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Waves in a conductor

  1. Oct 25, 2012 #1
    I don't really understand how electromagnetic waves in wires are created. Sure you can see from the Maxwell equations that the fields satisfy the wave equation. But if you plug some cables onto a battery isn't the situation more or less static. I mean the electric field from the battery has existed since t=-∞ so I don't see why it should take time for the electric field to reach the other end of the cables.
    Also it seems that there in general two ways to reach the equations for transmission in a cable. the telegraph equations. One goes by simply solving maxwell equations and applying the boundary conditions that a linear media gives. Another seems to be to view a cable as a sum of small capacitors and conductors. Either way you find precisely the same speed for the signal. Why is that? Surely Maxwells equations don't incorporate anything about the capacitance or inductance of the linear media.
     
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  3. Oct 25, 2012 #2

    tiny-tim

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    hi aaaa202! :smile:
    no, when you first connected the battery to the circuit, it took a finite time for the charge to get round :wink:
    Maxwell's equations include the ampere-maxwell-law …

    curlB = µ jfree + µε ∂E/∂t :wink:

    which include µ and ε, the permeability and permittivity, which could be (but aren't) called "inductivity" and "capacitivity" respectively :wink:
     
  4. Oct 25, 2012 #3
    Electromagnetic waves are not created in conductors they are created on and around conductors.
    The conductance of conductors is so high that any waves attenuate very very rapidly.

    This is known as the skin effect.
     
  5. Oct 25, 2012 #4
    studiot: okay that makes sense, although I don't see what the telegraph equations describe then. Is the potential in it the potential around the conducting wires?

    Tim: I do realize that it takes time for the charges to move. But we are conserned about how the field from our battery drives the current around right? And that field has existed always so I don't understand how it should take time for the field to propagate information around. Or what field are we really looking it when we study this apparant wavel like behaviour?
     
  6. Oct 25, 2012 #5
    We do not usually calculate this way but the wave dues to electric power at 50/60 Hz have wavelengths of thousands of kilometers in air but nanometres in copper.

    The telegraph equations and the transmission line equations describe waves in a transmission medium between two conductors, not in the conductors.

    The waves, of course propagate at the local speed of light, which is close to c in air but much slower in copper.

    If you search the forums I posted some calculations and figures at PF about this.
    I do not have more time now.
     
  7. Oct 26, 2012 #6
    To continue this discussion: What is it that is actually responsible for the wave motion of the field. At first I thought that it's the electrons bouncing into each other but that would be a transversal wave and it doesn't seem right since electromagnetic waves don't need a medium.
     
  8. Oct 26, 2012 #7
    This question caused controversy for a century or so.

    In terms of wave motion one way to think of it is to consider the electromagnetic wave as carrying its medium with it, in a manner that feeds on itself.

    It is a fundamental experimental observation that a changing electric field gives rise a magnetic one and a changing magnetic field gives rise to an electric one. There is no theoretical requirement for this in classical physics but it is observed to be so.
     
    Last edited: Oct 26, 2012
  9. Oct 26, 2012 #8
    Several people have already said this but you seem to be missing it.

    A transmission line is not a wire. It's (usually) a coaxial cable used to transmit AC signals.

    The theory is about EM waves travelling down such a cable - not about electricity from a battery running down a wire.
     
  10. Oct 26, 2012 #9
    Hi Studiot,

    You bring up an important observation but it's resolution may not be what you believe it to be. We should remember that none of the Maxwellians (and Faraday and Maxwell himself) assumed that the cause of a changing magnetic field is a changing electric field (and vice versa). That seems to be a 20th century bit of confusion. Please see Jefimenko's clear analysis of the actual causal relationships (or a secondary source such as Jackson's textbook).

    http://en.wikipedia.org/wiki/Jefimenko's_equations

    In regard to the OP, in Maxwell theory there are 2 kinds of current:

    conduction current - the movement of electrons or other charged particles
    displacement current - the movement of energy whose characteristics are described by 'fields'

    The Maxwell equations give us the rules for determining how both types of current affect each other.
     
