Energy flux

Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context:
 * 1) Total rate of energy transfer (not per unit area); SI units: W = J⋅s−1.
 * 2) Specific rate of energy transfer (total normalized per unit area); SI units: W⋅m−2 = J⋅m−2⋅s−1:
 * 3) * This is a vector quantity, its components being determined in terms of the normal (perpendicular) direction to the surface of measurement.
 * 4) * This is sometimes called energy flux density, to distinguish it from the first definition.
 * 5) * Radiative flux, heat flux, and sound energy flux are specific cases of this meaning.