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Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context:
- Rate of energy transfer per unit area. (SI units: W·m−2 = J·m−2·s−1)
- This is a vector quantity, its components being determined in terms of the normal (perpendicular) direction to the surface of measurement.
- This is sometimes called energy flux density, to distinguish it from the second definition.
- Radiative flux, heat flux, and sound energy flux are specific cases of energy flux density.
- Total rate of energy transfer. (SI units: W = J·s−1)
- This is sometimes informally called energy current.