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A medium is classed as a good conductor at a given frequency when

  • A.the permittivity greatly exceeds the conductivity
  • B.the permeability greatly exceeds that of free space
  • C.the frequency exceeds the plasma frequency of the material
  • D.the conductivity greatly exceeds the product of angular frequency and permittivity

Answer: D

The dimensionless ratio \( \sigma/(\omega\epsilon) \) decides the classification, and it is frequency dependent: sea water is a conductor at kilohertz and a lossy dielectric at gigahertz. Only the metals remain conductors across the whole radio spectrum.

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