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EEVBlog 1439 - Analyzing Veritasium's electricity video

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Sredni:
Dave, you should correct your video, the errors are too big.

At minute 3 something ops 9:30 you say that at DC the Poynting vector would be pointing towards the battery. This is wrong.
At minute 41 something you show a picture of a resistive wire from Feynman that shows the Poynting vector going inward, into the wire. Well, that's a resistor, power is getting in to be dissipated.

Even at DC the Poynting vector is going out from the battery, then goes nearly parallel to the copper wires, and then finally points inside the resistor. You misread Feynman (who, by the way, for fig 27-5 explicitly talks about 'resistive wire' which has a resistance and a voltage drop).

Simple experiment:
9V battery shorted by copper wire: the wire gets hot. Poynting vector pointing out of the battery and into the wire. (the wire is the dissipative element in this context).
9V battery with a 100 ohm resistor connected via copper wires: the resistor gets hot, the wire doesn't. Poynting vector directed out of the battery, nearly parallel to the wires, and then plunging into the resistor. As correctly shown by Veritasium (for DC)

And no, skin effect has nothing to do with that.
 :palm:


EEVblog 1439 - Analysing Veritasium's Electricity Video


Dave analyses Veritasium's video "The Big Misconception About Electricity" and how energy flows in the Poynting vector in the electromagnetic field OUTSIDE the wire instead of inside the wire.

00:00 - Veritasium's video "The Big Misconception About Electricity"
00:32 - Rection to the points in the video
01:11 - This is a bit MISLEADING!
02:28 - Electron drift
03:51 - Engineers use different tools and theorems
04:27 - Every electrical engineer knows this
05:17 - Everything he says is correct
08:24 - What is current?
09:30 - He doesn't address this in the video. Poynting vectors at DC
11:12 - How the lightbulb works
12:41 - At the physics level, it's correct
14:11 - My only problem with this is...
15:08 - Is it just an academic discussion?
16:17 - The undersea cable is just early transmission line theory
17:20 - So what is the answer to the question?
22:06 - What about skin effect and DC?
25:44 - Let's simulate this and answer the question
29:18 - Transient analysis
33:00 - DC Steady State analysis
34:28 - The quantitative values don't matter
38:25 - But what about DC steady state?
40:24 - What does Richard Feynman think?

golden_labels:
One thing Derek was not wrong about: that video will spawn a deluge of comments, with everyone correcting everybody else.

Has anyone so far realized that the video ventured deep into the territory of epistemology and metaphysics? A large part of the whole discussion is caused by making assumptions on the meaning of “truth”. To be more specific: confusing models with absolutely true essence, the inherent nature of the matter discussed. Neither of those explanations is more true. At best they may differ in their practical usefulness or how close they are to the most detailed views of the phenomenon. The Poynting vector approach is closer to the more detailed model, but by no means closer to any kind of truth.

Both explanations may be misunderstood and lead to wrong interpretations. It’s not hard to imagine that electrons’ movement play completely no role, after watching just that Veritasium video, if one doesn’t ask a question: how comes magnetic field is non-zero? Also the energy transfer lines (yellow) are reduced to direction only and miss magnitude, which — if considered — would draw a less surprising picture.

Sredni:
Well, the role of surface charge is very often not mentioned in most introductory books. Chabal and Sherwood have started a new trend in that sense. But one thing is trying to analyze the phenomena with different tools (propagation of fields and redistribution of surface charge, use of transmission line model in the transient phase, even antennas if you will) and another is getting basic physics completely wrong.
To say that at DC the Poynting vector is directed toward the battery, or even towards the wires when the complete circuit has a resistor that will sink the power, is just plainly, unmistakenly and uncontroversially wrong.
It's not a matter of intepretation. It's wrong.

(I corrected the minute in the video were Dave says that)

Amaruk:

--- Quote from: Sredni on November 24, 2021, 03:15:57 pm ---Dave, you should correct your video, the errors are too big.

--- End quote ---

I am a big fan of Dave and have watch so many of his videos and I am always amazed by the amount of knowledge shared in them.  Thank you! This video is a bit different though but it was kind of forced upon Dave by his viewers. The result of this is that this video does not feel as solid as the others I have watched... But who cares, we all learn stuff on here and who has not said things that are not correct when put on the spot anyway? These are difficult theoretical discussions so it just shows that Dave is human after all! :)

Kalvin:
When people are saying "At DC this and that ..." what do they really mean?

- Do they mean the static situation when there is an electric field present but no current is flowing (DC = 0)? In this experiment: Battery connected to the circuit, but no closed circuit present, circuit is in steady state. Because there is no charge moving in the circuit, there is no magnetic field present, thus there cannot be any Poynting vector.

- Do they mean that there is an electric field present and there is a [constant] current flowing in the circuit (|DC| > 0)? In this experiment: Battery connected to the circuit, switch closed, and constant current flowing into the load [after transitions have disappeared ie. circuit is in steady state again). Now, as there is charge moving in the circuit creating a magnetic field, there is a Poynting vector, too. The Poynting vector will show the direction of the energy transferred. In this experiment, and if the signs are correct, the direction should be from the battery into the load.

When the conductor is lossless, the Poynting vector is parallel to the conductor.

How about a situation when there is a good conductor, but somewhat lossy nevertheless? If I understood Feynman correctly, the Poynting vector should tilt a little towards the conductor, creating two vector components. The first component is parallel to the conductor, and which is transferring energy towards the load. And the second one, which is into the conductor, representing the losses.

Makes any sense?

What is the situation when we have a resistive wire connected directly on the battery, ie. the load is this resistive wire? This should not be solved using a traditional lumped model, because the load is now distributed along this resistive wire: How the Poynting vector is represented along the wire in this case?

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