Editing Dielectric (section)

===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.