Discussion
A transmitter radiates 1 kW isotropically. The power density at 1 km is about
Answer: A
Correct answer: 80 \( \mu \)W/m²
An isotropic radiator spreads its power evenly over a sphere: \[ S = \frac{P}{4\pi r^2} = \frac{1000}{4\pi (10^3)^2} = \frac{1000}{1.257\times10^7} = 8.0\times10^{-5}\ \mathrm{W/m^2}. \] That is 80 microwatts per square metre, and the inverse square law follows from the same expression - the sphere’s area grows as \( r^2 \) while the power crossing it stays the same.
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