 in classical electromagnetism, polarization density (or electric polarization, or simply polarization) is the vector field that expresses the density of permanent or induced electric dipole moments 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. the electric dipole moment induced per unit volume of the dielectric material is called the electric polarization of the dielectric.introduction to electrodynamics (3rd edition), d.j. griffiths, pearson education, dorling kindersley, 2007, isbn 81-7758-293-3mcgraw hill encyclopaedia of physics (2nd edition), c.b. parker, 1994, isbn 0-07-051400-3 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. the si unit of measure is coulombs per square metre, and polarization density is represented by a vector p. ==definition== the polarization density p is defined as the average electric dipole moment p per unit volume v of the dielectric material:electromagnetism (2nd edition), i.s. grant, w.r. phillips, manchester physics, john wiley & sons, 2008, isbn 978-0-471-92712-9 :\mathbf{p}=\frac{\langle\mathbf{p}\rangle}{v} which can be interpreted as a measure of how strong and how aligned the dipoles are in a region of the material. for the calculation of p due to an applied electric field, the electric susceptibility Ï of the dielectric must be known (see below). ==polarization density in maxwell's equations== the behavior of electric fields (e and d), magnetic fields (b, h), charge density (Ï) and current density (j) are described by maxwell's equations in matter. the role of the polarization density p is described below. ===relations between e, d and p=== the polarization density p yields the electric displacement field d via  :\mathbf{d} = \varepsilon_0\mathbf{e} + \mathbf{p}. here Îµ0 is the electric permittivity of empty space. in this equation, p is the (negative of the) field induced in the material when the "fixed" charges, the dipoles, shift in response to the total underlying field e, whereas d is the field due to the remaining charg
Electric_and_magnetic_fields_in_matter