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Old 13th Feb 2022, 3:52 pm   #61
regenfreak
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Default Re: How Electricity Works? By Electrons or by Fields?

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In time domain terms, what does the characteristic impedance of a line mean?
Ok this student is going to fail the test question again. This is hard, man. I don't have a degree in electronics. No matter, try again, fail again fail better:

The time-domain waveforms show impedance characteristics down the line; could be matched, mismatched, shorted or open.

In Time domain:

reflection coefficient = r=(ZL-Zo)/(ZL +Zo)=Vreflected/Vincident

Matched load, ZL=Zo, r = 0,no reflected power

Short: ZL = 0, r = -1, The reflected wave equal in magnitude but opposite to polarity to indicent wave. So it cancels the incident wave.

Open : Zl = infinity, ZL>>Zo r = ZL/ZL = 1, the reflected wave is equal both in magnitude and direction as the incident wave, so it reinforces the original pulse in the time trace (like two stair steps)
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Old 13th Feb 2022, 3:56 pm   #62
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Default Re: How Electricity Works? By Electrons or by Fields?

All very interesting.

It would be nice if someone could offer a talk on this topic to a science festival,
my physics is too rusty.
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Old 13th Feb 2022, 3:57 pm   #63
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Default Re: How Electricity Works? By Electrons or by Fields?

Hi Again

I think that the clouds are lifting! Possibly.

I think the key (to my confusion) lies in the fact (?) that the electric field is contained in the conductor and so it isn't a case of having fields all over the show which the start image of the video implies.

I have to admit too that I had never really thought about what causes 'current flow' when a wire is connected across a battery (say). Does the potential difference immediately set up an electrostatic field along the wire? Why is there a potential difference?

If the field is contained within the wire it would follow the twists and turns of the wire and so come back to my experience of designing with current flow 'in wires' rather than in fields (which can be all over the place).

Hmmm, but if the wire were resistance wire then a lower current flows but the electric field strength should be exactly the same as with a normal wire because it is voltage and physical arrangement dependant only. Lower current means lower magnetic field strength. It still looks like the electrons are still a big factor in the physics.

Sparks and Arcs thoughts - If we have a gap in a high current circuit we can draw an 'arc'. I am inclined to believe that what leaps across the gap are electrons pulled across by an electrostatic field. They don't dawdle - the speed is terrific. As they reach the other side they must lose linear speed or they would all pile up in a heap. This is OK as it is explained by the concept that they jump into a massive lattice and spend most of their time banging into the lattice. It also demonstrates the real speed of the electron I would think. Also it shows where the electrostatic field is.

I'm happier and I've learned a few things, but I have questions still, but I'll read around them later. Thanks all for your input!

Cheers
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Old 13th Feb 2022, 4:10 pm   #64
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Attached is the first ever image of an electron from the Lund University research team's paper:
An image of what the measuring equipment saw, it isn't an electron just the artefacts of what was thought to be one. I look at a building, I am not seeing it only the light reflected from it. All a bit weird, once you get over the weird bit it is logical and useable, still it doesn't make sense. Our brains are too small in both size and dimensions to cope.
 
Old 13th Feb 2022, 4:28 pm   #65
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Default Re: How Electricity Works? By Electrons or by Fields?

I have to cheat with this test, quotes from the link below:

The input impedance of the transmission line in the time domain is the impedance, looking between the signal and the return path, at the beginning of the transmission line, when we apply a step voltage signal into the transmission line. The input impedance, in the time domain is not constant. It varies with time, and varies depending on the source impedance, rise time of the signal and time delay of the transmission line.

The surge impedance of the transmission line is an older term, not used much today, which is the input impedance of the transmission line in the time domain, during the initial “surge time” the signal is traveling down the transmission line before, it reflects back to the source.

When we just refer to “the impedance” of a transmission line, most of the time, I think we mean the characteristic impedance of the line. This is the one value of instantaneous impedance a signal would see as it propagates down the line.

The input impedance of the transmission line in the frequency domain is the impedance, looking between the signal and return path, at the beginning of the transmission line, in steady state, at any single frequency at a time.


https://www.signalintegrityjournal.c...gnal-integrity

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Old 13th Feb 2022, 5:41 pm   #66
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Default Re: How Electricity Works? By Electrons or by Fields?

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Originally Posted by regenfreak View Post
Ok this student is going to fail the test question again. This is hard, man. I don't have a degree in electronics. No matter, try again, fail again fail better:
Nope, you're not going to fail. This bit isn't hard.

