Author Topic: EEVBlog 1439 - Analyzing Veritasium's electricity video  (Read 13941 times)

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Offline Kalvin

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #25 on: November 25, 2021, 07:51:27 pm »
I remain of the opinion that the problem has nothing to do with " transmission line" approach and has to be explained in classic electromagnetic theory terms. If anyone based his explanation on  "parallel wires 1 meter apart" , or " X mm trace on an FR4 PCB, the line impedance and such, if they instead wrap the wires around the Globe in a circular shape rather than running them in parallel, their answer to the challenge may differ.  This will right away indicate this approach is wrong.

This and other EM-field problems can be solved using classic electromagnetic methods, no question about that. Typically solving EM-problems requires numerical methods, unless the problem has a simple geometry for which there exists a closed form solution or for which it is possible to create a simple numerical approximation.

Looking at this particular experiment, it was very clear that the setup and circuit topology looks like a transmission line. Because there are already known, well established methods for dealing with transmission lines, why not then use a simple transmission line-model for solving this particular experiment? It is not by accident why transmission lines are being used whenever there is a need to move/guide EM-fields form point A to point B.

For some other circuit geometry a simple transmission line-model would not work, and another model would had to be created.
 
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Offline Bud

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #26 on: November 25, 2021, 09:30:21 pm »
Then we need to decide if the plumbing model with pipes and shut off valves is all that is needed to undedstand how transistors work. Why go to the trouble of the science.
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Offline David Hess

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #27 on: November 25, 2021, 10:56:21 pm »
Then we need to decide if the plumbing model with pipes and shut off valves is all that is needed to undedstand how transistors work. Why go to the trouble of the science.

And how hall effect sensors work.

Isn't drift velocity of the charge carrier critical to understanding how they work?
 

Online EEVblog

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #28 on: November 25, 2021, 11:28:35 pm »
Dave, you should correct your video, the errors are too big.

No, I am not going to pull the video and nuking all the comments.
 
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Offline SredniTopic starter

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #29 on: November 26, 2021, 02:52:30 am »
Dave, you should correct your video, the errors are too big.

No, I am not going to pull the video and nuking all the comments.

No need to pull it.
If YT no longer allows to add corrections, I think you should amend the description to address those two big blunders:

- Poynting vector at DC is no different than at AC (where did you get the idea it would point toward the battery or the highly conductive wires?) - timestamp 09:30
- Feynman was expressely showing a resistor in the figure you mention and show from his Lectures, not a highly conductive wire in a circuit with bigger resistors. timestamps 38:25 and 40:24

Also, maybe removing the timestamp for "Every electrical engineer knows this?"  ;-)

I mean, the people you called BS on for their dodgy tech could have a field day with this video if you do not correct it yourself in a clear manner.
All instruments lie. Usually on the bench.
 

Offline Kalvin

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #30 on: November 26, 2021, 08:55:46 am »
Then we need to decide if the plumbing model with pipes and shut off valves is all that is needed to undedstand how transistors work. Why go to the trouble of the science.

It is not a question whether we have to select exclusively between a plumbing model and most complex transistor model available. We just need to know when to use and apply a specific transistor model at given situation. There are multiple models for a transistor, each providing increasingly better approximation how the transistor will behave within the limits of the model. A very basic transistor model with pipes and shut off-valves is sufficient for understanding how to control LEDs, actuators etc. in practice, but for AC or RF circuits this simplified model is insufficient and better model shall be used.

I bet when analyzing transistor circuits you do not calculate EM-fields in the transistor (the science way), but you are using whatever approximation and transistor model there is suitable for that particular situation (practical way). After all, the art and science of engineering is to understand when to apply particular model and approximation, and understand the limits of these simplified models.
« Last Edit: November 26, 2021, 10:31:36 am by Kalvin »
 
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Offline AlienRelics

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #31 on: November 26, 2021, 01:34:51 pm »
Anyone linked to this video yet by Professor Walter Lewin, "Poynting Vector, Oscillating Charges, Radiation Pressure, Comet Tails, Polarization (Linear, Elliptical, and Circular)"?

https://youtu.be/6lb040GCs2M
Steven J Greenfield AE7HD
 
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Offline AlienRelics

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #32 on: November 26, 2021, 01:55:44 pm »
Then we need to decide if the plumbing model with pipes and shut off valves is all that is needed to undedstand how transistors work. Why go to the trouble of the science.

