General > General Technical Chat
Veritasium -- How Special Relativity Makes Magnets Work.
aetherist:
--- Quote from: TimFox on April 03, 2022, 10:45:18 pm ---
--- Quote from: aetherist on April 03, 2022, 10:36:38 pm ---
--- Quote from: TimFox on April 03, 2022, 09:37:24 pm ---
--- Quote from: aetherist on April 03, 2022, 09:26:35 pm ---
--- Quote from: TimFox on April 03, 2022, 09:16:52 pm ---Drifting electrons and holes make transistors work.
Flying electrons make vacuum tubes work.
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Yes, i am ok with drifting electrons making transistors work. And flying electrons too.
But how do drifting electrons in a wire know that the wire is or isnt insulated?
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In a coaxial cable, see how the dielectric effects the speed of propagation of the voltage from center conductor to coaxial shield in any normal textbook.
In such a cable, the relevant variables as a function of time at a given plane cutting the cable are the voltage from center to outer conductors, and the net current in the center conductor.
Note that the coaxial transmission line is a generalization of Heaviside's triumphant demonstration of adding inductive loading with lumped-constant inductors to telegraph lines.
A commercial cable (e.g., RG-58/U) is an example of a good transmission line. An insulated wire in an undefined environment (like from the bench to the floor) is a crummy ill-defined transmission line.
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There is still no explanation of how internal drifting electrons (in the central wire in the coax)(& in the ordinary wire that is not a coax) change their speed, or the speed of the wavefront, based on
(1) whether there is internal dielectric (in the coax), or
(2) whether there is external insulation on the outside of the shield of the coax, or
(3) whether there is insulation on the outside of an ordinary wire.
Which raises an interesting question. What is the speed of electricity along a coax if the outer shield duznt have insulation on the outside?
I think that the speed of electricity along the central wire would depend on the speed of electricity in the dielectric (usually 2c/3).
And the speed along the outer shield would be the speed of electricity in air (ie c/1).
Two different speeds. Fast going out, slow coming back.
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In a transmission line, the observable variables are the voltage across and the current down the cable, that I discussed in my reply.
That is "electricity" moving through the cable. The electrons respond in their own manner.
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If the electrons merely respond (which i agree with) then ok this avoids the "insulation catastrophe". But, it then contradicts any old electricity theory that claims that drifting electrons cause the electricity.
aetherist:
Here is one attempt to wave away the "insulation catastrophe".
William Beaty
Electrical Engineer 35yrs, electrostatics hobbyist, Amasci siteUpdated Jun 16
Is it true that electric currents are normally much faster than the actual electrons?
Well …is it true that air currents are much faster than the actual air? Yep.
• If you blow into a hose, the air in the far end of the hose starts moving instantly. Actually there’s a tiny delay before it starts moving: the speed of sound. Currents of air have a “startup-wave,” which is the same as a sound wave. The current moves at hundreds of KPH, while the air itself moves slow. (And, if rather than blowing, instead you sucked on the end of the hose, then what happens? Instead the air-currents travel opposite to the direction of the air! The air moves toward your lips, while the “current” races in the opposite direction, going out to the far end of the hose.)
• Is it true that water currents are much faster than actual water? Yes, water currents travel at the speed of sound. Step into a pool or pond, and the water level rises everywhere at the same time (after a speed-of-sound delay.) All the water moved slightly outward, away from your intruding foot. Now lift your foot back out. A “wave of decrease” races outwards, as all the water is slightly moved towards your foot.
• Is it true that “wood currents” are much faster than the actual wood? Yes, if you pick up a broom from one end, all the wood seems to move instantly. The wood-currents seem to appear everywhere in the wood. But there’s actually a small delay …from the speed of sound inside wood! The wood-current travels faster than the actual wood. (Heh, and if you lifted the broom handle sideways, then you generated a transverse wave, which travels slightly slower than pressure-waves inside wood. S-waves versus P-waves in solids.)
• Is it true that “train-car currents” are faster than actual train-cars? Yep. If you’ve heard a long freight-train starting up, maybe you’ve heard the “booms” as the couplings between cars are suddenly tightened. The train itself moves slowly forwards, and the “boom-boom-booms” races backwards along the chain of cars. Train-currents are very fast! Also, they’re backwards!
