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Topic Title: Velocity of propagation
Topic Summary: Transmission lines
Created On: 23 August 2013 04:48 PM
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 23 August 2013 04:48 PM
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ebanner

Posts: 48
Joined: 18 January 2003

One thing that has always puzzled me, the Vp seems to be how quickly the transmission line conductors take to charge up. This is modelled for instance by the lossless LC lumped model. Therefore taking this to the limit when the length decreases to zero would be how quickly an element would take to charge up. However current flow through a wire is how fast an electron takes to transit throught the conductor which is sort of conflicting with the theory of TLs?
 24 August 2013 08:57 AM
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IanDarney

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If a step voltage is applied to a conductor, a current pulse propagates along that conductor, laying down charge on the surface. Since the velocity of propagation is that of an electromagnetic wave, the entities which move must be the same as those which constitute light; photons. Electromagnetic waves propagate through solid material (i.e. glass). They also propagate along a conductor (i.e. copper). Electrons just move from the inner material of the conductor to the surface.
Ian Darney.
 24 August 2013 11:31 AM
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jencam

Posts: 608
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Transmission line theory connects the electromagnetic representation of a transmission line in terms of its E and H fields with a circuit model in terms of its lumped L and C per unit length. The derivation of formulae for the circuit representation of transmission lines from electromagnetic formulae for its physical structure can be found in any book about electromagnetism or microwave electronics.

The current flow through a conductor isn't about how fast an electron takes to transit through the conductor. It is (heavily simplified) about the time between an electron entering at the negative end and another electron leaving at the positive end.
 28 August 2013 02:23 AM
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kengreen

Posts: 400
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Yes indeed, all are very correct if you read textbooks but unfortunately most writers of such are nearly as confused as their readers.

The reason that a transmission line knows what is at both the beginning and the end of such line is thata correct termination prevents any reflection from the receiving end and so the input appears as a pure resistance; any mismatch at the far end means that a standing wave appears (due if you wish to a reflection of power that the load cannot absorb) and this standing wave causes the input impedance to have a complex value i.e. of the form (a + jx). This means that the driver cannot deliver power as intended and so the combination of driver - transmission line - load settle for a good old British compromise.
 28 August 2013 03:23 AM
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ebanner

Posts: 48
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I was asking more of a basic question that most writers in books miss when this subject is discussed. They jump from talking about electrons moving through wires to the transmission line circuit. I have hit a eureka moment from searching around on the internet with articles and got this from wikipedia:

Drift Velocity
The low drift velocity of charge carriers is analogous to air motion; in other words, winds.

The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas (these waves move through the medium much faster than any individual particles do)

The random motion of charges is analogous to heat - the thermal velocity of randomly vibrating gas particles.
 28 August 2013 08:29 AM
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ectophile

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The way I think of the distinction between drift velocity and the speed of electricity is to imagine a pipe stuffed full of balls.

If you push another ball in one end, then a ball will pop out of the other end almost immediately. So it looks like the balls travel really fast. But in reality, each one has just shuffled along a little bit. The one that came out of the end wasn't the one you pushed in.

-------------------------
S P Barker BSc PhD MIET
 28 August 2013 09:07 AM
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IanDarney

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Another analogy could be that between the wind and the waves on the sea. The air moves horizontally along the surface; the water particles move up and down. The photons propagate along the conductor, whilst the electrons move up and down at the surface of the conductor. (High frequency current concentrates at the skin.)
Ian Darney.
 28 August 2013 10:10 PM
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ebanner

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These all seem really good analogies thanks.

I was taught that skin effect happens because there is more inductance at the centre of a conductor than at the surface hence there is a higher impedance and majority of current flows at the surface increasing with frequency untill most travels along the surface.
 30 August 2013 07:53 PM
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kengreen

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Hmmm. Quite a mixed bag. In particular I am impressed by the idea of photons(whatever they may prove to be) actually travelling along wires.

