How Electricity Works? By Electrons or by Fields?

[Craig Sawyers]

Originally Posted by emeritus

I recall that there was a lot of correspondence about "displacement current" in charging and discharging capacitors with large plates in the pages of "Wireless World" in the 1980's. I don't recall a consensus being reached, but then again I didn't read every issue.

December 1978, page 51-. And a follow up in March 1979 page 67- . At the time of these articles I was at Southampton University, and was a real Maxwell's Equations theoretical jockey at that time. Catt's et al's rather simplistic treatment took a single cup of coffee between several of us to debunk the articles.

The authors tried to get rid of displacement current by making capacitor plates look like transmission lines with reflections at the plate boundaries. It did not stand the test of time as a concept.

There is a Wiki page about displacement current https://en.wikipedia.org/wiki/Displacement_current which indicates the need for displacement current in time varying fields.

What Catt et al missed in their alternative treatment of current flow in a (parallel plate) capacitor missed of course is that the theory of displacement current knows nothing of a capacitor. It is a quite general treatment of electrical fields in dielectric media, including anisotropic and non-linear media. In fact it finds its way into non-linear optics, where the non-linear displacement fields generate harmonics and frequency shifting in high power lasers. With high efficiency - typically 50%.
In these processes, think of the atoms as being a local oscillator which also behaves as a mixer generating an intermediate frequency in a radio receiver. Same sort of process, but at optical frequcies (~10^14Hz)

Craig

[Radio Wrangler]

The Catt theories sank without a trace, and he got enmeshed in wafer-scale stuff with (later sir) Clive. I thought the guy using the Ouida Dogg nom-de-ploom was unkind, maybe as unkind as Catt's parents?

If you have a capacitor, you have capacitance.

Now, take away the plates... does the capacitance remain, like some smile of a cheshire cat?

I suspect that capacitance remains, as a property of space, and dielectric, as a distributed quantity. The plates are just our way of controlling how much space we are playing with. This fits with fringing effects around plates and dielectrics, and also fits with what Craig says above.

David

[GrimJosef]

Certainly a single plate has capacitance. There's a 1st-year electromag exercise in which students get to calculate the capacitance of planet earth, allowing the electric field to decay to zero at infinite distance. It's about 700uF.

Cheers,

GJ

[Craig Sawyers]

That is what Nikola Tesla was trying to use for long distance communication in his Wardenclyffe tower. His intention was "to shake the earth", in other words to use the 700uF of the earth's capacitance. There was an alternative mode using Schumann resonances in the atmosphere.

I don't think he quite understood it in those terms, but that in effect of what his tower was supposed to do - long distance comms somehow. Demolished to settle debts when backers foreclosed.

https://en.wikipedia.org/wiki/Wardenclyffe_Tower

Craig

[GMB]

Yes, what a pathetic capacitor the earth makes. But what is its voltage rating I wonder? And the leakage, which may be quite large due to the solar wind?

[David Simpson]

My oh my, the academics on this forum site are having a "field day" - delving into Quantum & Atomic physics.
Like GMB, I think the American lecturer, in his video, gave a poor presentation. Does anyone know of a decent British based Youtube video which could answer James' questions more clearly? And perhaps, for the forum folk who are just learning radio/electronics as they work away at their growing collection of projects - a decent British Youtube video which explains basic DC & AC theory leading up to whats being discussed here ?

Regards, David

[Radio Wrangler]

We're just playing around and having a bit of fun.

Physics (and, by extension, other sciences and pretty much everything else) has a wee problem.

There are theories and models about what stuff is, but they don't seem to go all the way to the bottom, and they don't all seem to fit together. What we've got can at least be used at various levels. We have tools which work, at least for designing radios and nuclear reactors. But if we want to go to further things, we run into their limitations. If we want to think about the tiniest details or look for a faster-than-light spaceship drive, or time machine then we're off into the unknown.

In any analysis, everything we make or have or use is made out of <unknown>

You might find it disquieting.

