Discussion
The relation between power density and electric field strength in a plane wave in free space is
Answer: B
Correct answer: \( S = E^2/377 \) for peak or rms consistently used
In free space \( E/H = \eta_0 = 377\ \Omega \), so \[ S = EH = \frac{E^2}{\eta_0} = \frac{E^2}{377}. \] The intrinsic impedance divides rather than multiplies, exactly as power is \( V^2/R \) in a circuit. The proviso about peak and rms matters: using rms fields gives the average power directly, while peak fields require a further factor of one half.
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