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Local field at an atom

  1. Nov 29, 2009 #1
    I just have a small question regarding lorentz cavity:

    Refer to a small lorentz cavity in a uniformly polarised dielectric. as shown in fig.
    [tex]E_{ex}[/tex]: External electric field.
    [tex]E_{P}[/tex]: Electric field in the uniformly polarised dielectric (when sphere has NOT been cut out)
    [tex]E_{L}[/tex]:Electric field due to surface charge on cavity
    [tex]E_{near}[/tex]:Field due to dipoles inside cavity.

    Now, [tex]E=E_{ex}+E_{P}+E_{L}+E_{near}[/tex]

    But, does not [tex]E_{P}[/tex] change if we cut out a sphere from the polarised dielectric?
    Is it that we are neglecting the small change in [tex]E_{P}[/tex] due to cut out sphere and our result is an useful approximation, but not exact?

    Please, any help will be appreciated. I need to understand this for my term exam.

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  2. jcsd
  3. Dec 1, 2009 #2


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    Science Advisor

    I'm not sure, but offhand: If you form the cavity adiabatically, then EL would compensate for the change in EP, wouldn't it? So I suspect that's the approximation involved.
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