Or you might take a pragmatic view and decide that for the things we use in normal life, our tools and ideas are sufficient to make things work.

Why do research into things so far from our natural scale? Well the results can be a seeming waste of money and effort, but every now and then, the unexpected is found.

Different people have different interests and viewpoints, it stops the world being boring.

David

[kalee20]

Originally Posted by Radio Wrangler

The Catt theories sank without a trace...

Yes, I remember the correspondence (the Catt Anomaly), although I didn't follow it closely.

I remember one letter in EWW, pointing out that the correspondence had run for 10 years, surely it should be commemorated in some way? Immediately after this letter was an Editor's comment: Yes - the correspondence is now closed.

Originally Posted by Radio Wrangler

If you have a capacitor, you have capacitance.

Now, take away the plates... does the capacitance remain, like some smile of a cheshire cat?

I suspect that capacitance remains, as a property of space, and dielectric, as a distributed quantity. The plates are just our way of controlling how much space we are playing with.

I'd agree - the capacitance is always there. The plates merely provide a way to connect wires to it.

[David Simpson]

Right enough David, I thoroughly admire & respect the depth of knowledge of many folk on this Forum site. We have Professors, Doctors, and a good handful of BSc's & MSc's. I think there is even a Knight in VMARS.
I often feel that its a pity that much what is said in the multitude of pages here could not be expressed more pro-actively. Face to faces, so to speak. Perhaps once Covid restrictions have eventually passed for good, experienced fellows can take the oportunity to give talks, do a bit of mentoring, etc. - to up & coming novice members of this site & BVWS or VMARS. Greater London has masses of VR folk, going by folk's given details & membership lists. Places like Sheffield, Stafford, Manchester, Edinburgh, Newcastle seem also to be hot-spots.
I personally know of two highly qualified chaps from this forum site who do such as I suggest. An ARS chap I know locally, an ex physics lecturer, does uni student & ARS mentoring.
Old fart me - for a short time in the RAF, taught trainees & apprentices up to C&G &/or ONC level - many years later have mentored one or two local ARS or VR younger folk in past years. (Not, mind you, rotating matricies, or la plas transforms, or thevenin's theorum - - just bog standard basics).

Regards, David

[Craig Sawyers]

Originally Posted by Radio Wrangler

We're just playing around and having a bit of fun.

Physics (and, by extension, other sciences and pretty much everything else) has a wee problem.

David

Indeed. At very large scales, gravitational lensing by massive galaxy clusters needs ten times the mass of the visible stuff. That is so-called Dark Matter. And no one knows what the darned stuff is. There have been theoretical attempts, but so far with no, or little success. And Dark Energy is causing the expansion of the universe to accelerate. And again, no one knows what this stuff actually is.

At the other scale of quantum phenomena, particle physics is now rather well understood - a rather nice symmetrical arrangement where theory and experiment agree to high accuracy (like 5 digits or more).

The problem is that the macroscopic universe - even without considering Dark Energy and Dark Matter has no theoretical link whatever with quantum physics of the very small.

So as David suggested - there are some really fundamental physics problems in understanding the Universe!

Craig

[jamesinnewcastl]

Hi All,

The video states that fields carry power but in the case of DC it clearly (?) doesn't. To my mind that makes the theory stand in tatters.

My next concern is that if fields are only generated by a moving current then you absolutely need a current to support the concept of the power being carried by fields - so what is the current doing if the fields carry all the power? Sort of just along for the ride?

When energising the light bulb at the end of the wires my intuitive thought is that, using a voltage, you push the electrons in one wire and pull at the electrons in the other. I would assume that the electrons at the far end are as yet unmoved. Now as the electrons all get agitated along the wire and move, so fields appear riding along with (caused by) the now moving electrons in that part of the wire. Thus a tidal wave rushes down the wires producing a field as they go.

Another - Consider a resistance wire with a DC current in it - the wire gets hot - is that the field or the electrons doing the heating?

