Discussion
The magnetic field of a plane wave whose power density is 1 W/m² in free space has an rms value of about
Answer: C
Correct answer: 51.5 mA/m
\[ S = \eta_0 H^2 \quad\Rightarrow\quad H = \sqrt{\frac{S}{377}} = \sqrt{\frac{1}{377}} = 0.0515\ \mathrm{A/m}. \] Note that the impedance divides for \( E \) and multiplies for \( H \), since \( S=EH \) and \( E=\eta_0 H \). Getting that the wrong way round gives 19.4 A/m, which is offered here.
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