hi,
The current carriers are free electrons from the valency band of the conductor atoms and they do drift along the conductor towards the positive terminal.

As they are free electrons, the atomic structure of the conductor does not change.

If you could mark an electron entering the conductor, eventually it would arrive at the other end.

EDIT: if the conductor was say 300mtrs long it would take 1uSEc before current started 'to flow'

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Eric " Good enough is Perfect "
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Is there a relationship between the length of a wire through which AC can travel and the cycle of the AC? In other words, is it possible for whatever is alternating in AC to alternate so quickly that whatever it is starting out on one end of the wire comes back, or alternates, so quickly that it never reaches the other end of the wire?

 

Well, yes and it manifests itself in a very commonly measured manner...the standing wave on a transmission line.

The alternating current, from a transmitter, often (usually) has a wavelength shorter than the transmission line length and the AC travels along the line at some speed (don't quote me on this but, I've heard about 70% of the speed of light in free space). If the line termination is perfect, the AC just continues and either radiates or burns up in the load. But, if there's a discontinuity, the wave will reflect off the discontinuity back to the source and it's the summing of reflected and forward waves that create the standing waves on the line.

It's like any other echo. If you call into the wild and there's nothing to reflect your voice (no discontinuities like a cliff or cavern wall), you voice just continues outward until it dissipates it's energy and does so at the speed of sound. But, if it hits a discontinuity, then it reflects. That's why the echo doesn't return instantly, just as the reflection on a transmission line doesn't happen instantly. They both propagate at some finite speed.

There's a very strong analogy between acoustical and electronic reflections. In the audio world, the impedance mis-match is a function of the hardness of the discontinuity (ie: a brick wall reflects better than a padded wall). It's also related to the shape of things and their ability to allow acoustical energy to more or less easily propagate. That's why you see that horn on the old phonographs. The horn is a device that provides a smooth transition from the needle to the air in the room, making the transfer of energy more efficient and thus the sound louder. In electrical terms the discontinuity is based on electrical impedances and you often see the horn used, espeicially in microwave applications where the horn size is more manageable. But, it does the same thing as the acoustical horn. It' allows the radio waves to travel smoothly from one impedance level to another.

As a side note, that's the basis of speaker enclosure design. An effort to fool the speaker into thinking it's sitting in the middle of free space even though it's actually confined to a small box.

I know that's kind of "rambly" but, hopefully, some or all of it makes some sense.

 

Hi, Eric
I still insist upon my version of explanation, I might be wrong but if you can make it more clear then please try.
I do agree that the electron which are responsible for conduction are free electrons but, it actually doesn't mean it not literally, by free electron it mans that the an electron SHIFTS in to the atomic structure of the NEXT atom and the next atom's electron SHIFTS in to it's next atom and so on thus the number of electron that SHOULD be in the outer orbit of copper always remains the same, THOUGH IT DOES TAKE TIME TO HAVE THAT SHIFT receiving an electron and hence current at the other end of conductor but ,THAT WOULD ALWAYS BE EQUAL TO TIME TAKEN BY ONE ELECTRON TO SHIFT AT THE SPEED OF DRIFT VELOCITY,
Say a conductor has 10 atoms in it and say with 10 free electron in total (1 in each atom) then even the total time taken to start current will be Length of conductor / Drift velocity of a single atom rather than Length of conductor/ TOTAL of drift velocities of all 10 electron.

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CHRIS

 

hi Chris,

Look thru these links.
http://en.wikipedia.org/wiki/Electric_current

http://www.electronics-tutorials.ws/diode/diode_1.html
http://www.tpub.com/content/doe/h101...h1011v1_30.htm
http://www.ndt-ed.org/EducationResou...eeelectron.htm

The free electron does not shift to the atomic structure of the next atom it moves thru the crystal lattice formed by the atoms of the conductor..
thats quite different.
A good conductor has an excess of free electrons an ideal insulator has no free electrons.

Think about the analogy of an electron beam in a CRT.

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Hi Eric,
YOU ARE ABSOLUTELY RIGHT, but you actually need not to put those link to explain me basic electronics ( Though I did sound like I need ) That CRT comment strike my mind and opened my eyes, I'm sorry I was wrong, Thanks for nice response.

