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Homework Help: Quasi-static Approximation for coaxial wires

  1. Dec 29, 2013 #1
    1. The problem statement, all variables and given/known data

    A coaxial cable with inner radius a and outer radius b lies on the z-axis (such that the cable's axis merges with the z-axis). its length (along z-axis) is L. at z=-L there are voltage sources that are distributed uniformly connecting the inner wire to the outer one. at z=0 there is a short circuit. all conductors are perfect.

    what is the dominant field?

    2. Relevant equations

    the dominant field is the one that exists in DC conditions

    3. The attempt at a solution

    Im totally stuck. in a previos question instead of the voltage sources there were current sources so that was easy - at DC only current flows so the magnetic field is the dominant. I think that since there is a short circuit and all conductors are perfect than changing the source does not change anything since current still flows in DC and the magnetic field is the dominant but in the solution it says that its the electric field that is dominant.

  2. jcsd
  3. Dec 30, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    "voltage sources at z = -L"? How can you have more gthan one source at one point?

    Anyway, the situation is impossible in the steady state since the cable, being a perfect conducror and shorted at the z=0 end, would result in infinite current.

    However, if you assume the voltage is appplied as a step at t=0 and L is long enough, then the current is limited to the characteristic impedance of the cable until the return voltage hits the source, or T = 2L/v with v the velocity of propagation.

    Ayway, I think the question of what is "dominant" is extremely imprecise.
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