1
$\begingroup$

Take an object that radiates thermal radiation. In which direction does the thermal radiation go? I would think it equally distributed through all directions, but I can't seem to find it mentioned.

Thanks for any answers :)

(let's hope my first question here wasn't too stupid ;) )

$\endgroup$
0
$\begingroup$

Thermal emission is isotropic if the object is (that is, if it is a sphere of constant temperature, and reflectivity). Obviously, something that is hot on one side and cold on the other will have non isotropic emmission.

Of course, for the total energy flux around an object you need to take into account other sources as well.

$\endgroup$
0
0
$\begingroup$

It depends on the background gradient of temperature and gradient of temperature of your object's surface. Imagine that the background were all at the same temperature as your object. There would be no gradient (high to low) in any direction - so no flow of radiation.

But then consider a satellite orbiting a star. On the side of the satellite facing the star, the flow of radiation would be from the star to that side of the satellite - warming the satellite. But on the side of the satellite facing deep space which is only a few degrees Kelvin, the radiation would flow outward away from the satellite.

$\endgroup$
3
  • $\begingroup$ While there is no net flow of radiation in the first case, there is a dynamic equilibrium of radiation emitting from the object/the walls resp. being absorbed by the object/the walls. In other words: The cavity between the object and the walls will reach thermal equilibrium with the object and the walls, while there is no net flow in equilibrium, there certainly is a flow of radiation. $\endgroup$ Oct 16 '15 at 18:24
  • $\begingroup$ I agree; that is a more exact explanation, I understand it's a matter of probabilities, but more difficult for someone struggling with basic concepts. $\endgroup$
    – docscience
    Oct 16 '15 at 18:30
  • $\begingroup$ My question was perhaps awkwardly put. I wanted to confirm to myself that the thermal radiation was isotropic. (And couldn't find it mentioned in my books :/). Fully agree on the heat transfer remarks (and the fact that radiation goes between objects of equal temperature), tho :) $\endgroup$
    – Petter TB
    Oct 19 '15 at 11:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.