Another - Imagine the wires to the bulb are very long but after 10 metres or so they are pulled widely apart - the current is exactly the same but the fields are not - so if these fields are carrying all the power - how does the seperation and weakening (?) of the fields affect the power transmission? No effect? seems unlikely to me. What if the wires are seperated at right angles?


Cheers
James

[emeritus]

When my son was early on in junior school, I was tucking him in one night and he asked me how radio waves travelled through space? I replied that I didn't think anyone actually knew. A clever man called Maxwell had developed some equations that meant that, if you put an electric signal on an aerial, you could calculate how much signal would be generated in another aerial at a specified distance away, that Maxwell's equations were based on wave motion like you get when you drop a pebble in a still pond, and that the waves needed a medium to propagate in. That was why the early scientists came up with the concept of the aether, a mysterious medium that existed everywhere, for the waves to propagate in. Some other scientists had done some experiments that they reckoned proved that the aether didn't exist, yet Maxwell's equations worked, and they were based on its existence. I said it was just an example of something that we could decribe by what it did, but didn't know what it was, like electricity.

[G0HZU_JMR]

The video states that fields carry power but in the case of DC it clearly (?) doesn't. To my mind that makes the theory stand in tatters.

If you are comfy with how an extremely narrow pulse from a pulse gen travels down a transmission line (could be coax cable or a wire over ground) then I think you can expand on this to look at what might be thought of as the DC case.

If the pulse was sent again but was twice as fat then the leading half of the fatter pulse doesn't get pushed along by the half behind it. The pulse doesn't get any faster in other words. For a bit of fun watch the classic Dave Allen sketch below and think of every worker as a pulse of energy travelling along the 'wire' line. The pulse gradually gets fatter as each worker joins the line. If the sketch went on forever would it become the equivalent of DC?


https://www.youtube.com/watch?v=x_NM09rfFYI

Would it matter to the workers at the start of the line if the first worker arrived at the factory and was converted to heat? They wouldn't walk any faster or slower. If the workers stopped joining would it affect the workers at the head of the line?

[Craig Sawyers]

Originally Posted by jamesinnewcastl

Hi All,

The video states that fields carry power but in the case of DC it clearly (?) doesn't. To my mind that makes the theory stand in tatters.

My next concern is that if fields are only generated by a moving current then you absolutely need a current to support the concept of the power being carried by fields - so what is the current doing if the fields carry all the power? Sort of just along for the ride?

When energising the light bulb at the end of the wires my intuitive thought is that, using a voltage, you push the electrons in one wire and pull at the electrons in the other. I would assume that the electrons at the far end are as yet unmoved. Now as the electrons all get agitated along the wire and move, so fields appear riding along with (caused by) the now moving electrons in that part of the wire. Thus a tidal wave rushes down the wires producing a field as they go.

Another - Consider a resistance wire with a DC current in it - the wire gets hot - is that the field or the electrons doing the heating?

Another - Imagine the wires to the bulb are very long but after 10 metres or so they are pulled widely apart - the current is exactly the same but the fields are not - so if these fields are carrying all the power - how does the seperation and weakening (?) of the fields affect the power transmission? No effect? seems unlikely to me. What if the wires are seperated at right angles?


Cheers
James

To deal with all of those in turn:

1. At DC, where a static current flows down a wire, there is still a static magnetic field associated with the wire (put the wire through a piece of card, and sprinkle iron filings - they line up with the magnetic field lines). Someone mentioned Poynting Vector somewhere above. That defines the power flow in terms of the current and magnetic fields, which is an "easy" form of the notion.

2. To develop this a bit further, DC current in a wire generates an electric field (where the field lines are radial coming out from the wire) and a circumferential magnetic field. The power flow along the wire itself is equal to the electric field x the magnetic field - and perpendicular to both of those. Which is along the wire. So power flow in a real sense is carried by the fields (electric and magnetic).

