(a) Determine the net power flux P(t) entering the box due to a plane wave in air given by \begin{array}{l} \mathbf{E}=\hat{\mathbf{x}} 100 \mathrm{e}^{-20 \mathrm{y}} \cos \left(2 \pi \times 10^{9} t-40

y\right) \quad(\mathrm{V} / \mathrm{m}) \\ \mathbf{H}=-\hat{\mathbf{z}} 0.64 \mathrm{e}^{-20 \mathrm{y}} \cos \left(2 \pi \times 10^{9} t-40 y-36.85^{\circ}\right) \quad(\mathrm{A} / \mathrm{m}) \end{array} (b) Determine the net time-average power entering the box.The box has dimensions: a = 1 cm, b = 2 cm, and c = 0.5 cm. Problem 7.4 Consider the imaginary rectangular box shown in Fig. P7.4.

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