Author Topic: "Veritasium" (YT) - "The Big Misconception About Electricity" ?  (Read 210289 times)

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Offline emece67

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #375 on: November 27, 2021, 03:09:35 pm »
.
« Last Edit: August 19, 2022, 04:47:17 pm by emece67 »
 

Offline bpiphany

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #376 on: November 27, 2021, 04:45:37 pm »
Energy goes where energy is needed… Initially when the switch is closed, the outgoing and returning end of the loop are at different potential. The information of the change can’t travel faster than the speed of light. During this phase there will be an S-flux going into the wire at the front of this information wave, accelerating the electrons in the wire, both going away and coming back. No matter if the wire is broken or not at the turnaround point. It’s going to take a second before the system "knows" it’s possible to re-supply the electrons already sent on their way. It’s going to be messy, and not instantly full power, just like people in this thread have simulated.

The wires aren’t needed to guide the energy, that are needed to re-supply the charges.

A long coax cable on the other hand you can very much use as a delay line even rolled up in a tidy bunch going nowhere far. The E-field impulse is contained within the cable and has to travel down its length before noticed at the receiver. Energy will also flow along and within the cable where the E-field lives.
« Last Edit: November 27, 2021, 04:49:42 pm by bpiphany »
 
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Offline Sredni

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #377 on: November 27, 2021, 04:59:50 pm »
https://electronics.stackexchange.com/questions/532541/is-the-electric-field-in-a-wire-constant/532550#532550
...
John D. Jackson
Surface charges on circuit wires and resistors play three different roles
American Journal of Physics 64 (7), July 1996
Is a copy of this available on the webs?

If you dig deep enough...
I don't have (and I guess even if I had it I could not post) the link, though.

Quote
Quote
Bruce A. Sherwood, Ruth W. Chabay
A unified treatment of electrostatics and circuits
American Journal of Physics

This can be find online.
Hmm. Seems to be fairly well hidden.

Naaaa... It's linked at the end of my Stack Exchage answer.
Here's the link (file: circuit.pdf)
https://www.compadre.org/introphys/items/detail.cfm?ID=10026

Quote
Anyway, I'm glad to see that a ferret is really making something of his life -  and in Antarctica too. Must be tough.

The fur helps a lot.
« Last Edit: November 27, 2021, 05:02:26 pm by Sredni »
All instruments lie. Usually on the bench.
 

Offline aneevuser

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #378 on: November 27, 2021, 05:12:13 pm »

If the battery and bulb are close together, but the wires go off to the sides, there is going to be very little E-field at all along the wires. The S-field has to, mainly, jump across the gap between the source and sink where both the fields have any strength.


I'd be interested to see how you have derived this. It's all very well stating that this is the correct arrangement of fields, but I think that this is a situation where we need to see the detailed working, from the underlying physical principles.
 

Offline rfeecs

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #379 on: November 27, 2021, 06:36:34 pm »

If the battery and bulb are close together, but the wires go off to the sides, there is going to be very little E-field at all along the wires. The S-field has to, mainly, jump across the gap between the source and sink where both the fields have any strength.

I'd be interested to see how you have derived this. It's all very well stating that this is the correct arrangement of fields, but I think that this is a situation where we need to see the detailed working, from the underlying physical principles.

First, realize there are three time periods here:

1.  Before the switch is thrown.  The initial condition is there is no current.

2.  The seconds after the switch is thrown.  There is a transient condition and current and voltage waves bounce back and forth.

3.  Some seconds later, things settle down.  The steady state condition.

The diagram only refers to period three.  Steady state DC.

The long pairs of wires are shorted at the ends.  So the voltage on the top and bottom wires are the same.  So there is no potential difference between top and bottom wires.  So there is no electric field there.

Edit:  In general there will be an electric field perpendicular to the surface of the wire, so there will be some energy flow along the outside surface of the wires.  But at any point along the pair of wires, energy flows away from the battery along the bottom wire and towards the load along the top wire, so there is no net power flow horizontally at that point.

Power = IV.  Along the wires, V=0.  So there is no power flowing horizontally along the wires.

According to the theory, you need both an electric field and a magnetic field to have energy flow.  The only region with an appreciable electric field is between the battery and the load.  That's where the energy must be flowing.

Of course you need a magnetic field also.  That requires a current.  The role of the wires is to supply a path to enable current to flow.

The static surface charge in the wire also arranges itself to provide the same potential at both ends of the wire, so creating the electric field across the load that causes the current.

