Why Dielectric Constant is Constant ?

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Discussion Overview

The discussion revolves around the behavior of dielectric materials in electric fields, specifically addressing the concept of the dielectric constant and its variability under different conditions. Participants explore the mechanisms of polarization, the relationship between applied electric fields and induced dipole moments, and the factors that influence dielectric strength.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how the opposite electric field generated by the insulator increases when the applied electric field is doubled, suggesting that the distance between charged particles in polarized molecules should increase, which seems counterintuitive.
  • Another participant explains that when a dielectric is placed in an electric field, it reduces the field due to energy loss from polarization, with the induced dipole moment being proportional to the applied field and the permittivity of the dielectric.
  • A different participant notes that dielectric strength is not constant and varies with temperature and voltage, particularly approaching the dielectric breakdown limit, which differs by material.
  • One participant asserts that many materials exhibit non-linear behavior, with permittivity depending on the actual applied field.

Areas of Agreement / Disagreement

Participants express differing views on the constancy of the dielectric constant, with some acknowledging variability due to factors like temperature and voltage, while others focus on the mechanisms of polarization without reaching a consensus on the implications.

Contextual Notes

Limitations include the dependence of dielectric properties on external conditions such as temperature and voltage, as well as the complexity of material behavior under varying electric fields.

onurbeyaz
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What I meant to ask is; When we double the electrical field passing through the insulator, the opposite electrical field that caused by the insulator is doubled too. How can this happen, what happens to the molecules in that time to increase the opposite electrical field?

In my opinion, the polarized molecules have to strech (Distance between + and - charged particles in the molecules have to increase) But it doesn't make sense, because this means the generated electrical field have to decrease.
 
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When we place a dielectric in an electric field, the field is reduced. This is because it loses energy that is stored by polarizing the dielectric. In the dielectric, a dipole moment is induced whose strength is proportional to the applied field and the permittivity of the dielectric. This dipole moment produces a secondary field that weakens the applied field. So if we increase the applied field, we increase the polarization field which weakens the applied field. However, this weakening is always a fraction of the applied field.
 
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Please note that the dielectric strength is not quite a constant; it varies with temperature, and will change slowly with increased voltage ... slowly, that is, until the dielectric breakdown limit is reached! This limit varies by material, and how the material is made.

For optical materials the index of refraction depends upon the dielectric constant; changes in temperature can be readily measured via the changes in the index of refraction.

Here is nice description of the origin of the temperature dependence: http://www.doitpoms.ac.uk/tlplib/dielectrics/temperature.php
http://www.doitpoms.ac.uk/tlplib/dielectrics/dielectric_constant.php
 
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Many materials are not linear. Their permittivity depends upon the actual field applied.
 
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onurbeyaz said:
In my opinion, the polarized molecules have to stretch (Distance between + and - charged particles in the molecules have to increase)
The dipoles that give rise to the polarization field are initially randomly arranged and the applied field rotates them so more of them have components that counter the applied field.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dielec.html
 
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