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Electromagnetism - intrinsic impedance and poynting vector

  1. Aug 22, 2012 #1
    1. I know that 1/2 of the real part of the complex poynting vector gives the average power flow per unit area. But what is the significance of its imaginary part ?
    2. What is the significance of the complex intrinsic impedance ? For lossless medium (i.e. with no ohmic losses) it is real , but in circuit theory real impedance signifies ohmic losses. Then how the concept differs in both cases ? And also tell the significance of both the real and imaginary part of complex intrinsic impedance ?
  2. jcsd
  3. Aug 22, 2012 #2
    I can't do these questions proper justice, so I'll leave them for someone else, but maybe this quote sheds some light on your first question:
    Perhaps you can tell me what to make of that—my EM books are covered in metaphorical dust.
    Last edited by a moderator: May 6, 2017
  4. Aug 22, 2012 #3
    Let me try as see whether this help:

    [tex] e^{jx}=\cos x +j\sin x\; \Rightarrow \; \cos x =Re [e^{jx}][/tex]

    cos x is not a complex, it has no imaginary part. It is just the real part of the [itex] e^{jx}\;[/itex]. there is no jsin x.

    Remember for EM propagates in z direction

    [tex]\vec E_{(z,t)}=\hat x E_0 e^{-\alpha z} \cos(\omega t -\beta z) [/tex]

    This can be represented by

    [tex]\vec E_{(z,t)}=Re[\tilde E_{(z)}e^{j\omega t}] \;\hbox { Where }\;\tilde E_{(z)}= \hat x E_0 e^{-\alpha z} e^{-j\beta z} [/tex]

    You can write out the [itex] \vec H_{(z,t)}\; [/itex] the same way.

    Remember these are not complex number. They are just the real part of a complex representation, not that they are a complex number.

    Poynting vector is defined as:

    [tex] \vec E_{(z,t)}\times \vec H_{(z,t)} [/tex]

    Which is also a real number, not a complex number. You need to throw out the complex part as the reason given above.......That both E and H are real number represented only by the real part of the complex number. There is no imaginary part. It is just easier to do the calculation in complex form and throw away the imaginary part.

    And me too, I have to dust up my note book to give you this. I have to review my notes on lossless and lossy dielectric before I get back to you on the second part.
    Last edited: Aug 22, 2012
  5. Aug 22, 2012 #4
    OK regarding to complex intrinsic impedance. You start out from the homogeneous wave equation:

    [tex] \nabla ^2 \vec E -\mu σ \frac{\partial \vec E}{\partial t}-\mu\epsilon \frac{\partial^2\vec E}{\partial t^2}= 0 \;\hbox { which in phasor form becomes }\; \nabla ^2 \vec E - j\omega\mu σ \tilde E + \omega^2 \mu\epsilon \tilde E = \nabla ^2 \tilde E +\omega^2\mu(\epsilon -j\frac {\sigma}{\omega}) \tilde E = 0[/tex]

    You let [itex] \epsilon_c=(\epsilon -j\frac {\sigma}{\omega}) [/itex] so the wave equation becomes:

    [tex]\nabla ^2 \tilde E + \omega^2\mu \epsilon_c \tilde E=0[/tex]

    The intrinsic impedance:

    [tex] η_0=\sqrt{\frac{\mu}{\epsilon_c}}[/tex]

    So the impedance is complex. For lossless dielectric, [itex]\sigma=0\;[/itex]. The impedance become real.

    Remember this is characteristic impedance, unless it is lossy, there is no conduction.

    I am skipping a lot of steps as it is quite long in both of your question.
    Last edited: Aug 22, 2012
  6. Aug 22, 2012 #5
    so for lossless medium intrinsic impedance is real, but real imedance signifies pure resistance and losses . So how this is possible in lossless medium ?
  7. Aug 22, 2012 #6
    I suspect your confusion is caused by you thinking about this as being the same as the electrical impedance of e.g. a wire. It's not. Also, I really need to get out of this thread. Good luck.
  8. Aug 22, 2012 #7
    The real caracteristic impedance does mean a real power as in a circuit, but this power is transmitted further, not dissipated.

    Fun: is this power is light from a star that disappeared a billion years ago, we can still detect it, which means the energy has been stored since it was emitted: vacuum stores energy. I fact, the energy of a vacuum insulated capacitor is stored in the vacuum, not in the electrodes. Same for a coil.
  9. Aug 22, 2012 #8
    This is not real resistance, it is from the wave equation defined as intrinsic impedance η. The resistance part is only in the imaginary part with σ. For lossless dielectric ( air and others), σ=0 and the imaginary term disappeared and the resistance is infinite.

    It is important to know intrinsic impedance is not resistance in electrical means, that's the reason I pulled out the wave equation. It is really a definition. For example, for vacuum:


    This don't mean you can measure the vacuum and see 377Ω. The kind of question you asked is not easily explained by common sense terms. From what I study so far, Electromagnetics and Maxwell's equations cannot be easily explained in common sense terms that you can like tell a story. It started out as an observation and the original Maxwell's equation came out. Then the whole vector calculus was invented to help explain the Maxwell's equations. That's the reason it is in vector calculus form. Explanations in English usually are an interpretation of the calculus equations..........or at least I am not good enough to explain it better. That's the reason I have to pull out those fancy equations to talk.

    I am not an expert in EM theory, If anyone have better explanation, I would love to hear. This is not an easy subject, I studied three books, worked through most of the problems in three separate times and I still have more questions than answers. It is like peeling an onion, one layer at a time.
    Last edited: Aug 22, 2012
  10. Aug 22, 2012 #9
    got that
  11. Aug 22, 2012 #10
    The caracteristic impedance is the ratio between E and H for a plane wave; it's real when E and H are in phase. If you understand transmission lines first (U and I in phase) then free-space propagation is easier, to my taste at least.
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