So, even though the power doesn't flow "through the wires", they are still important. :D
« Last Edit: November 29, 2021, 06:02:25 pm by rfeecs »
 
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Offline Vtile

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #380 on: November 28, 2021, 07:44:22 pm »
One thing bothers me, it must be langauge thing. People are speaking how power is not transmitted through wires, but "flying in free-space". In similar sense power is spoken as current or potential difference or their fields. In my understanding power is byproduct not primary 'force' so to speak.  :-//

Edit. Also some statements here do give impression that DC is different (partly is), but ie. solenoid and hall current clamps (and ie. CRTs) still work on DC, so it is obvious that even with DC the electricity does not stay inside conductor per se.

Also some statements said that in house appliances this do not matter and it is usually handled as DC, which is correct . Usually only said it is indifferent under IIRC 200km (and then thrown T-model for local distribution calculations), but that also is customary that is inherited from times before non-standard loads (power electronics). However, nobody is cared to say first brick of understanding. That is 6000km, which is approximate wavelength of 50 Hz AC in vacuum.
« Last Edit: November 28, 2021, 10:41:46 pm by Vtile »
 
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Offline rs20

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #381 on: November 29, 2021, 12:15:39 am »
One thing bothers me, it must be langauge thing. People are speaking how power is not transmitted through wires, but "flying in free-space". In similar sense power is spoken as current or potential difference or their fields. In my understanding power is byproduct not primary 'force' so to speak.  :-//

This is a fantastic point that I hadn't even really considered, thank you so much for pointing this out. And I don't think it's necessarily just "a language thing" -- there's a conceptual leap that we're making as to whether it's even physically meaningful to describe power as a field... I'd love to hear a physicist's justification for why power is any less of a human-contrived convenience than pressure or temperature fields (imperfect analogy because those things are emergent macroscopic phenomena unlike the Poynting vector).
 

Offline Trader

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #382 on: November 29, 2021, 02:55:09 am »


 

Offline etiTopic starter

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #383 on: November 29, 2021, 05:03:48 am »
Gosh this topic got busy
 

Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #384 on: November 29, 2021, 09:04:40 am »


There is a fundamental flaw in the video from RSD Academy. Starting at 8:20 he is explaining the situation using first a small circuit loop, and then extends the the loop to a very long loop at 9:00. Fair enough. He then describes the circuit operation stating (9.30-10:00) that as he has added more wires, there will be more antenna, more coupling between the wires, there will be more energy flowing in the circuit, thus the light bulb will light up immediately with full power (10:05). Basically he is suggesting that the the EM-field will start flowing along the WHOLE 300 000km wire as soon as he has closed the switch.

The light bulb will not get full power when the switch is closed, even if there is "much more wiring and more capacitive coupling in the circuit", because the EM-fields will be flowing only at limited speed along the wire. When the switch is closed, the impedance of the wiring, due to wiring's capacitance and inductance, will limit the initial current which is flowing through the bulb.

Of course, if the impedance of the wiring would be zero, then there would be full power available as soon as the switch was closed. But, due to wiring's inductance and capacitance distributed along the total length of the wiring, there will be a finite impedance limiting the initial current flow through the light bulb.

Thus when the switch is closed, there will be a step in EM-field, and the EM-field is starting to propagate at limited speed along the wires, and there will be only limited amount of power available to the light bulb while the EM-field is traveling away from the light bulb and battery, and reflecting back at the end of the wires. Only after the circuit has reached steady state, the light bulb will get full power.

How long will it take that the circuit is able to reach the steady state? It depends on the impedance of the wiring and the impedance of the light bulb. In optimal case when the impedance of the light bulb matches with the impedance of the wiring, the time will be twice the time EM-field travels from the battery to the end of wiring.
 

Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #385 on: November 29, 2021, 09:12:01 am »
RSD Academy is using a term "pulse" in his video. More appropriate term would be a "step", because that would indicate that closing the switch creates a step in the EM-field which starts traveling along the wiring at limited speed [close to the speed of light c].
 

Offline etiTopic starter

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #386 on: November 29, 2021, 09:13:43 am »
You wait, I’ll come back in 5 years and this thread will be 13,000 pages long 😁
 
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Offline bpiphany

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #387 on: November 29, 2021, 10:01:49 am »
This is the most i depth technical discussion video I've seen on the question. It's on the long side, but interesting. I'm pretty sure it was linked earlier in the thread...

