Editing Polarization density (section)

{{Short description|Vector field describing the density of electric dipole moments in a dielectric material}}
{{About|the polarization of dielectric media by electric fields|the polarization of electromagnetc waves themselves|Polarization (waves)|other uses|Polarization (disambiguation)}}
{{Electromagnetism|cTopic=Electrostatics}}

In [[classical electromagnetism]], '''polarization density''' (or '''electric polarization''', or simply '''polarization''') is the [[vector field]] that expresses the volumetric density of permanent or induced [[electric dipole moment]]s in a [[dielectric]] material. When a dielectric is placed in an external [[electric field]], its molecules gain [[electric dipole moment]] and the dielectric is said to be polarized.

Electric polarization of a given dielectric material sample is defined as the quotient of electric dipole moment (a vector quantity, expressed as [[coulomb]]s*meters (C*m) in [[SI unit]]s) to volume (meters cubed).<ref name="Gri07">{{ cite book | title = Introduction to Electrodynamics | edition = 3rd | first = D.J. | last = Griffiths | publisher = Pearson Education | location = Dorling Kindersley | year = 2007 | isbn = 978-81-7758-293-2 }}</ref><ref name="Enc94">{{ cite book | title = McGraw Hill Encyclopaedia of Physics | edition = 2nd | publisher = McGraw-Hill | editor-last = Parker | editor-first = Sybil P. | date = 1994 | location = New York | isbn = 978-0-07-051400-3 }}</ref>
Polarization density is denoted mathematically by '''P''';<ref name="Enc94" /> in SI units, it is expressed in coulombs per square meter (C/m<sup>2</sup>).

Polarization density also describes how a material responds to an applied electric field as well as the way the material changes the electric field, and can be used to calculate the forces that result from those interactions. It can be compared to [[magnetization]], which is the measure of the corresponding response of a material to a [[magnetic field]] in [[magnetism]].

Similar to [[ferromagnets]], which have a non-zero permanent magnetization even if no external magnetic field is applied, [[ferroelectric]] materials have a non-zero polarization in the absence of external electric field.