# Describing material media

## Introduction

The material media are described by material regions :

• mainly volume regions in 3D problems; surface regions and line regions are also possible in 3D
• mainly surface regions in 2D problems; line regions and point regions are also possible in 2D

For additional information about the role of the regions, see chapter Physics: principles.

## Material regions: overview

Volume, surface or line material regions enable the modeling of the material media. The physical properties of the medium are those of the corresponding material region.

A region… enables the modeling…
air or vacuum

of the air or of the vacuum (permittivity εr = 1)

perfect conductor*

of a medium of perfect conductor: equipotential frontier (floating or fixed value of electric potential) with a normal electric field

dielectric and non-conducting

of a dielectric medium without losses (real εr permittivity) non-conducting

dielectric and conducting

of a dielectric medium with losses (complex εr permittivity) conducting or non-conducting (resistivity ρ)

Note: *Refer to the § Describing perfect conductors for more details on this type of region.

## Thin regions (3D)

Thin regions enable the modeling of dielectric regions of slight thickness.

In 3D problems:

• for dielectric and non-conducting regions, the direction of the electric field is selected by the user, as indicated in the table below
• for dielectric and conducting regions, the direction of the electric field is imposed by Flux. These regions are used to model pollution surfaces in insulators. The electric field is considered tangent to the face.
Thin region Direction of fields E and D
no restriction quasi tangential
dielectric and non-conducting thin region with random value of permittivity ε

thin region with: ε2 >> ε1

## Filiform regions (3D)

Filiform regions enable the modeling of the regions of small cross-section: dielectric and conducting / dielectric and non-conducting.

In 3D problems, the direction of the electric field is imposed by Flux. The electric field is considered tangent to the line that models the filiform region.