« Last Edit: November 29, 2021, 10:03:26 am by bpiphany »
 

Offline MIS42N

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #388 on: November 29, 2021, 12:12:23 pm »
This is a very artificial problem. It requires zero resistance wires, and an unobtainium light bulb that lights on any current other than zero. But given those starting points it is possible to work out what will happen.

Before the switch is closed, no current is flowing.

Switch closes, battery sees a charge deficit (switch is on the -ve pole of the battery, has an electron surplus) so current flows to remove the deficit. How much current? ignoring the return wire for the moment, the current is moving for at least a second into what looks like an infinite wire with no resistance. The only thing stopping the current going to infinity is the intrinsic inductance of a straight wire - so it will rise as fast as that inductance allows. With 12V it will rise to thousands of Amps. The charge front will move along the wire, as the current rises behind the front it creates an increasing magnetic flux around the wire, which creates a back emf.  So after a fraction of a second the current out of the battery is high and constant.

What happens in the return wire? - Almost all of the magnetic flux created by the switched wire will be concentrated around that wire and only a small amount will go around both wires. This is why it is OK to ignore the return wire in the initial analysis - the effective impedance of the switched wire is micro ohms. The impedance of the "transmission line" is hundreds of ohms so almost no energy is coupled into the return wire. However, that small amount is enough to start a current in the return wire. If there were no light bulb, the return wire, also with zero resistance will increase its current until the magnetic flux it creates is as large as flux around the powered wire, and the flux encircles both wires.

Eventually, both current fronts reach the short at the end, the current will increase and the increase is reflected back to the battery. The battery is then able to increase the current by the original amount again, back to the short etc. Causing the current to ramp to infinity in one second steps.

That is, assuming a zero resistance or non existent light bulb.

With a light bulb with some resistance, the return wire current for the first second is being generated by a tiny fraction of the flux created by the switched wire and the current will not be able to increase to match the outgoing current. It will also "light" the bulb (with a small current) . When the charge front in the switched wire reaches the shorted end, there is no (well, a tiny one) charge front coming up the return wire to increase the current in the switched wire (and cause a reflection). The current just continues back down the return wire (inducing a tiny effect in the switched wire) until it reaches the light bulb. The light bulb will now be assaulted by a massive current of thousands of amps with 600,000km of inductance behind it. It better be robust.

I think the misconception here is confusing what happens with AC and what happens with DC. With AC, EM effects are ongoing. With DC, EM effects are transient. Try using EM to couple two DC circuits. Can't be done. The almost instantaneous lighting of the bulb in the video is dependent on transients when DC is switched.
 

Offline snarkysparky

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #389 on: November 29, 2021, 12:47:16 pm »
The transient lighting of the bulb due to parasitic capacitance and inductance in the attached transmission lines is not interesting to me.  Obviously it will occur to some extent.  And it would not illuminate any real light bulb.  But technically some power appears nearly immediately.

The claim that steady state DC power flow does not happen in the wires is interesting.   Is that being claimed in the video ?

If so how does that happen?      I know about the poynting vector but what is this in a steady state DC circuit.   At a point in space B and E fields are constant.  Is power flowing through this point as indicated by poynting ?

Does Ver actually claim this is true ?
 

Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #390 on: November 29, 2021, 01:15:26 pm »
This is a very artificial problem. It requires zero resistance wires, and an unobtainium light bulb that lights on any current other than zero.

Let's replace that light bulb with a 200 ohm resistor, so that it is easier to calculate the current through the load after the switch is closed. Depending of the impedance of the wiring, there may be significant current flowing into the resistor after (3.3ns) the switch is closed. 

Before the switch is closed, no current is flowing.

Switch closes, battery sees a charge deficit (switch is on the -ve pole of the battery, has an electron surplus) so current flows to remove the deficit. How much current? ignoring the return wire for the moment, the current is moving for at least a second into what looks like an infinite wire with no resistance. The only thing stopping the current going to infinity is the intrinsic inductance of a straight wire - so it will rise as fast as that inductance allows. With 12V it will rise to thousands of Amps.

Even if the wiring is lossless, the wiring will still exhibit finite impedance due to the distributed inductance and capacitance along the wiring, thus limiting this current flowing in the circuit after the switch is closed.

