With reference to the Fig 6.3,

.......................... (6.20)

where is the conduction current density and is displacement current density. The loss tangent gives a measure of how much lossy is the medium under consideration. For a good dielectric medium is very small and the medium is a good conductor if . A material may be a good conductor at low frequencies but behave as lossy dielectric at higher frequencies.

For a source free lossy medium we can write

........................... (6.21)