General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
TimFox:
I know that the speed of a signal down a transmission line is the usual function of the inductance and capacitance per unit length.
In my career, I have measured transmission lines to verify this.
I have built resonant circuits using coax lines.
I have built delay lines from discrete inductors and capacitors.
I have used coax cables with and without the PVC outer insulation.
I have never seen the mythical behavior you wave your hands about.
I am getting tired of stepping through your “pearls”.
Berni:
--- Quote from: aetherist on May 10, 2022, 02:35:20 am ---Berni & TimFox know that Velocity Factor duz not tell us the speed of electricity. Velocity Factor is simply a radio ham fudge factor that gives good numbers for antennas.
Berni & TimFox know that Velocity Factor changes with GHz. Hence it has little to do with the pure speed of electricity.
Berni & TimFox know that there is no experiment or test that has ever been carried out that links Velocity Factor to the speed of electricity. Not for bare wire, not for insulated wire.
Any/every proper test will show that insulated wire has a speed of electricity of 2c/3.
Berni & TimFox don’t know that it is not simply what is inside a coax that determines the speed of electricity.
The speed of electricity along a coax is the sum of the speed of electricity on the Cu wire, & the speed of electricity on the outside of the sheath (which is usually fully insulated).
Seeesh, i am getting tired of casting pearls.
--- End quote ---
Velocity factor is not just a term that only works in RF. it is just a more convenient way of saying the speed without having to write out the speed as a huge number.
If velocity factor was significantly different for various frequencies then you would get your signal distorted after going trough a long coax cable as parts of the signal would separate out in time. This would be particularly visible when sending a square wave trough it, instead what tend to happen is the square wave getting rounded off, this is because the higher frequencies tend to cause more loss in the dielectric. This is why coax for >10GHz is special and usually insanely expensive.
You can do the experiment yourself if you don't believe it. Take a few different kinds of line such as classic solid RG58, foam core RG6 or some 300Ohm ladderline. Feed a fast square wave pulse into one end (with proper matching for the line impedance) and measure the time delay with a scope. You will see that the RG58 is very close to the 2/3c yet the ladder line is more like 9/10c while the foam RG6 is somewhere in between. As long as you have 10s of meters of cable you don't even need a particularly fast scope to measure it. You can also use a LCR meter to verify the equation that velocity factor is determined by the inductance and capacitance. In particular the capacitance part is what changes when you introduce a plastic dielectric.
aetherist:
--- Quote from: Berni on May 10, 2022, 05:29:06 am ---
--- Quote from: aetherist on May 10, 2022, 02:35:20 am ---Berni & TimFox know that Velocity Factor duz not tell us the speed of electricity. Velocity Factor is simply a radio ham fudge factor that gives good numbers for antennas.
Berni & TimFox know that Velocity Factor changes with GHz. Hence it has little to do with the pure speed of elekticity.
Berni & TimFox know that there is no experiment or test that has ever been carried out that links Velocity Factor to the speed of elekticity. Not for bare wire, not for insulated wire.
Any/every proper test will show that insulated wire has a speed of elekticity of 2c/3.
Berni & TimFox don’t know that it is not simply what is inside a coax that determines the speed of elekticity.
The speed of elekticity along a coax is the sum of the speed of elekticity on the Cu wire, & the speed of elekticity on the outside of the sheath (which is usually fully insulated).
Seeesh, i am getting tired of casting pearls.
--- End quote ---
Velocity factor is not just a term that only works in RF. it is just a more convenient way of saying the speed without having to write out the speed as a huge number.
If velocity factor was significantly different for various frequencies then you would get your signal distorted after going trough a long coax cable as parts of the signal would separate out in time. This would be particularly visible when sending a square wave trough it, instead what tend to happen is the square wave getting rounded off, this is because the higher frequencies tend to cause more loss in the dielectric. This is why coax for >10GHz is special and usually insanely expensive.
You can do the experiment yourself if you don't believe it. Take a few different kinds of line such as classic solid RG58, foam core RG6 or some 300Ohm ladderline. Feed a fast square wave pulse into one end (with proper matching for the line impedance) and measure the time delay with a scope. You will see that the RG58 is very close to the 2/3c yet the ladder line is more like 9/10c while the foam RG6 is somewhere in between. As long as you have 10s of meters of cable you don't even need a particularly fast scope to measure it. You can also use a LCR meter to verify the equation that velocity factor is determined by the inductance and capacitance. In particular the capacitance part is what changes when you introduce a plastic dielectric.
--- End quote ---
A measure of the delay for the reflexion of a signal or pulse or something for a straight (single) bit of insulated Cu would do the job (say 10 m long if a good scope).
Not coax. Not twin. Not ladderline.
I say the speed of elekticity will be 5.0 ns/m, which is 2c/3.
A bare wire would be 3.4 ns/m, which is c/1.
This is the speed of the fastest signal (which will probly be very strong with a steep rise, if the scope is a good scope).
But i haven't got a scope.
aetherist:
--- Quote from: TimFox on May 10, 2022, 05:08:54 am ---I know that the speed of a signal down a transmission line is the usual function of the inductance and capacitance per unit length.
In my career, I have measured transmission lines to verify this.
I have built resonant circuits using coax lines.
I have built delay lines from discrete inductors and capacitors.
I have used coax cables with and without the PVC outer insulation.
I have never seen the mythical behavior you wave your hands about.
I am getting tired of stepping through your “pearls”.
--- End quote ---
If the leading edge of a (DC) signal is slowed due to the presence of a parallel wire then that would be a big problem for my new elekton elekticity.
adx:
--- Quote from: aetherist on May 09, 2022, 11:35:53 pm ---On my copy i wrote a delay of 4.1 ns. But this was not an accurate estimate, your 3.0 ns is probly more accurate.
Actually i had another go at it myself & it came to 2.2 ns.
--- End quote ---
All reasonable. I think mine was just luck, but I have done this before and tend to know how to eyeball it to get it a bit tighter.
--- Quote from: aetherist on May 09, 2022, 11:35:53 pm ---What is your own explanation for the rise & plateau?
--- End quote ---
Probing (is my guess).
Probes like that are not supposed to be used (for accuracy) at GHz - they might be set to x1 (if they do that at all) which adds significant capacititve loading (what we like* to call complex impedance). They can't accurately measure voltage without applying a low impedance at HF (whilch will slow the rise), and they can't be something like a 50 Ohm load if they are measuring a 560R resistor or whatever it is. Many pages back we went over ways to measure it into a 50R input of a very high speed scope for this test, and differentially (or isolated). I'm not surprised it wasn't done (except the differential with channel math) - showing probes hanging like this, after a scope is lugged to the top of a ladder, is much more accessible than some perplexing RF apparatus with baluns and calibration (VNA, IFFT). Even the screenshot is just from pressing the stop button on the live signal (you can see it in real time in the video - the FM radio stations get visually averaged or at least fuzzed out).
* except me a couple of pages back
I say this because the other tests and simulations, the direct signal was sharp, and 'long way round' (including reflections) had progressively slowed rise (someone said dispersion, also skin effect). In this one you can see it is about the same - limited by probing and maybe drive a little, but not the scope.
It's also driven single-ended (almost certainly from the way the coax is hooked up behind the probe clip), and that will introduce the funny ground effects that so perplexed AlphaPhoenix in the pre-math channels.
All the funny little ripples (not the FM radio regular + beating sinewaves, I mean the little peaks near the edges) I don't know, only way to know is to tinker with the setup and work it out. Bit risky up a ladder with $ $ $ $ $ scope. (Forum SW didn't like all those $es together)
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