Using this online calculator for example, the impedance of the wiring can be estimated https://www.easycalculation.com/engineering/electrical/parallel-wire-impedance-calculator.php : ​Parallel wire impedance, Wire separation 1000mm, Wire diameter 10mm, Relative er 1, will result an impedance of 635 ohms. Increasing the wire diameter will reduce the impedance, and reducing the wire diameter will increase the impedance. The impedance of a wire with 100mm diameter is about 350 ohms, and the impedance of a wire with 1mm diameter is about 900 ohms.

We can therefore calculate the initial current thought the 200 ohm load using Ohms law:

- For the wiring impedance of 350 ohms, the current through 200 ohm resistor will be 12 / (200+350+350) = 13 mA.
- For the wiring impedance of 600 ohms, the current through 200 ohm resistor will be 12 / (200+600+600) = 8.5 mA.
- For the wiring impedance of 900 ohms, the current through 200 ohm resistor will be 12 / (200+900+900) = 6 mA.

The full/final current through the 200 ohm load will be 12 / 200 = 60 mA after the circuit has reached the steady state, thus there will be significant current of 6 mA ... 13 mA ie. 10% ... 20% of the steady state current flowing through the load after 3.3ns after the switch is closed, and the lamp will light up. It will be dim, but there is significant current flowing through the lamp after the switch is closed.
 
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Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #391 on: November 29, 2021, 01:47:40 pm »
Here a simple simulation using a 200 ohm resistive load, and different impedances for the wirings of 350 ohms (green), 600 ohms (blue), and 900 ohms (red). It can clearly be seen how the current through the load is significant (6mA ... 13mA) right from the start, and how the current is increasing in steps towards the steady state current of 60mA as the EM-fields are traveling back and forth along the wiring.
 
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Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #392 on: November 29, 2021, 02:33:23 pm »
The transient lighting of the bulb due to parasitic capacitance and inductance in the attached transmission lines is not interesting to me.  Obviously it will occur to some extent.  And it would not illuminate any real light bulb.  But technically some power appears nearly immediately.

There will be significant current flowing through the light bulb right after (3.3ns) the switch is closed. See my post above.

The claim that steady state DC power flow does not happen in the wires is interesting.   Is that being claimed in the video ?

If so how does that happen?      I know about the poynting vector but what is this in a steady state DC circuit.   At a point in space B and E fields are constant.  Is power flowing through this point as indicated by poynting ?

Yes, at DC there is power flowing in the circuit in EM-field, and there is a Poynting vector towards the load. Check this paper for more detailed explanation:
http://sharif.edu/~aborji/25733/files/Energy%20transfer%20in%20electrical%20circuits.pdf
« Last Edit: November 29, 2021, 02:35:09 pm by Kalvin »
 

Offline SilverSolder

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #393 on: November 29, 2021, 02:59:19 pm »
[...]  the current through the load is significant [...] right from the start  [...]

That makes sense:  the nearest part of the cable capacitance begins to get charged up immediately when the switch is thrown.  That "charging current" can only go through the load.
 

Offline snarkysparky

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #394 on: November 29, 2021, 03:09:29 pm »
My EE curriculum used the term "electromagnetic"  to indicate energy transfer by self propagating wave.  Not near field behavior.   

Poynting vector literature is full of this term.   Does my taught definition match common usage.   If so then none of the literature using "electromagnetic"  addresses my question.

I have read the article linked by Kalvin. It is concerned with charge on the surface of the conductor,  not in the space between.

So I ask what to make of the poynting vector in the space between the conductors where the E and B fields are nonzero and constant indicating power flow through the point by way of poynting vector.
 

Offline MIS42N

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #395 on: November 29, 2021, 09:11:15 pm »
Kalvin's model, like most models, ignores the inductance of a straight wire. Understandable in a model because it isn't easy to determine and is usually negligible. Models aren't designed to handle 300,000km of zero resistance wire. They don't crop up much.

The first question that came to mind is where did the 12V come from to drive current through the 200 ohm load. Anyone who says capacitive coupling is welcome to calculate the capacitance per meter of two parallel wires a meter apart. But yes there will be 12V, it comes from the tiny amount of shared magnetic flux as the current in the switched wire ramps up from zero to a few thousand amps, enough to create a few milliamps in the return wire.

In the extremely unlikely event that this problem needs to be solved in real life, it would not be a good idea to put a team on it. They will be in deadlock. Reminds me of a quote about economists. 'If all the economist in the world were laid end to end, they still wouldn't reach a conclusion'.
 

