How Electricity Works? By Electrons or by Fields?

[GrimJosef]

Originally Posted by kalee20

... Very likely! The material chosen for the fibre has to have a high refractive index ...

These things can be a strong function of wavelength. At optical wavelengths fused silica fibre has a refractive index a bit below 1.5. Co-ax typically handles wavelengths at least 10,000 times longer. The effective refractive index of the dielectric (e.g. teflon, or polythene) depends on how much air is mixed with it, either simply by using thin plastic supports in an otherwise air-filled gap between inner and outer, or by 'foaming' the plastic. It can vary from the 100% plastic figure, again of around 1.5, down to below 1.1 if there's enough air in it.

I was once in charge of some few-metre lengths of 72cm (if I remember rightly) diameter 'co-ax' whose dielectric was de-ionised water (quite a good insulator on short timescales). Given the nanosecond timescales I was dealing with, I was interested in wavelengths of a less than a metre in air. But in this regime water has a 'refractive index' of around 9 ! So the wavelengths drop by that factor inside the transmission lines. At optical wavelengths, on the other hand the same material - water - has a refractive index of just 1.33, as we can quite easily see if we look into a stream and notice that it appears 25% shallower than it really is.

Cheers,

GJ

[Craig Sawyers]

One of the tricks with particular high-power pulsed gas lasers, which need an electrical discharge to work, was to use a deionised water dielectric capacitor, charged to many kV. It doubled as a transmission line, so when the discharge was initiated, the capacitor emptied itself in a time determined by the physical dimensions of the capacitor.

Craig

[Craig Sawyers]

For anyone interested in this stuff, the most efficient and high power lasers have always been carbon dioxide. From wall socket to laser power is around 10% efficiency, and the laser wavelength is 10.6 microns (or 0.0106mm). They were invented in the early days of lasers back in the 60's.

They are used extensively for material processing - cutting, welding, annealing.

This http://www.sintecoptronics.com/CO2HP.asp is pretty much what you can buy. Note that these things weigh over a ton and can chuck out over 4kW.

Needless to say these things are exceptionally dangerous. Like any machine tool of course. But with one of these you cannot see the beam at all. The visible range is 400 to 700nm, so 10.6 microns (10,600nm) is well outside the visible range.

Craig

[GrimJosef]

Originally Posted by Craig Sawyers

For anyone interested in this stuff, the most efficient and high power lasers have always been carbon dioxide ...

Apart from diode lasers

https://www.laserfocusworld.com/lase...w-capabilities

Cheers,

GJ

[regenfreak]

Bell Lab 1959, mechanical vs electrical vs acoustic waves, excellent visual demos

https://youtu.be/DovunOxlY1k

[regenfreak]

Latest breakthrough in semiconductor technology in terms of speeds.

Photonic transistors (AI chips):
https://youtu.be/L6oXAfvmJQ8

Interaction of photons and sound waves to speed up signal speed:
https://www.sciencedaily.com/release...0926110935.htm

VTFET (Vertical FET):
https://youtu.be/IHxv8ehrx2Q

[Craig Sawyers]

Originally Posted by GrimJosef

Apart from diode lasers

https://www.laserfocusworld.com/lase...w-capabilities

Cheers,

GJ

Well that was interesting! Very efficient, and with powers starting to rival CO2.

Not as easy to use as CO2 though, and with the energy densities in a rather compact semiconductor structure, I'd be concerned about lifetime.

Craig

[GrimJosef]

Ease of use is a matter of the detail. Diode-pumped solid-state lasers have been commercial turn-key systems since the late 1990s. Their much shorter wavelength than CO2 allows a wider range of materials to be used both in the beam transport and in diagnosis and control. And diffraction is much less of an issue. Importantly the only moving parts are in the water cooling system. Properly big CO2 lasers, on the other hand, have to have fast laminar gas flow inside the laser volume.