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PS
Electric currents have another name: ELECTROMAGNETIC WAVES! That’s why currents (and voltage) in wires will always travel at the speed of light. [1]
When the electricity inside a wire starts to flow, it doesn’t actually all flow at once. Instead, all the movable charges are using e-fields to “talk” with their neighbors. When one electron is forced to move, the surrounding e-fields change, and this moves the next electron in the chain …which changes that electron’s surrounding fields, which then moves the next one. It’s like a long row of train-cars, going “boom-boom-boom-boom.” Wires contain long columns of electrons. Fast EM waves travel along these columns, “informing” all the electrons to start moving.
Heh, electric companies don’t sell “electricity.” Instead they sell electromagnetic waves! (The EM waves at 60Hz frequency.) The electricity just sits inside the wires and wiggles back and forth. Electricity doesn’t travel to your home. The electric companies are actually selling us some 60Hz photons.
Also, these waves of e-fields are not moving inside the metal wires. Instead, the fields are out in the air, in the space surrounding the wires. The “startup wave” is leapfrogging through space, skipping across billions of electrons on the surface of the wire.
In other words, wires are like energy-ducts. Wires behave something like hoses full of air, with sound waves racing along, while the “air” inside moves slowly. Also, the “air” in the hose can wiggle back and forth, while the energy zooms along in just one direction.
Too complicated?
Just remember that the flow-rate of these energy-waves is measured in watts, while the flow-rate of the electricity inside the copper is measured in amperes. Two kinds of flow, with two different units of measurement. For AC circuitry, the amperes are a back-and-forth wiggle, while the watts are an EM wave at the speed of light. (It’s a lot like wiggling air …versus moving sound waves.)
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[1] In wires the current usually propagates slightly slower than light in a vacuum, going slower than “c”. This happens whenever the wire is encased in plastic, not in vacuum. The plastic insulation slows down the waves of current …much like the glass in a prism slows down the light waves passing through it. The electrical waves will propagate at 2/3 of “c,” or even slower, depending on the type of plastic. Search for… “velocity factor” in cables.
TimFox:
Something causes the electrons to drift, not the other way 'round.
Applied voltage, time-changing magnetic flux, photons, thermoelectric effects, etc.
I am speaking of physical observables, like when you connect a transmission line to an oscilloscope, or a wire to an ammeter and voltmeter.
Maxwell and theories derived from his theories treat electricity.
Solid-state physics, working from quantum mechanics, discusses how the electrons behave in metal conductors in response to electrical causes.
aetherist:
Here is another explanation by William Beaty.
Is the speed of electricity in a wire (signal speed) related to the speed of light (a function of the permeability and permittivity of space) or the speed of sound (based on nearest neighbor interactions of electrons)?
Signals on wires aren’t a “nearest neighbor” phenomenon. Instead, when electrons move, they create altered EM fields and altered attraction/repulsion forces. These forces are experienced by distant electrons in the wire, not just by the close neighbors in adjacent atoms. Next, those distant electrons are, again, moved by those long-range EM field-forces …which then send out new fields, which affect even more distant electrons in the wire.
In other words, the vibrations of one electron can “leapfrog” across large distances and pass over immense numbers of electrons. If electrons are like a chain, then the “yank” isn’t going from link to link, instead it’s an EM wave which ripples through the space outside the links, yet is guided by the row of links (the column of mobile electrons in the wire surface.)
This effect, plus the extremely low mass of electrons, leads to signal-velocities closely approaching lightspeed. And, since these fields DON’T travel inside the metal of the wires …if we place some ferrite or some plastic insulation just outside the wires, the “leapfrogging fields” must pass through that material, and this has an enormous effect on the speed of the signals
penfold:
--- Quote from: aetherist on April 03, 2022, 11:10:28 pm ---[...]
If the electrons merely respond (which i agree with) then ok this avoids the "insulation catastrophe". But, it then contradicts any old electricity theory that claims that drifting electrons cause the electricity.
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Err... you still haven't defined "the electricity"... that is an incredibly important definition to get right. Conventionally, the arrival of power emitted from point A arriving at point B, does not depend on a continuous movement of electrons along the length of the wire... it can happen, but it's not essential. Current is just one of the fields within Maxwell that so happens to only occur in the presence of charges. Poynting's theorem explains this phenomenon (reasonably) exactly, the exactness depends on how much of the physical circuit is included in the mathematical model.
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