onsdag 22 maj 2013

Lost in semantics

Following the long argument about "back-radiation" and its newly invented derivative "back-conduction" I have started to speculate if most of the disagreement cannot be traced back to more or less semantic confusions and misunderstandings. First of all, below I have listed the various modes of heat transfer most commonly thought of:


Now consider instead the following list

Diffusive heat transfer
Convective heat transfer

What is the difference? I would say that the first list ties on to the "actual" physical mechanisms, whereas the second list is a classification into different mathematical forms. Convection probably belongs to convective heat transfer, conduction is usually thought of as diffusive, but what about radiation? My guess is that radiation should be considered diffusive too. Now let's add some more confusion: 

Thermal conductivity

The very name seems to imply that it refers only to conduction. But let's suppose you want to experimentally measure the thermal conductivity of a gas. It is possible to tell the molecules "Hey, guys! Could you stop radiating for a while, I only want to measure the conductive heat transfer." Of course it is impossible, yet we stick to the misnomer "conductivity". Now let's move on


This is perhaps one of the most infuriating concepts of modern time. Who invented it? I don't know. If we look again at the first list, radiation occurs as one particular heat transfer. Hence, accepting back-radiation we also ought to be able to speak of


But then the protagonists of back-radiation say "Hey, wait a minute, when I speak of back-radiation I am simply speaking of down welling electromagnetic radiation which we can measure". Ok, so in order to avoid confusion let's call it back-photons instead:


(People who don't like photons can instead think of "Back-electromagnetic rays".) But here comes the final nail in the coffin:


What do you say now? "Well, well. Ok. But photons doesn't stick out their fingers to measure the surrounding temperature".

I rest my case.  

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