Displacement polarization

The negative center of charge (electron shell) is to the right of the atomic nucleus (positive center of charge)

In displacement polarization (also called deformation polarization ), dipoles induced by an external electric field are formed by ${\displaystyle {\vec {P}}}$

• the electrons of an atom or molecule are changed ("shifted") in such a way that the center of gravity of the negative charges no longer coincides with the center of gravity of the positive charges ( atomic nuclei ) ( electron polarization )

or

• positive ions are shifted relative to negative ions ( ion polarization ).

The displacement polarization is defined (for linear isotropic media) as:

${\displaystyle {\begin{aligned}{\vec {P}}&=\chi \cdot \varepsilon _{0}\cdot {\vec {E}}\\&=(\varepsilon _{r}-1)\cdot \varepsilon _{0}\cdot {\vec {E}}\end{aligned}}}$

in which

• ${\displaystyle \chi }$the (di-) electrical susceptibility
• ${\displaystyle \varepsilon _{0}}$the electric field constant
• ${\displaystyle {\vec {E}}}$the electric field strength and
• ${\displaystyle \varepsilon _{r}}$is the relative permittivity .

The relative permittivity for ion crystals is

${\displaystyle \varepsilon _{r}=1+\chi _{\text{ion}}+\chi _{\text{el geb}}}$

in which

• ${\displaystyle \chi _{\text{ion}}}$ the ionic susceptibility and
• ${\displaystyle \chi _{\text{el geb}}}$ is the electronic susceptibility.

The relative permittivity for metals is

${\displaystyle \varepsilon _{r}=1+\chi _{\text{el frei}}+\chi _{\text{el geb}}}$

in which

• ${\displaystyle \chi _{\text{el frei}}}$the susceptibility of the conduction electrons and
• ${\displaystyle \chi _{\text{el geb}}}$ is the susceptibility of the quasi-bound electrons.

The displacement polarization is the sum of all induced dipole moments divided by the volume:

${\displaystyle {\vec {P}}={\frac {\sum {\vec {p}}_{ind}}{V}}}$

With bound electrons, the strength of the induced dipole moments depends on the polarizability of the molecule / atom. The connection between microscopically relevant polarizability and macroscopically relevant permittivity is established by the Clausius-Mossotti equation .

In alternating electric fields (e.g. light), matter is repolarized with the frequency of the oscillating E-field. For higher frequencies (greater than those of typical molecular vibrations , e.g. from the infrared range ), the ion polarization can no longer follow due to the greater inertia of the massive ions and can therefore be neglected. The much lighter electrons, on the other hand, follow the alternating field even at higher frequencies (up to about the UV range ).

See also

• Orientation polarization - here existing so-called permanent dipoles (e.g. water) are aligned by an electric field.

literature

• Dieter Meschede (Ed.): Gerthsen Physik . 23rd edition. Springer, Heidelberg 2006, ISBN 3-540-25421-8 .