Discussion
A plane wave in free space has an rms electric field of 10 V/m. The power density is about
Answer: D
Correct answer: 0.265 W/m²
\[ S = \frac{E^2}{377} = \frac{10^2}{377} = 0.265\ \mathrm{W/m^2}. \] Because the field is given as rms this is directly the average power density, with no further factor of one half. Multiplying by 377 instead of dividing gives the 3770 offered, which is wrong by a factor of \( 377^2 \).
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