatible with special relativity.  We use it all the time to deal with problems where waves move at the speed of light.  Also, we have only one reference frame here.  Relativity doesn't come into it.

Of course if you want to account for gravity considering we are going half way to the moon and back, then general relativity may come in.  But I gather that is not compatible with QFT?

Supposedly QFT should give basically (with a probability of 99.99...%?)  the same answer as classical theory so I'm not seeing the point, I guess.
adx:

--- Quote from: SandyCox on January 10, 2022, 01:59:52 pm ---According to transmission line theory, the bulb current is constant between t=0ns and t=100ns. So, the COMSOL simulation is delayed by 3.3ns compared to transmission line theory. I attach the full explanation.

Edit: I updated the note which now includes a comparison over the first 500ns.

--- End quote ---

Ah your note takes me back - is the sort of thing I have not done since uni (decades). Nothing personal against Laplace, but I found the transforms and representation extremely cumbersome - unbelievably so, I went to university believing I knew a fair bit about electronics as a hobby and knowing things like Ohm's law, so this was surprising at the time. It would have been extremely useful in the 1700s - 1800s. Z transform made a little more practical sense to me, closer to actual numbers. Oh - looking at Wikipedia, it was originally (due to Laplace) the discrete Z-transform. All this stuff I didn't know.

It might still be good to see it shifted 3.33ns (just an idea, not really necessary).
adx:

--- Quote from: SandyCox on January 10, 2022, 03:14:20 pm ---If the bulb is properly matched to the line (R=2Zo), we will get 25% of full power in the bulb after 3.3 ns. I wonder how QED explains this. Is the probability time dependent?

--- End quote ---

As far as I remember, it is pretty much the same as classical. Time and space are treated essentially the same, so the probabilities of something moving to new coordinates in x, y, z as well as t, is part of the equation. But relativistic EM is more general than classical EM, the latter relies on magnetic field to describe most but not all of the possible relativistic effects. If the charges are moving at relativistic speeds, then classical EM does not work? "I don't really know" is a good caveat here.
adx:

--- Quote from: rfeecs on January 10, 2022, 05:48:02 pm ---
--- Quote from: Uttamattamakin on January 08, 2022, 10:05:53 pm ---It's not "at odds" with it.  All QFT predicts is that while energy can flow via the path Pyonting predicts it can also flow via an Infinite number of other paths.  Incuding paths via the wire.  When one carries out the computation, taking account the presence of existence of the wire as a path of charges which are very close toegher for the battery to interact with, the path of highest probability is along the wire.  The path suggested by Pyonting also exist but the probability of conduction via that path is low. 

The classical theory is not "wrong" it is just too limited for this situation.  The very size of it makes the speed of light relevant and so relativity has to be accounted for.


--- End quote ---

I don't understand this argument.  The classical theory (Maxwell's equations) is compatible with special relativity.  We use it all the time to deal with problems where waves move at the speed of light.  Also, we have only one reference frame here.  Relativity doesn't come into it.

Of course if you want to account for gravity considering we are going half way to the moon and back, then general relativity may come in.  But I gather that is not compatible with QFT?

Supposedly QFT should give basically (with a probability of 99.99...%?)  the same answer as classical theory so I'm not seeing the point, I guess.

--- End quote ---

At the risk of (continuing?) to talk through a hole in my hat or whatever that idiom is...

I see it as the same, but different semantics and perhaps historical direction.

In classical land... Voltage stands in for energy, being potential energy. An electron 'has' energy by virtue of the physical pressure it is under (how close it is to other electrons). As it moves around the circuit it gains or loses this energy. If you believe the electron carries the energy, then the argument is done. But if you believe the energy is coming from the force being conducted through the collection of electrons, it is the closest ones in the wire directly next to it that feel this force the strongest.

However if you look at a 'unit area' of wire pair crossing space, the connections are where energy is injected and removed from. A small source of energy would have to cross space to get to these, implying that it is impossible to get energy to flow from one place to another wihout going to the extents of this system. That could mean "the wires", or the field/s. It should work just as well at DC without a magnetic field, unavoidable, but it chucks a bit of a bucket of cold water over the Poynting energy flux idea (except for the fact it works and no one seems to know why).

Probability is an interesting concept. It allows the system to be run entirely in parallel, without knowledge of the state (or indeed existence) of its other parts, while coming up with the same result.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod