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Why is power density of RF waves measured in m2 or cm (Area) and not in cm3 or m3 (volume) ?

I mean aren't those waves travelling (radiating) in 3D space?

I am new to this topic so please pardon if there are any problems in the question.

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This is sometimes called intensity, to avoid confusion with the power density of devices such as batteries and capacitors, where it is indeed measured in $W/m^3$. Radiation intensity on the other hand measures how much energy hits (or passes through) a unit of area per unit of time. This is the same as the energy volume density times the radiation's speed, so it is measured in:

$$J/m^3 \cdot m/s = J/m^2/s = W/m^2$$

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