All of engineering is selecting and using simplifications that are just good enough for that particular situation. To design a guitar amp, I don't need to understand or use a lick of quantum chromodynamics, but the water analogy just isn't good enough. If I'm wiring up switches and lights in my house, the water analogy is good enough. But if I'm to design a new IC, I'd better understand and use a lot more accurate models!

I'm reminded of an early episode of "The Big Bang Theory" where Sheldon assumes that because he is extremely good at theoretical physics, he must therefore find engineering something mechanically simple a doddle. Then he fails spectacularly.

I see a LOT of engineers, technicians, and hobbyists who learn the simplifications and rules of thumb, who then think that the most complex of the simplifications they've learned is actually how things work. The simplifications are analogies. Analogies are not the truth. If you see a sign that says "Watch For Falling Rocks", the analogy of "the Earth sucks" is good enough, you don't need to know the (current) theory of gravity that it isn't a force, but due to differential time dilation caused by mass. Einstein's warped space is an analogy, a model. There is no rubber sheet.

This is an example of a type the Dunning-Kruger effect, where someone assumes they are very smart about everything because they are very smart about something.
Steven J Greenfield AE7HD
 

Offline Fungus

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #33 on: November 26, 2021, 03:18:55 pm »
Better question: How long before the bulb reaches full brightness?

 
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Offline vad

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #34 on: November 26, 2021, 05:58:56 pm »
Better question: How long before the bulb reaches full brightness?
Using transmission line model: it can take many seconds until all reflected waves attenuate and the circuit reaches steady state.
 
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Online nctnico

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #35 on: November 26, 2021, 06:25:46 pm »
Better question: How long before the bulb reaches full brightness?
Using transmission line model: it can take many seconds until all reflected waves attenuate and the circuit reaches steady state.
Without any resistivity in the circuit, that state is likely never reached because there is nothing to dampen the oscillations.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Fungus

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #36 on: November 26, 2021, 07:43:40 pm »
Better question: How long before the bulb reaches full brightness?
Using transmission line model: it can take many seconds until all reflected waves attenuate and the circuit reaches steady state.
Without any resistivity in the circuit, that state is likely never reached because there is nothing to dampen the oscillations.

OK, how long until 98%?  :P
 

Offline thm_w

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #37 on: November 26, 2021, 09:56:23 pm »
Better question: How long before the bulb reaches full brightness?

Which is intuitively what the majority of people watching the video would think the question is. IMO.
I mean look at the simulation Sredni posted, you'd see that and go "oh some energy reaches the bulb, but its not really well lit until it propagates the length of the wires.
Profile -> Modify profile -> Look and Layout ->  Don't show users' signatures
 
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Offline David Hess

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #38 on: November 26, 2021, 10:00:32 pm »
Better question: How long before the bulb reaches full brightness?

Using transmission line model: it can take many seconds until all reflected waves attenuate and the circuit reaches steady state.

Without any resistivity in the circuit, that state is likely never reached because there is nothing to dampen the oscillations.

If the transmission lines are shorted at the ends, then the bulb alone provides termination of the transmission lines, so proper termination depends on the bulb resistance.  There is no requirement for a transmission line to be properly terminated at both ends; a single termination at either end is sufficient to prevent reflections.

With a proper termination by the bulb, and shorts at the ends of the two transmission lines, the bulb sees twice the transmission line impedance in series until the wave propagates to the ends and reflects at which point the bulb sees the full voltage.  If the bulb resistance is a mismatch, then there will be reflections.

There are some (weird) RF power sensors that work like that with the transmission line terminated into a thermister which is DC biased to operate at a constant temperature, and thereby resistance for proper termination.  The required DC bias indicates the RF power level.
 
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Online nctnico

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #39 on: November 26, 2021, 10:21:37 pm »
Better question: How long before the bulb reaches full brightness?

Using transmission line model: it can take many seconds until all reflected waves attenuate and the circuit reaches steady state.

Without any resistivity in the circuit, that state is likely never reached because there is nothing to dampen the oscillations.

If the transmission lines are shorted at the ends, then the bulb alone provides termination of the transmission lines, so proper termination depends on the bulb resistance.  There is no requirement for a transmission line to be properly terminated at both ends; a single termination at either end is sufficient to prevent reflections.
But the experiment claims ideal wires (0 resistance) so it also implies ideal battery (0 Ohm), ideal switch and an ideal lamp (infinite resistance).
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline vad

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #40 on: November 27, 2021, 01:09:21 am »
… and ideal shorts (0 Ohm) at the end of each transmission line, causing ideal reflection. And when reflected wave travels back, it will pass through the ideal switch and will proceed into the opposite transmission line…
 

Offline David Hess

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #41 on: November 27, 2021, 05:44:17 am »
But the experiment claims ideal wires (0 resistance) so it also implies ideal battery (0 Ohm), ideal switch and an ideal lamp (infinite resistance).

