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A floodlight filtered to emit only red light produces a sinusoidal plane electromagnetic wave with electric field Eₓ = 36*sin(1.20*10⁷ * z - 3.6*10¹⁵ * t) V/m. What is the average intensity of this beam?

1. 0.86 W/m²
2. 1.72 W/m²
3. 3.44 W/m²
4. 6.88 W/m²

Correct answer: 1.72 W/m²

Solution

The amplitude of the electric field is E₀ = 36 V/m. Average intensity = E₀² / (2*Z₀) where Z₀ = mu₀*c = 377 ohm. I = 36² / (2*377) = 1296/754 ≈ 1.72 W/m².

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