First of all, we have a speed limit in our universe; the speed of light. No electrical signal can travel along you transmission line or through space any faster than that. In fact, the presence of insulation increases the capacitance per unit length and slows it down a bit. The higher the dielectric constant (capacitance multiplying factor) of the insulation, the slower it gets. The semi-air-spaced coaxial cable usually used to connect your TV antenna to the set will pass signals at about 80% of the speed of light.

This means that if I connect a signal or a DC voltage to the end of a piece of cable, whatever is at the far end can have no influence on what the cable looks like at the driven end until enough time has gone by for the signal to have reached the far end, and some reflection to have come back.

So we've established that the cable's immediate appearance is independent of whatever is going on at the far end - at least for a while. So the only thing which can influence the impedance the cable presents in that instant, is the very cable itself. This is what the 'Characteristic Impedance' means.

Buy a reel of twenty seven and two thirds ohms cable (just a stupid value for fun) and when you hit it with your pulse generator, twenty seven and two thirds Ohms, resistive, is what your pulse generator sees it as, at least until it can see any returned echo. It really is this simple.

In Dover Castle, there is a well. 400 feet deep. It has an iron grille across the top and you can stand on it and look down. There is a long long cable dangled down the well and it has fluorescent strip lights at various depths. A timer turns them on and off in sequence, going down just to help you feel comfortable and not worry about the security of the grille.... So you clap your hands or shout and some of sound waves from you will reflect off of the grill, the floor, walls and ceiling, but some will go down the well. This sound wave will propagate smoothly downwards and keep on going. If there is a discontinuity in the wall like a change in diameter, or a side-tunnel, some of your sound will be reflected back. The rest will continue. Eventually you'll get a strong reflection from the surface of the water. Some will go into the water and continue, some will be absorbed, but the reflection will be strong.

If your hearing is very fast or if you cheat with a microphone and oscilloscope, you'll hear your initial sound being made, then a series of echoes from successive changes in the well, and then the big echo from the water. There will be smaller later echoes from paths which have bounced to-and-fro between reflective things, but the main ones illustrate what's going on.

The same sorts of things go on with electrical transmission lines.

The sound wave propagating down the well sees it as a sort of acoustic impedance. Any ledges, side tunnels or changes in diameter represent a change in acoustic impedance. and these changes go hand in hand with reflections.

But back to characteristic impedance:

Applying DC abruptly is an interesting thought .he line has capacitance, so many picofarads per metre. But it's distributed. The capacitance does not all live right at the entry terminal. much of it is distant, and the speed of light gets in on the act. The wire of your line is also inductive, so many microhenries per metre.

You could model your line as a lot of little capacitors, each representing the C of an increment of length, across the two conductors. Not to be left out, there needs to be similar small inductors in series with the wires between the little capacitors. It looks like a lowpass filter. The L and C values set its roll-off frequency. Let's do a finer detail model, with twice as many capacitors and inductors, each representing half the distance increment of before, so they are half the values. Filter analysis tells us that we've doubled the cutoff frequency, but also doubled the speed of roll-off above it.

Fast forward to a real transmission line, and we have to think of an infinite number of infinitesimal L and C values. Extrapolate the filter theory, and our transmission line bandwidth is limitless. WOW! In reality, resistive losses and dielectric losses stop this, but you can make good signal transporters.

When your DC edge hits the line, it can't charge all the little Cs at once, the Ls slow it down. The L and C per unit length sets the speed theDC edge moves down the cable... to the speed of light, reduced by the velocity factor. The Ls stop your pulse gen seeing all the Cs at once, the Cs stop it seeing all the Ls at once. A magical balancing act.

This should help get you a bit further. Things aren't as complicated as they seem.

David
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Old 13th Feb 2022, 5:48 pm   #67
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Default Re: How Electricity Works? By Electrons or by Fields?

I did a quick simulation of the circuit I showed back in post #55 and I've show the circuit again below.

For the transmission line I used about 2.2 metres of twisted enamelled wire to try and make up a transmission line that had a Zo somewhere in the region of 50R. This line has a delay time of about 10ns and this is the same delay used in the simulation. It's a very lossy line and the Zo will only be in the ballpark of 50R.

I dug out my old HP 54520C digital scope as this has a fast pulse generator on the rear panel. This does produce a long duration negative pulse but the 'rise-time' is typically <500ps.

