tisdag 20 juli 2010
Conduction vs Radiation
In heat transfer by conduction particles interact locally, with more energetic particles feeding less energetic neighbors in a local spatial averaging process. Heat conduction is the result of local interaction by averaging. This process has a direction in time given by decreasing differences or smoothing, or quick damping of high frequencies.
Radiative heat transfer is carried by electromagnetic waves at the speed of light over distance.
This process may be viewed as a form of action at distance, but it appears to be more fruitful to view it (partly) as a local process where an incoming Planck spectrum is transformed into an outgoing spectrum of lower temperature by spectral cut-off of high frequencies, at the surface of absorption/emission. This process has a direction in time by the high frequency cut-off: What has been cut-off is gone and cannot be retrieved.
We conclude that conduction and radiation share the property of high frequency damping/cut-off, while their underlying physics is different.