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I Electrical signals -- How do they actually propagate in real wires?

  1. Aug 3, 2017 #1
    I am an electrical engineer, and I know that my question is not directly relevant to the pure Physics but anyway, my question is that how electric signals move inside wires? how to initiate the electrical signals knowing that in order for the electron to move in a circuit, we need a continuous voltage which is not the case in the signals.
     
  2. jcsd
  3. Aug 3, 2017 #2

    ZapperZ

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    Why is this not relevant to "pure physics"? The largest number of practicing physicists are in the field of condensed matter/solid state physics, which deals exactly with topics that include charge transport in solids.

    To answer your question, electrical signals in a wire are transferred via the presence of mobile, "free-electrons" in the conductors. These electrons can easily be made to move or vibrate based on the electrical signals that are sent into one end of the conductor. The moving electrons then affect other electrons nearby, and this causes similar movement. This propagates along the wire, translating to the electrical signal at the other end of the wire. The electrons themselves may not move very far from their average position during this transfer of signal. So naively, one can imagine that all they do is simply relaying the information along the wire.

    Zz.
     
  4. Aug 3, 2017 #3
    thanks so much for your reply .. you have answered a part of my question .. the other part is that in a digital computer it is essential to understand the electrical signal.. how to initiate the signal

    is it by applying a voltage at the end of the wire for an instant?
     
  5. Aug 3, 2017 #4

    anorlunda

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    That's very interesting. I had not heard that before. Are there sources that publish breakdowns of the number of physicists by specialty?
     
  6. Aug 3, 2017 #5

    sophiecentaur

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    Not just applying a voltage for an instant but varying the voltage will carry information. Binary signalling with just two levels, ternary signalling with three levels etc. And there are many other more complex strategies, many of them involving a ' carrier wave' that is modulated to increase the channel capacity further.
     
  7. Aug 3, 2017 #6

    berkeman

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    Then you have taken classes that should have explained this, no?
    If the conductivity of the metal wires were infinite, I could understand some confusion. But the conductivity is finite, so the voltage drop and driving electric fields are ________? :smile:
     
  8. Aug 4, 2017 #7

    sophiecentaur

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    You are expressing one of the most common problems that people have with 'Electricity'. (I know you have a good answer, btw :smile:) The mechanism of Power transfer through a wire is always being confused with the loss mechanism. This is not a worry for other power / signal transfer systems. When we describe a bicycle chain transferring the Power from feet to wheel, do we consider the hysteresis and friction in the chain in order to describe how the system works? Do we instantly worry about internal losses in air or solids when we think of sound being transferred? (only way down the line of reasoning) There is something about 'Electricity' that makes us think differently. We can't just be happy with the fact that very high Electric forces between particles result in 'one in one out' for charges in a wire but for some reason we worry about losses. It's only in circuits where there is a problem putting in enough copper that the resistance of the wires is relevant. If we want to manufacture a "resistance" we have to go to some trouble and find a fancy alloy (or Carbon) with enough Resistivity to give us 10kΩ in a small package.
    I think it must be to do with the difficulty that people have with Potential Difference when the subject isn't taught properly or in the 'right' order.
     
  9. Aug 4, 2017 #8
    Hi,

    It is interesting to look at the fact that current density can be present in a wire without charge density. Basically this means you can have a constant current in a neutrally charged wire. The free electrons move down the wire and you have a direct current that can carry energy.

    However, to send a signal as opposed to power a changing current is involved. What can happen then if the frequencies are right is that the free charges can build up local charge densities - the free electron density is not constant in the wire. When that happens electrostatic repulsion moves the electrons to the surface of the conductor. That is why ribbons are used in the ground planes of hf radios. So that there is a higher surface to volume ratio.

    Also take a look at how the signals work in the helix of a Traveling Wave Tube Amplifier (TWTA):

    https://en.wikipedia.org/wiki/Traveling-wave_tube

    The signal in the helix wire is actually amplified because these local charge densities (bunched up free electrons) electrostatically couple to charge densities induced in the electron beam. The acceleration of the electron beams electrons due to the anode pushes on the wires bunched up electrons and you get amplification in the wire. Just as a gun recoils there is mechanical force exerted on the structure by the amplification via action and reaction.

    Another interesting thing is that electromagnetic radiation can remove energy from a wire. A co-axial metal shield is used around the wire so that the electromagnetic energy is reflected back into the conductor and you get less loss.

    One other interesting thing is related to a bull whip. If you try to send energy down a line that is not homogenous under some conditions you can get the energy reflecting and the signal bounces back instead of going through. If you tie a knot in a bull whip you can generate similar reflection.

    Here is a technical description of a "balanced circuit". It will go into some of the techniques used to get a signal to travel using free electrons and pairs of wires:

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

    Hope it helps. It is a simple problem as long as you do not need to look to close.
     
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