  11. Oct 26, 2012 #10
    Hello Philip.

    Look at the title of this thread.
    It is about waves.
    How much displacement current exists in a conductor?

    Thank you for the link,

    Does it not imply that it is theoretically impossible to avoid having a magnetic field without an electric one and vice versa?

    Yet the conventional view is that it is change of one that gives rise to the other.
    If this is true it, does it not preclude the possibility of a steady uniform field throught time and space?
     
  12. Oct 26, 2012 #11
    The displacement current is perfectly described by wave equations.

    Yes, I think you are pointing out that the observation of the changing of an electric field is very often linked to the changing of a magnetic field. So the difference between correspondence and a causal relationship is a bit subtle. But there are cases where one or both of the fields propagate as evanescent waves. That is, they aren't traveling waves and their changing values don't continuously propagate. In those situations you may find exceptions.

    P. S. The Poynting theorem is good to look at in conjunction with these questions. It shows the movement of energy (which is also described by wave equations related to field fluctuations). But the Poynting theorem shows that the energy moves at right angles to the flow of electrons in the wire - from outside the wire into it.
     
    Last edited: Oct 26, 2012
  13. Oct 26, 2012 #12
    I think all this is a digression. Do gauge theories (on which I am not an expert) have any place in the classical physics section?

    Further I don't see the connection between mass and conductivity in either view. Conductors are conductors because of their electron arrangement, not because of their proton arrangement. Neutron matter offers an enormous density but does it have high conductivity?
     
  14. Oct 26, 2012 #13
    The movement of charges in the wire induces a displacement current outside of the wire (and to some extent inside the wire). The displacement current propagates as an evanescent wave according to the Maxwell equations.
     
  15. Oct 26, 2012 #14
    Which is the point we have been trying to get over to aaaa2002.
     
  16. Oct 26, 2012 #15

    ZapperZ

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    I don't understand this at all. How does the "large amount of mass concentrated in a small volume" have anything to do with non-wave behavior?

    I can look at the current in an AC circuit, and I definitely see electron current being described as a wave.

    And if you are arguing about the actual physical behavior, then there's the circuit equivalent of the 2-slit experiment, such as in SQUIDs. Those are certainly wave-like description to me.

    Zz.
     
  17. Oct 26, 2012 #16
    Yes, I was thinking about what you said as I wrote it. The mass only changes the velocity that the free electron moves at (slower than c of course). The free electron should still move in the same manner as a wave more or less. But the Lorentz force law probably best describes its potential movement.
     
  18. Oct 26, 2012 #17

    ZapperZ

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    That still doesn't explain anything.

    A buckyball is many orders of magnitude more massive than an electron. No one can say now that a buckyball doesn't exhibit wave-like behavior after we've show that it can produce 2-slit interference pattern!

    But this is neither here nor there. The very fact that we have experimental observation of wavelike behavior of conduction electrons should be enough to falsify what you said. So if you disagree with this, you need to address directly these experimental facts, not some other conjectures.

    Zz.
     
  19. Oct 26, 2012 #18
    This does not address my objection to the same quote.

    I do not know Jackson, but Griffiths is oft quoted here.

    Section 9.4 of Griffiths is entitled Electromagnetic Waves in Conductors and follows the conventional path I described.

    P396 has a particularly good sketch of the rapid attenuation of an EM wave attempting to propagate in a conductor.
    Beneath is a good question
    "Find the skin depth in a good conductor in nanometers.......

    Chapter 10 of Griffiths relates to your earlier intervention, although I cannot find any dependence on mass in any of the equations presented, there is certainly none in Maxwell.


    However I think Plonus has a more comprehensive treatment of the subject (EM waves) in his chapter 13. It includes many practical examples, facts and figures.
     
  20. Oct 26, 2012 #19
    Yes, I agree. The word wave-like fits very well. What I really meant was that momentum needs to be factored into the equations of motion for the electron in addition to the wave equations.
     
  21. Oct 26, 2012 #20

    ZapperZ

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    That is a very strange statement. I can have momentum in a classical wave! The classical treatment of EM wave certainly has momentum in it, and this is without having to resort to having any mass either!

    Zz.
     
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