[FEM] fix handling of vacuum permittivity

- fix message that permittivity object was not handled
- set an expression, otherwise the user gets only "0.000" for the default of 8.8e-12
- use as unit the common "F/m" (this is also more user-friendly than "s^4*A^2 / (m^3*kg)")
- fix a ToDo

src/Base/UnitsSchemaInternal.cpp

@@ -435,6 +435,10 @@ QString UnitsSchemaInternal::schemaTranslate(const Quantity &quant, double &fact
             factor = 1e6;
         }
     }
+    else if (unit == Unit::VacuumPermittivity) {
+        unitString = QString::fromLatin1("F/m");
+        factor = 1;
+    }
     else if (unit == Unit::Frequency) {
         if (UnitValue < 1e3) {
             unitString = QString::fromLatin1("Hz");

src/Base/UnitsSchemaMKS.cpp

@@ -383,6 +383,10 @@ QString UnitsSchemaMKS::schemaTranslate(const Quantity &quant, double &factor, Q
             factor = 1e6;
         }
     }
+    else if (unit == Unit::VacuumPermittivity) {
+            unitString = QString::fromLatin1("F/m");
+            factor = 1;
+    }
     else if (unit == Unit::Work) {
         if (UnitValue < 1.602176634e-10) {
             unitString = QString::fromLatin1("eV");

src/Mod/Fem/femobjects/constant_vacuumpermittivity.py

@@ -45,6 +45,7 @@ class ConstantVacuumPermittivity(base_fempythonobject.BaseFemPythonObject):
             "App::PropertyVacuumPermittivity",
             "VacuumPermittivity",
             "Constants",
-            "Set the permittivity of vacuum"
+            "Overwrites default permittivity of vacuum"
         )
-        obj.VacuumPermittivity = Units.Quantity(constants.vacuum_permittivity())
+        # we must set an expression so that the small value can actually be entered
+        obj.setExpression("VacuumPermittivity", "8.85419e-12")

src/Mod/Fem/femsolver/elmer/writer.py

@@ -198,13 +198,6 @@ class Writer(object):
             "BoltzmannConstant": constants.boltzmann_constant(),
         }
 
-    def _setConstant(self, name, quantityStr):
-        # TODO without method directly use self.constsdef[name]
-        if name == "PermittivityOfVacuum":
-            theUnit = "s^4*A^2 / (m^3*kg)"
-            self.constsdef[name] = "{} {}".format(self._convert(quantityStr, theUnit), theUnit)
-        return True
-
     def _writeMesh(self):
         mesh = self._getSingleMember("Fem::FemMeshObject")
         unvPath = os.path.join(self.directory, "mesh.unv")
@@ -300,15 +293,19 @@ class Writer(object):
         """
         redefine constants in self.constsdef according constant redefine objects
         """
-        permittivity_objs = self._getMember("Fem::ConstantVacuumPermittivity")
-        if len(permittivity_objs) == 1:
-            Console.PrintLog("Constand permittivity overwriting.\n")
-            self._setConstant("PermittivityOfVacuum", permittivity_objs[0].VacuumPermittivity)
-        elif len(permittivity_objs) > 1:
+        objs = self._getMember("Fem::ConstantVacuumPermittivity")
+        if len(objs) == 1:
+            permittivity = float(objs[0].VacuumPermittivity.getValueAs("F/m"))
+            # since the base unit of FC is in mm, we must scale it to get plain SI
+            permittivity = permittivity * 1e-9
+            Console.PrintLog("Overwriting vacuum permittivity with: {}\n".format(permittivity))
+            self.constsdef["PermittivityOfVacuum"] = "{} {}".format(permittivity, "F/m")
+            self._handled(objs[0])
+        elif len(objs) > 1:
             Console.PrintError(
                 "More than one permittivity constant overwriting objects ({} objs). "
                 "The permittivity constant overwriting is ignored.\n"
-                .format(len(permittivity_objs))
+                .format(len(objs))
             )
 
     def _handleSimulation(self):

src/Mod/Fem/femtools/constants.py

@@ -45,7 +45,7 @@ def stefan_boltzmann():
 def vacuum_permittivity():
     # https://forum.freecadweb.org/viewtopic.php?f=18&p=400959#p400959
     # https://en.wikipedia.org/wiki/Permittivity#Vacuum_permittivity
-    return "8.85419e-12 s^4*A^2 / (m^3*kg)"
+    return "8.85419e-12 F/m"
 
 
 def boltzmann_constant():