You obviously don't understand electronics either. The current doesn't start flowing in one wire of the cable, work its way up to the load, then back down the other wire like gophers running through tunnels.
The single biggest factor in an AC line making an EM field is the fact that there is space between the two conductors. Twisting the wire creates fields from the different portions that cancel each other... for the most part. But there always remains some residual field.
-- Rick
But infra-red light would, by definition, only produce monochrome pictures.
-- Graham. %Profound_observation%
But it's bells in a belfry!
I bet it would be effectively monochrome in any lighting conditions!
John
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Yes, It does. Actually it is more like a tube full of ping pong balls, as you push in at one end one another pops out at the far end but there is a small delay as this takes place.
If it was instantaneous as you are implying they we would be able to communicate across distances faster that light.
-- There are three ways to get something done: (1) Do it yourself. (2) Hire someone to do it for you. (3) Forbid your kids to do it.
You really don't understand this "electricity" stuff, do you?
Dave
I didn't imply any such thing. I am simply stating what I am stating. Your model is inaccurate in every meaningful way. There are two wires and the wave propagates along both of them simultaneously. Do some reading on transmission lines.
-- Rick
That's a superb analogy.
-- Steve O'Hara-Smith | Directable Mirror Arrays C:>WIN | A better way to focus the sun The computer obeys and wins. | licences available see You lose and Bill collects. |
+1
I learned the mathematics of transmission lines a long time ago. Doing so has been very valuable during my career.
Dave
I have to say that was largely ignored in the context of digital signals when I was in college. I remember a professor mentioning the far from ideal waveforms while never talking about the "why". Then in my first real job the were trying to run a wire wrap backplane at 25 MHz knowing it would be hairy without knowing the "why". Then when I first saw the "why" it was very illuminating.
-- Rick
I apologize if I am coming across as a know-it-all jerk, but this is still not a good analogy because it doesn't help explain anything. What is really important is the propagation of the wavefront, not the movement of the electrons (ping-pong balls). Of course you won't have one without the other so they go hand in hand.
-- Rick
Actually as far as a scope probe stuck on he wires goes, thats exactly what it does do :-)
The deeper story is of course that like all science the story is only a story. A model. Whether its a EM wave, or discrete electrons or even holes, is a moot point.
The point is signals dont get to the end of wires immediately. It takes time.
No. That reduces magnetic 'radiation' in the near field but does little for electrostatic coupling.
For that,. we have shielding...
-- Everything you read in newspapers is absolutely true, except for the rare story of which you happen to have first-hand knowledge. ? Erwin Knoll
A DC circuit does not have a wavefront, but there is electron movement exactly as described so you are missing something. A signal running through a single conductor loop (essentially a cable with widely separated conductors) has a wavefront that follows the single conductor. The transmission line behaviour you describe only applies when the cable behaves as a transmission line. It's been a long time but I think this is not the case when the signal wavelength is long compared to the conductor wavelength which is certainly true for a 50Hz sine wave on a 60 metre cable.
-- Steve O'Hara-Smith | Directable Mirror Arrays C:>WIN | A better way to focus the sun The computer obeys and wins. | licences available see You lose and Bill collects. | http://www.sohara.org/
well that not true either - a transmission line is a transmission line and the distributed LC terms make it so. And increase delay so that its a bit slower than the speed of light for a given signal. 50Hz or 5GHz...
Interestingly it IS significant - delay - at 50/60Hz. The USA is not a fully connected single grid because the delays across it would lead to distinct losses if it was...
-- Everything you read in newspapers is absolutely true, except for the rare story of which you happen to have first-hand knowledge. ? Erwin Knoll
which is exactly what the analogy shows, the Ping pong balls may be moving quite slowly (& in fact electrons in a cable do move slowly) but the ball at the far end ops out quite quickly, but that wave front does take time to travel.
-- I'll burn my books. -- Christopher Marlowe
Give some thought to what you wrote. If the electrons travel slowly what is it that travels quickly? The wavefront does not move any faster than the electrons. The electrons create the wavefront. The wave front seems to move slowly because the electrons only move a short distance before a collision deflects their motion.
My issue was with the statement that the wave only propagates along one wire of the pair and the resulting lag in the current in the other wire is what gives rise to the EM field. The wave propagates along both wires and the EM field is due to the fact that the two wires are not superimposed.
-- Rick
Wow! I've found another UG that is a good at wandering off topics as uk.tech.digital-tv!!!!
-- Woody harrogate3 at ntlworld dot com
Nope the electrons drift *much* more slowly than the wavefront in response to an electric field (they move rather more quickly at random). The electron drift speed is in the range of a few millimetres per hour while the wavefront speed is a large proportion of the speed of light. The drift speed is imposed on a thermal random motion of about 0.3c.
Now what happens when the wires are widely separated, perhaps forming a large circle or shielded from one another ? At what point does the behaviour change and why ?
-- Steve O'Hara-Smith | Directable Mirror Arrays C:>WIN | A better way to focus the sun The computer obeys and wins. | licences available see You lose and Bill collects. | http://www.sohara.org/
60 feet . Not meters
This reminds me of me (as an 8-year old) switching on the light and try to see how long it would take to get light ...
I have some CFLs that take a long time to turn on.
It would be fun to setup a photodetector and watch a tungsten bulb turn on. I wonder how many 1/2 cycles it takes to get to 90% of brightness.
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