Online SiliconWizard

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #396 on: November 29, 2021, 10:14:26 pm »
To reach a conclusion, you must first define what EXACTLY is the question, and as I said a few posts ago, I think the initial question is ill-defined. So that can run in circles forever. Not that the points made about it all are not interesting. ;D
 
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Offline Vtile

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #397 on: November 30, 2021, 08:13:32 am »
To reach a conclusion, you must first define what EXACTLY is the question, and as I said a few posts ago, I think the initial question is ill-defined. So that can run in circles forever. Not that the points made about it all are not interesting. ;D
Ill defined or not, it is interesting to read and follow the discussion, by someone who is not (unfortunately) dealing with these at day job. Here is many that are much more accustomed in fields, transmission line theory and physics involved... or they at least have really good "consultant facade".  ::)
 

Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #398 on: November 30, 2021, 08:19:40 am »
Wikipedia has a brief reference to the first transatlantic telegraph cable and its poor performance mentioned in Veritasium's video: https://en.wikipedia.org/wiki/Transmission_line#History

Mathematical analysis of the behaviour of electrical transmission lines grew out of the work of James Clerk Maxwell, Lord Kelvin, and Oliver Heaviside. In 1855 Lord Kelvin formulated a diffusion model of the current in a submarine cable. The model correctly predicted the poor performance of the 1858 trans-Atlantic submarine telegraph cable. In 1885 Heaviside published the first papers that described his analysis of propagation in cables and the modern form of the telegrapher's equations.

Kalvin's model, like most models, ignores the inductance of a straight wire. Understandable in a model because it isn't easy to determine and is usually negligible. Models aren't designed to handle 300,000km of zero resistance wire. They don't crop up much.

When two [electrically long] parallel wires are modeled as a transmission line, the transmission line model includes distributed inductance and capacitance of the wires. So, the inductance is there, it is not missing or ignored. The inductance is not negligible, because without this inductance there won't be a transmission line.

These kind of two-wire feedlines have been used by amateur radio operators over a hundred years, so the properties of these transmission lines are well known and understood. In practice their characteristic impedances are typically in range 300 ohm ... 600ohm, and their characteristic impedance is determined by the distance of the wires and the diameter of the wires. There is a lot of literature available for amateur radio operators on transmission lines.

It really doesn't matter how long a transmission line is when the transmission line can be considered as lossless. When the transmission line is lossless, the shape and amplitude of the traveling EM-field will remain unaltered. This is also true in practice for short, real-life coaxial cables, feed lines, PCB strip lines etc., and this can be easily verified with a step generator and an oscilloscope.

The transmission line model and its derivation can be found here: https://en.wikipedia.org/wiki/Transmission_line#Telegrapher's_equations

The characteristic impedance of a transmission line can be calculated as follows: https://en.wikipedia.org/wiki/Twin-lead#Characteristic_impedance

Here are two online impedance calculators for parallel feed lines / transmission lines:

https://www.easycalculation.com/engineering/electrical/parallel-wire-impedance-calculator.php
https://hamwaves.com/zc.circular/en/

The first question that came to mind is where did the 12V come from to drive current through the 200 ohm load. Anyone who says capacitive coupling is welcome to calculate the capacitance per meter of two parallel wires a meter apart.

In this experiment, when the switch is closed, there will be two traveling steps of EM-field flowing along the two transmission lines (towards left from battery/switch/load and towards right from battery/switch/load), and the step-like shape of EM-fields will remain undistorted as the waves travel along the lossless wiring.

The current through the 200 ohm load [after 3.3ns when the switch is closed] is due to the circuit formed by a 12V battery, the two transmission lines and 200 ohm load. The characteristic impedance of the two wirings can be calculated, and the current can be calculated using Ohm's law (see my previous post above).

The transmission line model and the calculated characteristic impedance of the wires include the capacitance, too.

But yes there will be 12V, it comes from the tiny amount of shared magnetic flux as the current in the switched wire ramps up from zero to a few thousand amps, enough to create a few milliamps in the return wire.

After the switch is closed, there will not be a huge current surge in the wires. The current is limited by the wiring's characteristic impedance.
 
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Offline MIS42N

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #399 on: November 30, 2021, 09:59:05 am »
Anybody that thinks that the circuit will only show the characteristics of a transmission line is missing something. There are parallel wires in your street delivering loads of amps to all the houses. They don't behave like there's a few hundred ohms of impedance stopping the the flow of current. There isn't a huge drop of voltage because of reactance. They behave like low resistance wires with a tiny, tiny hint of a transmission line.

So there's no use in quoting transmission line formulae here. They really don't apply. 
 


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