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What is a Conductor and Dielectric? |
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The ratio of the conduction and displacement current densities for a given electric field decides whether the medium is a conductor or a dielectirc |
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This means the absolute value of conductivity is not very meaningful in calling a medium good conductor. |
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Any medium with finite conductivity behaves like a conductor as we reduce the frequency and behaves more like a dielectric if we increase the frequency. |
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Generally, we call copper( which has a conductivity  ) a good conductor because it behaves like a conductor upto frequency of  Hz. |
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Whereas, a medium like sea water which has a conductivity of  can behave like a conductor only below 100KHz. |
