Charge Conservation
The Charge Conservation node adds the equations for charge conservation according to Gauss’ law for the electric displacement field. It provides an interface for defining the constitutive relation and its associated properties such as the relative permittivity. There are two types of Charge Conservation available; Charge Conservation in Solids and Charge Conservation in Fluids. This distinction decides how materials behave and how material properties are interpreted when the mesh is deformed.
Charge Conservation in Solids applies to materials whose properties change as functions of material strain, material orientation, and other variables evaluated in a material reference configuration (material frame).
Charge Conservation in Fluids applies to materials whose properties are defined only as functions of the current local state at each point in the spatial frame, and for which no unique material reference configuration can be defined.
In COMSOL versions 6.2 and earlier, this Solids/Fluids distinction was controlled within the Charge Conservation node by specifying Solid/Nonsolid in the Material type setting.
Constitutive Relation D-E
Select a Dielectric model to describe the macroscopic properties of the medium (relating the electric displacement D with the electric field E) and the applicable material properties, such as the relative permittivity. Note that Charge Conservation in Fluids uses only Relative permittivity or Polarization. The other constitutive relations are only accessible in Charge Conservation in Solids. Select:
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Relative permittivity (the default) to use the constitutive relation D = ε0εrE. Then the default is to take the Relative permittivity εr (dimensionless) values From material.  For User defined, select Isotropic, Diagonal, Symmetric, or Full and enter values or expressions in the field or matrix. The default is 1.
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Polarization to use the constitutive relation D = ε0E + P. Then enter the components based on space dimension for the Polarization vector P (SI unit: C/m2). The defaults are 0 C/m2.
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Remanent electric displacement to use constitutive relation D = ε0εrE + Dr, where Dr is the remanent displacement (the displacement when no electric field is present). Then the default is to take the Relative permittivity εr (dimensionless) values From material.  For User defined, select Isotropic, Diagonal, Symmetric, or Full and enter values or expressions in the field or matrix. Then enter the components based on space dimension for the Remanent electric displacement Dr (SI unit: C/m2). The defaults are 0 C/m2.