3. The transient effects (what happens when you turn on a light bulb) are real. If you close an instantaneous switch, and the bulb is at the end of a long pair of wires is real and can be measured. What happens is that a wavefront moves down the wires until it gets to the other end. You won't see this effect with a light bulb, because it takes so long to get hot that these transient effects have died down by the time the bulb is hot. But if you use an oscilloscope, or use a fast LED in place of a bulb, you can detect these effects by looking at the LED using a fast photodiode and oscilloscope.

4. These transient effects need the concept of a transmission line. This has a notion called Characteristic Impedance. Think of a pair of wires in this circuit we're talking about. The capacitance C between the two wires is due to the electric field, and the inductance L between the two wires is due to the magnetic field. The Characteristic Impedance is root (L/C) and is resistive. For an RF coax, that might be 75 or 50 ohms, and is determined by the L and C of the coax.

5. Then all your questions about what happens if the wire spacing changes in the circuit, of they cross in some way and be understood that the Characteristic Impedance is not constant (because L and C are changing as you go down the wires. So the transient flow down the wires is a complicated thing, and instead of a nice step waveform going down the wire, it gets smeared out.

6. The transient effect die out very quickly for short wires (and even for quite long ones). In the DC condition, moving the wires around generates a new transient because you are moving the fields around. Then they quickly settle down again (for a 10 metre length of wires we're talking about less than a microsecond). But with an oscilloscope you could measure this by looking at the voltage at the bulb (or resistor, or LED etc).

Craig

[G0HZU_JMR]

Yes, I'd encourage anyone to consider any waveform (pulse, sinewave, triangle wave or even DC) as a series of incredibly thin pulses of energy each making their own way down the transmission line. The fact that the series of pulses make up a longer waveform that resembles a sinewave or a squarewave is kind of irrelevant to the progress of each individual fine pulse within the waveform.

The transmission line consists of distributed L and C and if you can get comfy with how the pulse travels down the line in terms of fields then DC becomes just another waveform. You can apply this analysis to everyday structures like a coupled line directional coupler and (very quickly and easily) work out why it has a null response at DC at the FWD coupled port and why it has other null responses at the FWD port at regular intervals in the frequency domain. I think if you tried to describe this behaviour of the directional coupler at every frequency using just classic equations it would be a lot of hard work.

[regenfreak]

In the film Full Metal Jacket, Private Joker had a "born to kill" writing on his helmet and also wore a Greenpeace button, he gave a philosophical answer to colonel on the "Duality of Man". Now there is the "Duality of Electron", behaving like particles and waves at the same time. Reading quantum mechanics is reading philosophy. It is worth watching the responses to the video:

experimental demo:

https://youtu.be/2Vrhk5OjBP8

EE blog
https://youtu.be/VQsoG45Y_00

RDS academy:
https://youtu.be/MDOYPQQmuco

[regenfreak]

The link below gives the most easy to understand explanation. I often ponder how electron transient time in a copper wire could be measured.


"In the case of electrical currents traveling through metal wires, there are three different velocities present, all of them physically meaningful:

The individual electron velocity
The electron drift velocity
The signal velocity"

Link below:
https://www.wtamu.edu/~cbaird/sq/201...f-electricity/

[G0HZU_JMR]

I have done some experiments like this in the past. A few years back I built a huge printed directional coupler using three A4 sized PCBs in series. The aim was to pass a very fine pulse that was <<1ns wide into the coupler and then monitor all the ports using a 4 channel 40GSa/s scope (13GHz bandwidth).

From this it was possible to examine how a backward wave coupler operates as it took a few ns for the pulse to travel down the coupler. The difficult bit was obtaining a reasonable match everywhere to minimise discontinuities but it was possible to see what happens with various terminations at the far end of the coupler. This can also be done in a simulator but it is kind of neat to see it for real even if the pulse shapes aren't as well defined as the simulation. It did prove to be educational if a bit wobbly.

[regenfreak]

using a 4 channel 40GSa/s scope (13GHz bandwidth).

woww!

[G0HZU_JMR]

Sadly, it's not mine. It belongs to the company I work for. It has fixed 50R inputs rather than 1Meg. I did check out the bandwidth with a 20GHz sweep generator and it was about 13GHz.