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CHRIS

 

hi Chris,
Dont forget, other less experienced members read our posts, so I find it helpful to give the 'students' a more detailed description,
thats why I include links.

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There are actaully a number of things to consider. One is the drift velocity of the electrons but, in practical circuit analysis, that's meaningless. It's basically a physics class problem. More important is the speed with which the electrical field propagates along the conductor.

If you have a garden hose, full of water and you turn on the tap at one end, the water will almost immediately start coming out the other end...but, it's not the same water that's going in. But, even there, the water does not come out instantly. There's a delay while the molecules jostle each other as the pressure effect propagates along the hose. THEN, the water starts coming out.

Same with a wire. The wire is full of electrons (the famous valence electrons being involved in these situations). When the EMF is applied across the ends of the wire the electrons act like the water molecules in a hose with the effect traveling quickly and the actual electrons moving more slowly.

The CRT is a completely different animal. In the CRT the electrons are emitted from the cathode and accelerated, as a formed beam, to some high + potential...but, through a vacuum. In a CRT the electrons themselves travel fast.

Again, as a side note, the electrons in the beam can actually be made to change velocity as they travel and that's the basis for the operation of vacuum tubes such as klystrons and magnetrons and traveling wave tubes.

 

No offence intended, but basically you have just repeated what I have said already.

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We've seen the plausible replies that repeat what you read in physics texts (which I believe), but if you were a skeptic, how would you measure it? Can anyone devise an experiment to differentiate between all the conduction electrons moving at an average speed of say 5mm/sec against half of them moving at 10mm/sec and the other half staying put?

 

Hi Eric,
Isn't this something and SOMEWHAT I talked about ?


If you have a garden hose, full of water and you turn on the tap at one end, the water will almost immediately start coming out the other end...but, it's not the same water that's going in. But, even there, the water does not come out instantly. There's a delay while the molecules jostle each other as the pressure effect propagates along the hose. THEN, the water starts coming out.

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CHRIS

 

hi Chris,
There's nothing wrong with that analogy.

Consider a simple loop of copper wire.

You connect a battery across the ends of the loop.

At the instant of connecting the battery, the negative charge on the battery will drive away,
along the wire, any close proxity free electrons.
At the same time the positive battery terminal will attract any free electrons from out of the end of the wire.

Now there is a potential gradient along the length of the wire and the free electrons will migrate down this gradient.

So the battery positive terminal keeps removing the electrons and the negative terminal keeps injecting eletrons, this is the current

Even in the hose pipe analogy, the water coming out of the end of the hose will eventually be from the tap end of the hose.
Stop injecting water by turning the tap off and the water will not come out at the far end.

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Eric " Good enough is Perfect "
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Hi Eric,
So all you are trying to say is that I was wrong only in believing that electron only SHIFTS and doesn't travel THROUGH up to the other end, am I right ? Is that the point you are making ? If so then this was also a wrong assumption..was it ?

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CHRIS

 

That's true but, there seems to be a continuing discussion of it so, I endeavored to put a slightly different slant on it.

However, in the case of the CRT, there's sort of another can of worms that might open up. As you rightly state, once you get out of the CRT electron beam, you are back into copper ("copper" being used generically, of course...I know you guys are sticklers) and, while you continue to have the same current, you now suddenly have verrrrrrrrrry slow electrons instead of very fast ones.

I've never had occasion to think about the physics of that but, it's an interesting phenomena to ponder.

 

hi Chris,
We should consider the effect the voltage has when applied to both ends of the conductor.

The battery completes the loop/path.

As I posted previously the +V removes electrons from the conductor and the -V injects electrons into the conductor as soon as the voltage is applied to the loop.
This causes the potential gradient in the conductor to be applied close to the speed of light and would exist as long as the voltage is applied.
I would say that the free electrons would drift along the conductor.
The physicists tell us this the drift in the order of inches/hour.

Taking the water pipe analogy, if the pipe is full of water and the feed tap is turned on water flows 'almost' instantly, thats IF the end of the pipe is open to a lower pressure.

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