Electric displacement field

In physics, electric induction, also called electric displacement field, is a vector field used in electromagnetism to describe the electric polarization of a dielectric material following the application of an electric field. It is a generalization of the electric field used in Maxwell’s equations to describe the effect of polarization charges on the spatial and temporal configuration of the electromagnetic field.

The electric induction also called dielectric induction, or electric displacement, is a vector quantity D that together with the vector quantity E electric field strength, completely describes the characteristics of the electric field in the presence of insulating materials (dielectric), taking into account the phenomenon of polarization. The fundamental property of the vector D is expressed by a special case of Gauss theorem: the total flux of the vector D through any closed surface is equal to the algebraic sum of all charges contained in it, excluding the polarization charges.

The magnitude of the D-vector is measured in coulomb/m2, its modulus on the surface of conductors is equal to the surface charge density. On this property is based a method for the measurement of the vector D. We introduce two very thin metal disks, of very small area, supported by two insulating handles, in the electric field of which we want to measure the electric field induction: the two disks are in contact with each other and are arranged orthogonally to the force lines of the field. For the phenomenon of electrostatic induction on the external faces of the two disks are manifested equal electrical charges, but of opposite sign. The disks are subsequently separated and taken out of the field; measuring the charge present on one or the other and dividing it by the surface area of the disk, we obtain the value of the modulus of D. The derivative of D with respect to time is called displacement current density and was introduced by C. Maxwell in the classical theory of electromagnetic phenomena that he elaborated.

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