:<math> \tan \delta_m = \mu^{\prime\prime}_r / \mu^{\prime}_r </math>
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Three special media are frequently encountered in electromagnetic problems:
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{| class="wikitable"
|-
! scope="col"| Medium
! scope="col"| \epsilon_rε<sub>r</sub>! scope="col"| \mu_rμ<sub>r</sub>! scope="col"| \&sigma ;! scope="col"| \sigma_m σ<sub>m</sub>
|-
! scope="row"| Free Space
| ∞
|}
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* '''Vacuum''' or '''Free Space''': ε<sub>r</sub> = μ<sub>r</sub> = 1 and σ = σ<sub>m</sub> = 0
* '''Perfect Electric Conductor (PEC)''': ε<sub>r</sub> = μ<sub>r</sub> = 1, σ = ∞, σ<sub>m</sub> = 0
* '''Perfect Magnetic Conductor (PMC)''': ε<sub>r</sub> = μ<sub>r</sub> = 1, σ = 0, σ<sub>m</sub> = ∞
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