Editing Dielectric (section)

==Dielectric polarisation==

===Basic atomic model===
[[Image:dielectric model.svg|right|thumb|400px|Electric field interaction with an atom under the classical dielectric model]]

In the classical approach to the dielectric, the material is made up of atoms. Each atom consists of a cloud of negative charge (electrons) bound to and surrounding a positive point charge at its center. In the presence of an electric field, the charge cloud is distorted, as shown in the top right of the figure.

This can be reduced to a simple [[dipole]] using the [[superposition principle]]. A dipole is characterised by its [[electrical dipole moment|dipole moment]], a vector quantity shown in the figure as the blue arrow labeled ''M''. It is the relationship between the electric field and the dipole moment that gives rise to the behaviour of the dielectric. (Note that the dipole moment points in the same direction as the electric field in the figure. This is not always the case, and is a major simplification, but is true for many materials.)

When the electric field is removed, the atom returns to its original state. The time required to do so is called [[Relaxation (physics)|relaxation]] time; an exponential decay.

This is the essence of the model in physics. The behaviour of the dielectric now depends on the situation. The more complicated the situation, the richer the model must be to accurately describe the behaviour. Important questions are:
*Is the electric field constant, or does it vary with time? At what rate?
*Does the response depend on the direction of the applied field ([[isotropy]] of the material)?
*Is the response the same everywhere ([[Homogeneity (physics)|homogeneity]] of the material)?
*Do any boundaries or interfaces have to be taken into account?
*Is the response [[Linear system|linear]] with respect to the field, or are there [[Nonlinear system|nonlinearities]]?

The relationship between the electric field '''E''' and the dipole moment '''M''' gives rise to the behaviour of the dielectric, which, for a given material, can be characterised by the function '''F''' defined by the equation:
<math display="block">\mathbf{M} = \mathbf{F}(\mathbf{E}).</math>

When both the type of electric field and the type of material have been defined, one then chooses the simplest function ''F'' that correctly predicts the phenomena of interest. Examples of phenomena that can be so modelled include:

*[[Refractive index]]
*[[Group velocity dispersion]]
*[[Birefringence]]
*[[Self-focusing]]
*[[Harmonic generation]]

===Dipolar polarisation===
Dipolar polarisation is a polarisation that is either inherent to [[polar molecule]]s (orientation polarisation), or can be induced in any molecule in which the asymmetric distortion of the nuclei is possible (distortion polarisation). Orientation polarisation results from a permanent dipole, e.g., that arises from the 104.45° angle between the asymmetric bonds between oxygen and hydrogen atoms in the water molecule, which retains polarisation in the absence of an external electric field. The assembly of these dipoles forms a macroscopic polarisation.

When an external electric field is applied, the distance between charges within each permanent dipole, which is related to [[chemical bond]]ing, remains constant in orientation polarisation; however, the direction of polarisation itself rotates. This rotation occurs on a timescale that depends on the [[torque]] and surrounding local [[viscosity]] of the molecules. Because the rotation is not instantaneous, dipolar polarisations lose the response to electric fields at the highest frequencies. A molecule rotates about 1 radian per picosecond in a fluid, thus this loss occurs at about 10<sup>11</sup> Hz (in the microwave region). The delay of the response to the change of the electric field causes [[friction]] and heat.

When an external electric field is applied at [[infrared]] frequencies or less, the molecules are bent and stretched by the field and the molecular dipole moment changes. The molecular vibration frequency is roughly the inverse of the time it takes for the molecules to bend, and this distortion polarisation disappears above the infrared.

===Ionic polarisation===
Ionic polarisation is polarisation caused by relative displacements between positive and negative [[ion]]s in [[ionic crystal]]s (for example, [[Sodium chloride|NaCl]]).

If a crystal or molecule consists of atoms of more than one kind, the distribution of charges around an atom in the crystal or molecule leans to positive or negative. As a result, when lattice vibrations or molecular vibrations induce relative displacements of the atoms, the centers of positive and negative charges are also displaced. The locations of these centers are affected by the symmetry of the displacements. When the centers do not correspond, polarisation arises in molecules or crystals. This polarisation is called '''ionic polarisation'''.

Ionic polarisation causes the [[ferroelectric effect]] as well as dipolar polarisation. The ferroelectric transition, which is caused by the lining up of the orientations of permanent dipoles along a particular direction, is called an '''order-disorder phase transition'''. The transition caused by ionic polarisations in crystals is called a '''displacive phase transition'''.

====In biological cells====
Ionic polarisation enables the production of energy-rich compounds in cells (the [[proton pump]] in [[mitochondrion|mitochondria]]) and, at the [[plasma membrane]], the establishment of the [[resting potential]], energetically unfavourable transport of ions, and cell-to-cell communication (the [[Na+/K+-ATPase]]).

All cells in animal body tissues are electrically polarised – in other words, they maintain a voltage difference across the cell's [[plasma membrane]], known as the [[membrane potential]]. This electrical polarisation results from a complex interplay between [[ion transporter]]s and [[ion channels]].

In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the [[dendrite]]s, [[axon]], and [[soma (biology)|cell body]] different electrical properties. As a result, some parts of the membrane of a neuron may be excitable (capable of generating action potentials), whereas others are not.