While the transmission line is charging or discharging before the wave is reflected, it appears to have a  pure resistance equal to its impedance.  It really does look like a resistor as far as the source is concerned.

… and ideal shorts (0 Ohm) at the end of each transmission line, causing ideal reflection. And when reflected wave travels back, it will pass through the ideal switch and will proceed into the opposite transmission line…

The bulb has a resistance and if it is the same as the transmission line impedance, it will completely absorb the reflection.
 
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Offline vad

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #42 on: November 27, 2021, 12:23:50 pm »
The bulb has a resistance and if it is the same as the transmission line impedance, it will completely absorb the reflection.
While the bulb has the resistance, impedance between its terminals is not completely resistive and neither it is infinite: there is parallel capacitance between bulb’s terminals, and there is impedance of free space. It will take several round trips until all reflections attenuate below noise floor.
 

Offline AlienRelics

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #43 on: November 27, 2021, 04:34:21 pm »
An ideal bulb is not infinite or zero resistance.

An ideal bulb is an ideal resistor. No parasitic capacitance or inductance, just resistance and no change with temperature.

So with all ideal parts, and assuming nothing near the wires to absorb near field radiation, the resistance of the bulb is all that absorbs energy. So it will eventually settle, still an asymptotic delay. I think you could model this as two paralleled transmission lines shorted at the far end, so simplify it to one shorted transmission line of half the characteristic impedance with the battery and bulb at one end.

So the wave has to travel to the other end and back again, say 5 times.

However, if we arbitrarily make the bulb resistance equal to the characteristic impedance of the paralleled transmission line, then it should immediately (1m/c) reach 1/2 power. And reach full current/power in the time it takes for the step change to propagate to and back from the shorted end.

Since it is all perfect parts, a shorted transmission line reflects back in opposite polarity, therefore the reflected step change is in exact peak value but opposite polarity. So as it propagates back to the bulb, it exactly cancels the voltage present.

When it reaches the bulb/battery end, it is zero. So full brightness after length of wire/c. So if the wires are 1 light second long, it takes 1 second to reach the end, 1 second for the reflection to return. So the bulb will reach 1/2 brightness in the time set by the spacing of the wires, and 2 seconds later it steps to full brightness.

Steven J Greenfield AE7HD
 
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Offline Fungus

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #44 on: November 27, 2021, 05:36:36 pm »
the bulb will reach 1/2 brightness in the time set by the spacing of the wires, and 2 seconds later it steps to full brightness.

The video should really have pointed that out IMHO.

Anything less is a bit dishonest.
 
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Online nctnico

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #45 on: November 27, 2021, 07:01:33 pm »
An ideal bulb is not infinite or zero resistance.
No, an ideal bulb has infinite resistance by definition! An ideal bulb draws zero current. Anything else is just moving goal posts.

The only real conclusion that you can draw is that Veritasium's video is a down to earth introduction to EM field theory but the circuit presented is so much dumbed down that any answer between 1m/c and infinite time is correct. It just depends on how you fill in the blanks.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Fungus

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #46 on: November 27, 2021, 08:13:48 pm »
An ideal bulb draws zero current.

The big lightbulb corporation will kill you if you try to manufacture them.
 

Offline Fungus

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #47 on: November 27, 2021, 08:27:28 pm »
Another question: How long does it take to turn off the light?  >:D

 

Offline David Hess

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #48 on: November 28, 2021, 08:20:01 pm »
The bulb has a resistance and if it is the same as the transmission line impedance, it will completely absorb the reflection.

While the bulb has the resistance, impedance between its terminals is not completely resistive and neither it is infinite: there is parallel capacitance between bulb’s terminals, and there is impedance of free space. It will take several round trips until all reflections attenuate below noise floor.

If someone else can assume lossless transmission lines, then I can assume a spherical bulb in a vacuum with no parasitic elements.  Besides which, maybe the bulb is constructed as part of the transmission line; there are real world parts constructed this way for exactly this reason including diodes and resistors and switches.
 
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Offline thinkfat

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Re: EEVBlog 1439 - Analyzing Veritasium's electricity video
« Reply #49 on: November 28, 2021, 08:24:06 pm »
Another question: How long does it take to turn off the light?  >:D

Calculate the current of the arc across the switch.
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