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Dielectric objects in electric field

  1. Jul 29, 2011 #1
    Hello,

    This is simple question, but I didnt find the clear answer e.g. on the web pages:

    How Faraday cage works in electric field is clear.
    1. The metal blown sphere = faraday cage in electric field - inside the sphere is not electric field.

    2. But how is it when the blown sphere is made from dielectric material ?Is there inside the sphere electric field or not?

    Thanks
    st_01
     
  2. jcsd
  3. Jul 29, 2011 #2
    I am not aware that it is possible to make a Faraday cage out of purely dielectric, non-conducting material. You need free or semi-free charges in a material in order to move around and cancel out the incident waves. You could make an electromagnetic shield out of dielectric if it is highly absorbent at a frequency of interest (a cardboard box is a shield to visible light) but that would not really be a Faraday cage. A Faraday cage blocks almost all frequencies of incident electromagnetic waves, whereas a plastic box painted black will not.
     
  4. Jul 29, 2011 #3
    Idea is to induce the surrounding emf back to electrical current and send it to ground. Denser the grid higher the performance. You can't make a Faraday cage out of pure dielectric, more you get close to perfect conductivity (not super conductivity) more you eliminate emf.

    Faraday cage doesn't necessarily block all spectrum, it will depend on the specifications and the application. Cage sizes, grid mesh sizes, material conductivity.
     
  5. Jul 30, 2011 #4
    Hi,

    Forget the faraday cage.

    The basic question is: The blown sphere from dielectric (isolant) material is in electrostatic field (no electromagnetic waves).Is there inside the sphere electrostatic field or not?
     
  6. Jul 30, 2011 #5
    There is.
     
  7. Jul 30, 2011 #6
    Yes, I agree with you (y33t), but how is it in situation:

    We have for example positive charged sphere with cavity made of isulation material.
    Is there electric field in the cavity?
     
  8. Jul 30, 2011 #7
    Yes there is but less then completely insulating surface case. Insulating surface will cause leakage which will end up static fields in the sphere (assuming electrostatic condition). If you need to know what will 'exactly' happen, youneed to simulate the system. Solution will require sphere's dispersion parameters and geometrical definition and surrounding field vectors/sources. Result will be electric and magnetic field components in/on/out of the sphere.
     
  9. Aug 1, 2011 #8
    Yes. Take a look at this problem involving a http://faculty.uml.edu/cbaird/all_homework_solutions/Jackson_4_8.pdf" [Broken].

    By the way, an "insulator" refers to the fact that a material insulates against electric current (e.g. keeps electricity in wires), not that it insulates against fields.
     
    Last edited by a moderator: May 5, 2017
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