It's hard to know where the real power limits are as they tend to be in military systems which tend to be classified. There were rumours back in the 1980s of a several-hundred kW CO2 system on the US west coast. I have seen this 67kW system https://lasers.llnl.gov/science/phot...d-energy/sshcl in the flesh, so to speak, and that's the power out of the Nd:YAG slabs. The diode pump power would have been rather more than 100kW (run from a surprisisngly compact MW-class battery !). Of course if you want real power then the way to go is with a chemical laser. The one here https://en.wikipedia.org/wiki/Boeing_YAL-1 was described as 'MW-class'. It's a bit hard to define efficiency when the laser power comes from a chemical reaction. There are also real operational issues with them. My boss at the time said "Essentially the operators are in an aircraft which has a continuous explosion going on in the back of it. They are trying to direct the power from that explosion into an enemy aircraft. It really isn't clear which aircraft you'd be better off in !".

Cheers,

GJ

[Craig Sawyers]

I can't remember where it was, perhaps Culham, perhaps elsewhere. This was one of the stupid class many tens of kW CO2 TEA lasers in the 80's sometime. The sort of thing with supersonic gas flow. During testing they has used a fire brick as a beam stop, not realising that the beam would make short work of a fire brick.

Next there was a scream from down the hall "Turn that thing off!"

Basically after the fire brick it had gone through the wall into an office. The first the guy in the office knew about it was when his filing cabinet side glowed white and a hole appeared. It then started on the opposite wall. Fortunately his head was not in the way.

Craig

[GrimJosef]

It was Culham and I think the (alleged) incident happened with their 15kW laser in the late 1970s. The version I heard had the beam get into a metal-walled corridor. It burnt the paint off at each contact point forming a crude mirror and causing the beam to zig-zag along the corridor. CO2 laser beams are invisible of course, so tricky to avoid, although in this case no harm was done to any people.

Cheers,

GJ

[Radio1950]

Originally Posted by regenfreak

Bell Lab 1959, mechanical vs electrical vs acoustic waves, excellent visual demos

https://youtu.be/DovunOxlY1k

The Bell Labs video from AT&T Archives.

I believe there may be an error at time 4:50 in this specific version of this video, where the blackboard drawing of the electrical analogy of the transmission line shows a "far end short circuit" giving a reflection "in phase", and an open circuit giving a reflection "out of phase".

The opposite is true in each case.

Please prove me wrong.

[Radio Wrangler]

Originally Posted by Radio1950

Please prove me wrong.

I'd rather prove you right.

Launch a wave along a transmission line. Zo has to be purely real, so there is a current wave and voltage wave, in phase, propagating together at the same speed, so in phase at all times and all places.

The end of the line is an open circuit. The voltage wave can express itself freely across an open. The current wave has nowhere further to go, but back up the line.

So, we have, when the voltage wave is positive, a positive voltage across the line and current going back up the line. The direction of flow of energy is reversed. Seen in the reverse direction the voltage and current are in the proper phase, as if the generator had been moved to the far end.

If we inspect the voltage and current very close to the open end, we see the signal voltage, but we see both forwards and a cancelling reverse current waves. So we see a high impedance. So this checks out OK.

If we try this with a short. the current does have somewhere to go at the end and flows back on the other wire of the transmission line pair. The voltage has to be zero, so we must have an inverted voltage wave to give cancellation, travelling in the reverse direction. In the reverse direction we therefore have both an inverted current wave and an inverted voltage wave, so the returned wave is still correct, with voltage and current components in phase with each other, but they suffered a phase inversion compared to the forward wave.

Seen close to the shorted end, we see current flow, but we see voltage cancellation, so a very low Z.

If we have a quarter-lambda line and short the far end. the wave gets 180 degrees of phase shift from the round trip, and if the short is an inverting reflection, that gives 360 degrees total, so the near end looks like an open circuit. This too checks out.

It all fits.

I haven't watched the film, but it looks like you've spotted a goof.

David

[Radio1950]

Yes,
I saw the possible problem quite a few years earlier, but thought it rather churlish to spoil a marvellous visual explanation of wave motion.