I then fed this to the transmission line as per the simulation (using the resistive splitter R4, R5, R6 and R7) and then measured the response time at R2. You can see that the simulation and the actual measurement agree quite well and show that current starts to flow in R2 as quickly as it does in R8 and this happens in under 1ns. The voltage seen at R2 is about -88mV and this rises to about -135mV once the system settles fairly well after maybe 20-30ns.

The imperfections in the twisted pair transmission line spoil things a bit because you can see some wiggles caused by discontinuities in the line. However, I think this is a useful test/demonstration.
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Old 13th Feb 2022, 7:06 pm   #68
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Default Re: How Electricity Works? By Electrons or by Fields?

Quote:
When your DC edge hits the line, it can't charge all the little Cs at once, the Ls slow it down. The L and C per unit length sets the speed theDC edge moves down the cable... to the speed of light, reduced by the velocity factor. The Ls stop your pulse gen seeing all the Cs at once, the Cs stop it seeing all the Ls at once. A magical balancing act.
Thanks. For each segment of the stray capacitance:

current = stray capacitance x rate of voltage change

The stray capacitance resists the rate of voltage change by drawing current and charging it up.

For each segment of the stray inductance:

voltage = stray inductance x rate of current change

The current creates a magnetic field; hence, energy is stored in the magnetic field. It resists the change of the charging current. The stray inductance tames the charging current of stray capacitance, keeping it in check.

I have just read that the speed of light inside fibre optic cable is slower than the best coxial cable (95% speed of light).
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Old 13th Feb 2022, 8:02 pm   #69
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In frequency domain, a transmission line can be modelled by a lumped LC model, each peak of the impedance graph (attachment 1) corresponds to a resonance peak. The first peak is half a wavelength fitting the length of the transmission line. In the second peak, one wavelength fits into the transmission line *(attachment 2).
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Old 13th Feb 2022, 8:04 pm   #70
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Default Re: How Electricity Works? By Electrons or by Fields?

Refractive index = 1/Velocity Factor

Velocity Factor = Speed of light in the medium/speed of light in vacuum.

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Old 13th Feb 2022, 8:47 pm   #71
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Default Re: How Electricity Works? By Electrons or by Fields?

David's explanation of characteristic impedance (post 66) of a transmission line is a lot clearer than many text books!

I generally consider the characteristic impedance of a cable type as the resistance I'd measure with an Avo 8 if I connected it to an infinitely long length.

An infinitely long length of coax or twin-pair will of course have infinite capacitance, but almost all of that infinite capacitance is at a considerable distance from my local end, so there's the inductance of the wires to isolate it.

I put some numbers in: typical coax is 80pF/m.

If I connect, say, 5V to my local end, the voltage shoots up to 5V but this 5V propagates no faster than the speed of light. So, after 1 second, any cable that's farther than 300,000km from my source end doesn't even 'know' that I've applied a voltage. Meanwhile, after 1 second the first 300,000km-worth of cable capacitance is charged to 5V.

300,000km of cable at 80pF/m has capacitance 0.024 farads. To charge 0.024F to 5V takes 0.12 coulombs (from Q = C x V).

In the next second, the next 300,000km of cable will be charged, while the first 300,000km sees no change in voltage so doesn't consume any more charge itself. So another 0.12 C is taken from my 5V source. And so on for every subsequent second.

A continuous drain of 0.12C per second is just a current of 0.12A of course. and breaking down into 150,000km every half-second; 300km every millisecond; 0.3m every nanosecond, you can see that the 0.12C per second isn't taken in 'gulps,' it's taken smoothly.

And something which consumes 0.12A when 5V is applied is equivalent to 1 41.7 ohm resistor.

That 41.7 ohms is pretty close to the characteristic impedance of many cables - add in the fact that insulation materials slow down the velocity of propagation and you soon get 50Ω, 75Ω, or whatever.
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Old 13th Feb 2022, 8:59 pm   #72
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Default Re: How Electricity Works? By Electrons or by Fields?

It's this 'transit time isolation' which allows you to build neat things like transmission line transformers. If you have, say, 10 equal lengths of co-ax cable you can connect them in parallel at one end and launch a pulse into all 10 of them at the same time. Then you can connect the other ends in series (inner of cable n to outer of cable n+1) and when the pulses arrive you get 10 times the input voltage between the outer of cable 1 and the inner of cable 10.

