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 defined by the relative permittivity.
Constitutive Relation D-E
Enter the Relative permittivity εr (dimensionless) to describe the macroscopic properties of the medium. The default is to take the Relative permittivity values From material.  For User defined, enter a value or expression in the field.
Notice that the boundary element method is based on the availability of an analytic Green’s function for the domain. Thus, the Relative permittivity must evaluate to a constant value for each Charge Conservation node.