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I Spreading of electric current -- start signals in a circuit

  1. Nov 30, 2016 #1
    Hi
    I have a quesiton about current start signals in a simple circuit such as explained in http://amasci.com/elect/poynt/poynt.html.
    Turning on a switch somewhere in the circuit, sphere of influences, i.e. motions of electrons in wire and generation of Poynting vector around the wire, start from the location of switch and spread by exactly or slightly less than speed of light c. Could you teach me about the mechanism of this spreading, e.g. EM waves generation ?
    Best.
     
    Last edited: Dec 1, 2016
  2. jcsd
  3. Dec 1, 2016 #2

    cnh1995

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    Look up "surface charge feedback mechanism" in electrical circuits. This discussion might help.
    https://www.physicsforums.com/threads/e-field-inside-wires-during-transient-state.846108/
     
  4. Dec 2, 2016 #3
    Thanks cnh1995. Now I share difficulties with the authors of that link :
    How the state transfer, e.g. starting from where, spreading with what speed and amplitude, how much relaxations and transient time, through
    1 turn-off steady state, 2 transient state and 3 Turn-on steady state ?
    It'a kind of progress anyway. Or Did I fail to find fine thoughts in that thread ?
     
  5. Dec 3, 2016 #4
    Hi. I now assume as follows.
    First electron motion at the switch contact generates the magnetic field in the vacuum with delay of light speed and dissipation by distance.
    Thus generated magnetic field is further transmitted to the position of wire nearby and make electrons there move.
    Such a feedback consequence takes place in the transient state. The first weak interaction, from the position of switch to the wire position concerned, comes with light speed. It is followed by stronger interactions that is retarded by further distance of vacuum-wire transmissions but enhanced by accumulation from nearer and multiple current sources.
    Does this make sense? Best
     
  6. Dec 3, 2016 #5

    cnh1995

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    I am not sure about that since magnetic field requires a steady current. But as the switch is closed, there is a change in electric field, so there can be a magnetic field.
    But as far as I know, the interactions inside the wire are purely governed by the electrons (as far as surface charge feedback is concerned). When you close the switch, electrons near the +ve terminal are attracted towards it and those near the -ve terminal are repelled. This creates a difference in charge densities and this difference propagates along the wire through the surface charge feedback. Excess charge in a particular section in the wire ends up on the surface of the wire, making "surface charge rings". The surface charge gradient is responsible for the electric field inside the wire. This surface charge formation continues until there is a steady current in the circuit, meaning no excess charge in any part of the circuit (KCL). If there is no gradient, there is no field inside the wire. Hence, for conductors, the surface charge gradient is very small compared to that for the resistive elements and we approximate the voltage across the conductors to zero.

    These videos might help.
    This one describes the surface charge feedback mechanism...
    https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=video&cd=2&cad=rja&uact=8&ved=0ahUKEwiDuqvv19nQAhVLwI8KHaAwDh4QtwIIHDAB&url=https://www.youtube.com/watch?v=Y-t6l8tVCt0&usg=AFQjCNE_rpu-XRpYAmYv-rdXWNTEpvtRIA&bvm=bv.139782543,d.c2I
    and this one is the experimental verification of the existence of surface charges in a circuit.
    https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=video&cd=5&cad=rja&uact=8&ved=0ahUKEwiDuqvv19nQAhVLwI8KHaAwDh4QtwIIKzAE&url=https://www.youtube.com/watch?v=8BQM_xw2Rfo&usg=AFQjCNFjwcn3xKGmNYOgb16L-iYP8c_p_w&bvm=bv.139782543,d.c2I
     
  7. Dec 3, 2016 #6
    I have no doubt about charged wire. I wonder whether the charge distribution difference can describe the propagation speed that is as fast as light.

    The propagation is not necessarily along the line in case of redundant wiring. I drew some figures
    #1 http://fphys.4rm.jp/modules/xelfinder/index.php/view/149/スライド1.jpg
    #2 http://fphys.4rm.jp/modules/xelfinder/index.php/view/150/スライド2.jpg
    #3 http://fphys.4rm.jp/modules/xelfinder/index.php/view/151/スライド3.jpg
    where yellow circle is the light speed sphere of current starting signal.
    Best
     
    Last edited: Dec 3, 2016
  8. Dec 4, 2016 #7

    cnh1995

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    Last edited: Dec 4, 2016
  9. Dec 4, 2016 #8
    Thanks cnh1995. I did not find a specific description of the transient state of my concern in your recommended page. Please tell me the exact lines if I failed.

    When current starts at the switch point, charged state there changes. In order to keep the charged state current should start at neighbors thus expands.
    So we might say the same thing in different ways.
    I attributed the speed of c to electromagnetic information speed as is in Lienart Wiechert Potential. That's enough or should you explain a great speed of charge density change propagation in wire?
     
  10. Dec 4, 2016 #9

    cnh1995

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