# Why Dielectric Constant is Constant ?

1. Jun 18, 2014

### onurbeyaz

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.

2. Jun 18, 2014

### Born2bwire

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.

3. Jun 18, 2014

### UltrafastPED

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

4. Jun 19, 2014

### sophiecentaur

Many materials are not linear. Their permittivity depends upon the actual field applied.

5. Jun 23, 2014

### Simon Bridge

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