Of course the impedance at the output is 100 times what you had to drive at the input, and in the real world if you use co-ax then cable n has to charge both the internal impedance of cable n+1 and also the stray capacitance from the outer of that cable to ground, so you lose a bit to that stray loading. It becomes a real issue if you're stacking more than a few cables. The things can be made to work quite effectively for suitably short pulses though.

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Old 13th Feb 2022, 9:24 pm   #73
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Default Re: How Electricity Works? By Electrons or by Fields?

The velocity of the signal propagation equation is incredibly elegant:
v = 1/ SQRT(LC)

When I first saw above equation, I found it unbelievably simple, almost mysterious and too good to be true.

How accurate the second method of measuring the speed of light described in the second method in this experiment using resonance LC?

https://facultyweb.cortland.edu/doug...eedOfLight.pdf



The other issue is: I am unable to establish the role of Fermi velocity on the transient time history of current across the resistor (after the first pulse returns)in AlphaPheonix's video. Everyone focuses on the speed of signal in the discussions.

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Old 13th Feb 2022, 10:20 pm   #74
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Default Re: How Electricity Works? By Electrons or by Fields?

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The velocity of the signal propagation equation is incredibly elegant:
v = 1/ SQRT(LC)
I d better write L' and C' (per unit meter) not to confuse with normal L and C in the resonance equation:

v = 1/ SQRT(L'C')
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Old 13th Feb 2022, 11:06 pm   #75
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Default Re: How Electricity Works? By Electrons or by Fields?

That makes it dimensionally correct! Capacitance per metre (or mile, or whatever) and inductance per metre (or mile, or whatever) gives velocity in metres (or miles, or whatever) per second.
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Old 14th Feb 2022, 12:13 am   #76
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Default Re: How Electricity Works? By Electrons or by Fields?

Don't overlook that the velocity factor can be simplified to 1/SQRT(Er).

If you make a transmission line using PTFE (Er=2) as the dielectric the velocity factor should be 1/SQRT(2) = 0.71

If you use polyethylene (PE Er=2.25) the velocity factor will be 1/SQRT(2.25) = 0.66.


The datasheets for various coax cables should show similar velocity factors if they use PTFE or polyethylene as the dielectric material.
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Old 14th Feb 2022, 12:16 am   #77
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Default Re: How Electricity Works? By Electrons or by Fields?

So the wave/particle duality of photons as per the double slit experiments mentioned earlier, extends out to electrons and even atoms/molecules then..? Curiouser and curiouser. As for 'virtual photons'....the deeper you dig, the weirder it gets.

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Old 14th Feb 2022, 12:31 am   #78
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Default Re: How Electricity Works? By Electrons or by Fields?

It's possible to do some neat transmission line testing using a VNA to work out the velocity factor and Zo. A few years ago I measured 59cm of the classic 300 ohm ribbon feeder on a 4 port VNA. Each end of the ribbon obviously has two connections so I needed a 4 port VNA to measure the twin feeder at both ends at the same time.

I have a decent test fixture that de-embeds the measurement so the reference plane is right at the connection to the ribbon cable. Once this is done the VNA data gets exported as a 4 port s4p (s-parameter) file and this becomes a component model for the 59cm long feeder and this can be entered into a simulator.

I put a quick youtube video up showing me playing with the s4p data. You can see I connect it up using simulated baluns and then measure the pF/59cm and the nH/59cm for the 59cm line.

The Zo can then be calculated. Note also that the simulator allows the port impedances to be changed and you can see that the optimal port impedance for low insertion loss is about 309 ohms and this agrees with the SQRT(L/C) calculation.

https://www.youtube.com/watch?v=aAtVTIVmrBc

I also compare the VNA measurement against a theoretical transmission line with Zo =309R and length 59cm. You can see the results are quite close. You can also see a plot of group delay when the 300R feeder is correctly terminated. Sorry, there is no sound on the video.
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Old 14th Feb 2022, 2:04 am   #79
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Default Re: How Electricity Works? By Electrons or by Fields?

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I have just read that the speed of light inside fibre optic cable is slower than the best coxial cable (95% speed of light).
Very likely! The material chosen for the fibre has to have a high refractive index, so that the light stays inside the fibre by total internal reflection. The critical angle, which is the angle at which an internal ray of light makes with the material surface, stays entirely within the material and none escapes, is dependent on the refractive index.

A high refractive index, of course, implies a slow propagation velocity.
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Old 14th Feb 2022, 3:10 am   #80
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Default Re: How Electricity Works? By Electrons or by Fields?

it is easy to measure velocity factor with nanovna:
https://youtu.be/aWvPB299U60
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