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DC propogation

  1. Nov 21, 2014 #1
    I've been stuck on what I would of thought was a simple/common question but I can't seem to find an answer anywhere. I'm confused as to how a DC current propagates along a conductor (wire) in terms of it's E/H field. I understand that it is these fields that cause the propagation of current. However for a DC current these fields are constant and in a direction other than the direction of propagation. How could they induce a current down a straight conductor. I found an answer that said the propagation in a DC sense is due to the H field, more so than the E field. This field however will be circular around an electron. How does this field propagate along a conductor other than being induced by the charge moving and then being restricted to the Fermi velocity.
  2. jcsd
  3. Nov 21, 2014 #2


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    Science Advisor

    Maybe you should look at the problem from a different point of view.
    http://www.matterandinteractions.org/Content/Articles/circuit.pdf [Broken]
    Last edited by a moderator: May 7, 2017
  4. Nov 21, 2014 #3


    Staff: Mentor

    In an Ohmic material the current density is always in the same direction as the E-field. That is the definition of an Ohmic material.

    That said, I am not sure I understand what you are asking. A DC current does not "propagate", by definition.
  5. Nov 21, 2014 #4
    My question is essentially, how does a DC current not travel at the Fermi velocity? If I have a long straight piece of wire with a dc source at one end and a light bulb the other, I will observe illumination well before the time it takes the electrons to travel according to the drift velocity. Thus there must be some E or H field traveling along the conductor. However as far as I can work out both the E field and H field are orientated at a normal to the direction of current flow. How then do these fields travel to the end of a conductor before the electrons would do so at the Fermi velocity.
  6. Nov 21, 2014 #5
    No, the E field is not oriented normal to the current flow (current density vector). Where did you get this idea?
    There are surface charges on the conductor which produce the electric field responsible for the current flow.
    They are established during the transitory regime, when you turn on the current. The field propagates with speeds comparable with the speed of light, during this transitory period.
  7. Nov 21, 2014 #6


    Staff: Mentor

    The energy delivered by a wire to a light bulb is actually carried by the fields surrounding the wire. Not by the electrons in the wire (except as they contribute to the field).


    The propagation of the fields is governed by Maxwells equations and happens at speeds near the speed of light, as mentioned by nasu.
  8. Nov 27, 2014 #7


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    Gold Member

    Note also that a DC current is -by definition- constant. If you flip a switch you introduce a transient which -again by definition- is a propagating wave (which in a copper line typically travel at about 80% of c). Hence, it is no different than an AC current.
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