==Defining The Physical Structure==
In [[EM.Cube]]'s [[FDTD Module]], a physical structure consists of one or several sets of objects that are grouped together and identified by their material types. Materials are divided into five seven categories that are listed under the '''Physical Structure''' node at the top of the Navigation Tree:
* [[#Perfect Conductors|Perfect Electric Conductor (PEC) Objects]]
* Thin Wires
Under each material node, you can create new material groups of the same type/category but with different properties (color, texture, or electric and magnetic constitutive [[parameters]]). These material groups are used to organize the CAD objects that you draw in the project workspace or import from external model files. When you create a new geometrical object such as a Box or a Sphere, it is inserted under the currently active material type. There is only one material group that is active at any time. It is normally recommended that you first create material groups, and then draw new objects as part of the currently active material group. However, if you start a new [[FDTD Module]] EM.Tempo project from scratch, and you start drawing a new object without having previously defined any material groups, a new default PEC group is created and added to the Navigation Tree navigation tree to hold your new CAD object.
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===Perfect Conductors===
[[EM.Cube]]'s [[FDTD Module]] Tempo offers two types of perfect conductors:
# '''Perfect Electric Conductor (PEC)Objects:''' The tangential electric field on the surface of this type of perfect conductor is zero. The electric and magnetic fields are assumed to vanish inside the volume of a PEC object. A PEC material is characterized by an infinite electric conductivity (σ = ∞). You can draw solid, surface and [[Curve Objects|curve objects]] as part of a PEC group.# '''Perfect Magnetic Conductor (PMC)Planes:''' The tangential magnetic field on the surface of this type of perfect conductor is zero. The electric and magnetic fields are assumed to vanish inside the volume of a PMC object. A PMC material is characterized by an infinite magnetic conductivity (σ<sub>m</sub> = ∞). EM.Tempo currently allows only PMC plates (rectangle strips objects) parallel to one of the three principal axes.
PEC and PMC materials do not have any constitutive material properties that you can modify except for their color or texture. Note that [[FDTD Module]]'s PMC materials are different in nature than the PMC traces you will find in [[EM.Cube]]'s [[Planar Module]]. In the latter, the PMC term is applied to slot traces, which represent finite-sized cut-out objects (areas) from an infinite horizontal PEC ground plane. [[FDTD Module]]'s PMC objects, by contrast, can be arbitrarily oriented [[Surface Objects|surface objects]] or impenetrable [[Solid Objects|solid objects]] of finite extents.