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The $\vec{H}$ field (in $\mathrm{A/m}$ ) of a plane wave propagating in free space is given by

\[ \vec{H}=\hat{x} \frac{5 \sqrt{3}}{\eta_{0}} \cos (\omega t-\beta z)+\hat{y} \frac{5}{\eta_{0}} \sin \left(\omega t-\beta z+\frac{\pi}{2}\right) \text {.}\]

The time average power flow density in Watts is

- $\frac{\eta_{0}}{100}$
- $\frac{100}{\eta_{0}}$
- $50 \eta_{0}^{2}$
- $\frac{50}{\eta_{0}}$