Dielectric objects in electric field

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Discussion Overview

The discussion revolves around the behavior of dielectric materials in electric fields, specifically addressing whether an electric field exists inside a dielectric sphere placed in an electrostatic field. The conversation touches on concepts related to Faraday cages, electromagnetic shielding, and the properties of insulating materials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether a Faraday cage can be made from dielectric materials, suggesting that free charges are necessary for effective shielding against electromagnetic waves.
  • Another participant proposes that while a Faraday cage blocks most frequencies, a dielectric material could serve as an electromagnetic shield if it is absorbent at specific frequencies.
  • It is noted that the effectiveness of a Faraday cage depends on factors such as cage size, grid mesh size, and material conductivity.
  • A participant clarifies that in an electrostatic field, a dielectric sphere will have an electric field inside it, challenging the initial assumption about the absence of an electric field.
  • Further discussion indicates that in the case of a positively charged sphere with a cavity made of insulating material, there will still be an electric field present in the cavity, albeit influenced by leakage effects from the insulating surface.
  • Another participant emphasizes the need for simulations to determine the exact behavior of electric and magnetic fields in such configurations, highlighting the importance of material parameters and geometrical definitions.
  • One participant provides a reference to a problem related to the topic, suggesting that insulators do not prevent electric fields but rather inhibit electric current.

Areas of Agreement / Disagreement

Participants express differing views on the nature of electric fields within dielectric materials and the applicability of Faraday cage principles to non-conductive materials. The discussion remains unresolved regarding the specifics of electric field behavior in dielectric spheres.

Contextual Notes

Participants acknowledge the complexity of the topic, noting that assumptions about material properties and external conditions significantly influence the outcomes. The discussion also highlights the need for precise definitions and parameters to fully understand the phenomena involved.

st_01
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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
 
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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.
 
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.
 
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?
 
There is.
 
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?
 
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.
 
st_01 said:
Hi,
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?

Yes. Take a look at this problem involving a http://faculty.uml.edu/cbaird/all_homework_solutions/Jackson_4_8.pdf" .

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.
 
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