Except for a particular concept, to convey a signal requires a pair of wires and these, regarded simply as two electrical conductors,form a physically long but thin capacitor which, usually,is referred to as a distributed-capacitance. Signal progression along such a device involves chargin g each capacitive section from the preceding section; from this comes the concept of a "charging wavefront" which progresss alonga transmission line; no matter the speed of light this transfer of charge from input to load requires a finite time? Follow the argument through and you should find that a failure of the load to absorb (i.e. to eliminate) the incident signal results in a reflected wave simply because the non-absorbed energy has to find somewhere to go. It is the interaction between incident and reflected waves that causes the standing wave and hence the transmission line looks to a puzzled driver to be causing a phase shift between applied voltage waveform and the resulting current waveform.

None of the components which are involved in this process can be regarded as "understanding" what the heck is going on - we are looking at a system of cause-and-effect. Indeed if you wish to understand the universe in which we exist you must acquire the habit of associating cause with an effect which in turn acts as a cause; to be understood in this technological world you must indeed read the books but always keep a salt cellar to hand.

Ken Green
 02 September 2013 08:44 AM
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IanDarney

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Textbook analysis of magnetic field shows that it penetrates the material of conductors. Maxwell's equations demonstrate that electric field and magnetic field are inseparable. So, if magnetic field exists inside a conductor, then there must be an electric field inside the conductor. Electromagnetic fields exist in conducting material.

According to Wikipedia 'A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation, and the force carrier for the electromagnetic force, even when static via virtual photons.'

When a step voltage is applied to a single conductor (i.e. the conductor is acting as an aerial) a current propagates along the conductor at a speed which is comparable to the speed of light in a vacuum. As it progresses, it charges up the surface of the conductor. It also creates an electromagnetic field which radiates out into the environment. When it reaches the end of the conductor, current is reflected back to the source. The incident and reflected currents both flow along the same conductor.

Since current has radiated away in the form of electromagnetic energy, less current arrives back at the source. This can be measured using a current transformer.

Current is a measure of the kinetic energy of the photons.
Ian Darney
 02 September 2013 11:16 AM
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kengreen

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Ian,
at risk of starting an argument I have to disagree with the main theme of your last post. When you say that" current is a measure of the kinetic energy of the photons" I will admit that I haven't the faintest idea exactly what you're talking about. I will challenge you to give one example - just one example - of an instrument which will measure the strength (or the direction) of what is alleged to be (an electric current)? I do strongly believe that you will describe to me an instrument that will measure the strength and direction of the magnetic field which we know always - repeat always - surrounds the path of a so-called electric current. Or perhaps you will describe an instrument that all indicate the appearance of infra-red radiation from the area within which the so-called electric current is travelling. You could also describe it in terms of the weights of certain chemicals which will be transported across an electrolytic cell - you might even report it as a body count:-)?

The one thing above all else that you cannot do is to measure that electric current directly. Please - please - do not bury me with the equations with which the redoubtable Mr. Maxwell amused himself.

You are also in serious error when you state that "a current propagates along the conductor at a speed which is comparable to the speed of light in a vacuum". I spent more than a few hours making allowances for the speed of propagation in practical set-ups.

Ken Green
 03 September 2013 10:18 AM
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IanDarney

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Ken,
The person who formulated the Wikipedia definition of photons put a lot of thought into the wording. I can accept this definition because it is plausible.

The statement 'Please - please - do not bury me with the equations with which the redoubtable Mr. Maxwell amused himself' is puzzling. It is almost as though you view the Maxwell Equations as a red herring.

Section 7.2 of the book 'Circuit Modeling for Electromagnetic Compatibility' (available from the IET bookshop) describes the design, construction, and calibration of a current transformer. Section 7.4 describes how to use it to characterize the properties of an isolated conductor as an aerial. Section 7.5 shows how the propagation velocity of differential-mode current in a twin-conductor cable can be measured.

The picture at the home page of www.designemc.info illustrates the use of a current transformer.

Ian Darney
 04 September 2013 03:02 AM
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kengreen

Posts: 400
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yes Ian,

I'm sure you are absolutely correct.

But then the most senior scientists of their day said much the same thing about Phlogiston?

Yes indeed, I do regard the Maxwell equations as being much ado about nothing.

Ken Green
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