Solving Inside or on the Surface
When you assign a material to an object, you can specify whether to generate a field solution inside the object or on the surface of the object. If you elect to generate a solution inside the object, a mesh is created inside the object and generate a solution from the mesh. If you elect to generate a solution on the surface of the object, only a surface mesh is created for the object.
If you want a solution to be generated inside an object, select Solve Inside in the Properties window. Conversely, if you want a solution to only be generated on the surface of an object, clear the Solve Inside option in the Properties window.
By default, Solve Inside is selected for all objects with a bulk conductivity less than 105 siemens/meter and for perfect insulators. By default, the Solve Inside option in the Properties window is clear for perfect conductors.
You can set the default values for perfect conductors and insulators in the Maxwell 2D or Maxwell 3D Material Threshold options panel ( refer to Setting Maxwell 2D Options, or Setting Maxwell 3D Options for details.