[dave cox]

Originally Posted by The Philpott

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.

Dave

Yes, you can certainly get interference patterns with electrons and they can be visible on a CRT, under the right conditions. Pretty tricky to explain that with classical physics! On the other hand some things are so simple to "explain" with classical physics, like magnetism There is an well known video of Richard Feynman being asked by an interviewer to explain magnetism but ultimately it involves virtual particle ... I imagine Mr Feynman would have a similar answer if questioned on the subject of the OP

Still, that's beginners level confused compared to entanglement

dc

[dave cox]

Originally Posted by The Philpott

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.

Dave

Yes, you can certainly get interference patterns with electrons and they can be visible on a CRT, under the right conditions. Pretty tricky to explain that with classical physics! On the other hand some things are so simple to "explain" with classical physics, like magnetism There is an well known video of Richard Feynman being asked by an interviewer to explain magnetism but ultimately it involves virtual particle ... I imagine Mr Feynman would have a similar answer if questioned on the subject of the OP

Still, that's beginners level confused compared to entanglement

dc

[Craig Sawyers]

Feynman declined to give a layman's answer to the force between two magnets. He said that the process involved atomic processes that could not be explained in simple terms.

And the Young's Slits experiment, when you turn down the light so that only a single photon at a time passes through. And you still get a diffraction pattern. Quantum effects.

And Feynman had a statement about that (paraphrased) "If anyone tells you they understand quantum mechanics they are lying. No one understands this stuff. All we know for certain is the equations work perfectly to describe nature".

Craig

[Radio Wrangler]

If you keep asking the question 'why', pretty soon you run out of physics.

Have I just made myself a new .sig one liner?

David

[G0HZU_JMR]

The Bell Labs video from AT&T Archives.

I believe there may be an error at time 4:50 in this specific version of this video, where the blackboard drawing of the electrical analogy of the transmission line shows a "far end short circuit" giving a reflection "in phase", and an open circuit giving a reflection "out of phase".

The opposite is true in each case.

Please prove me wrong.

I'd recommend you watch the video again because I think it is easy to misinterpret what is being described in the presentation. It's common knowledge that the reflection coefficient for an open circuit is 1 / 0deg so I can see why many people viewing this in recent decades might think the video is wrong.

Watch it again and listen to how he describes the role of the mechanical clamp and how he reads this across to the prevention of current in the case of the open shown below it.

Back in those days few people would had access to a scope let alone a fast pulse generator and the presenter is talking in terms of the effect of the discontinuity on current not on voltage. I agree the blackboard diagrams look strange because we are all used to using pulse generators and scopes but I think you should watch the video again and think in terms of current.

[Craig Sawyers]

Originally Posted by Radio Wrangler

If you keep asking the question 'why', pretty soon you run out of physics.

David

Or you run out of maths.

Hilbert tried to derive all of mathematics starting from self evident axioms, following the same principle as Euclid did in his Elements in deriving a complete set of geometry.

Alas for Hilbert, 11 years into this work Kurt Godel came along and proved that you could either have a consistent mathematical framework that was incomplete, of a complete mathematical framework that was inconsistent.

So Hilbert and his coworkers efforts were in vain.

Craig

[G0HZU_JMR]

Originally Posted by Radio1950

The Bell Labs video from AT&T Archives.

I believe there may be an error at time 4:50 in this specific version of this video, where the blackboard drawing of the electrical analogy of the transmission line shows a "far end short circuit" giving a reflection "in phase", and an open circuit giving a reflection "out of phase".

The opposite is true in each case.

Please prove me wrong.

Hi R1950 I think you are wrong and in case you are still unconvinced see the simulation below that shows the current pulse returns 'upside down' from an electrical open circuit just as described in the video. I've also shown the waveform on a screenshot from the video for the 'open' case.

I think the video is fine in this respect and you must have missed the presenter's reference to current rather than voltage.