A medium has r>1, r=1 and conductivity >0. It is semi-transparent to an electromagnetic wave when

Conduction current is much greater than displacement current.

Conduction current equals displacement current.

GATE 2020 PH – Question 19

Electromagnetic theory · Electromagnetic waves in free space, non-conducting and conducting media · 1 mark · Multiple choice

A medium has $\epsilon_r>1$, $\mu_r=1$ and conductivity $\sigma>0$. It is semi-transparent to an electromagnetic wave when

Conduction current is much greater than displacement current.
Conduction current is much less than displacement current.
Conduction current equals displacement current.
Both currents are zero.

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Show answer and explanation

Correct answer: (B) Conduction current is much less than displacement current.

Explanation

Conduction-current density has magnitude $\sigma E$ and displacement-current density has magnitude $\omega\epsilon E$. For a weakly absorbing dielectric, displacement current dominates, $\sigma\ll\omega\epsilon$, allowing wave propagation with limited attenuation. Conduction domination instead describes a strongly attenuating conductor. Thus B.

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