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Question on lossy materials

  1. Jul 26, 2011 #1
    Hi,

    I'm trying to understand what makes materials lossy and how to measure that lossiness - this is for either/or electromagnetics or acoustics. So far, I haven't really found good explanations/resources other than that a complex permittivity can cause loss. However, how do viscosity or antennas cause loss, for instance? And moreover, how can these values be measured?

    Sorry for the vague question, I'm not too sure on the subject itself.

    Thanks!
     
  2. jcsd
  3. Jul 27, 2011 #2
    An electrodynamic wave that passes through a medium induces oscillations in the electrons. They "ride the wave" in a sense, passing along the energy but not absorbing it. If something interferes with the electron's oscillation (such as bumping into a nearby atom), it is damped so that some of the energy it was going to pass along with the wave, it now looses to the thing it bumped into as kinetic energy. The more damping, the more the energy of the wave is converted to waste heat in the material and the weaker the traveling wave gets. This is lossiness and it is described mathematically by the imaginary part of the material's permittivity. Lossiness is a function of material density, temperature, bond strength, etc.

    You don't measure lossiness directly. Rather, you measure the refraction and attenuation of a wave as it goes through a material, then http://faculty.uml.edu/cbaird/95.658%282011%29/Lecture2.pdf" [Broken] from these parameters.
     
    Last edited by a moderator: May 5, 2017
  4. Jul 27, 2011 #3
    It's expressed in terms of dispersion. epsilon, mu and sigma ; permittivity,permeability and conductivity. They are related to Maxwell equations through constitutive relations. Send a wave which will obey reflection phenomena (wavelength of the wave should not be equal or greater than the length of the object you are sending the wave to, in other words no scattering) and measure the reflected part, subtract it from the original wave there you will have your true loss.

    For a liquid, lower viscosity means higher density which can be interpreted as wave will interact more rapidly in a liquid with higher viscosity thus induce more loss.

    Antennas can cause everything, they are the fundamental block of em radiation after all. Please be more specific.
     
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