Is the Net Flux from an Isotropic Source Zero?

[ed2288]

Hi everybody.

Apologies for asking what may be a very simple question, but in every textbook I've read, they say that the net flux through an area dA from an isotropic emitter is zero. But it also says that the sun is an isotropic emitter.

Now hold on, the net flux from the sun is clearly *not* zero, otherwise we'd get no energy from it

Can somebody explain to me where I'm missing the point?

Thanks

 

[Ken G]

I think you are confusing "isotropic emitter", which refers to the object doing the emitting, with "isotropic radiation field", which refers to the radiation itself. Isotropic radiation is something that occurs locally, at a point, and if the radiation at some point is isotropic, then there's no net flux through any tiny area at that point (speaking statistically). However, saying the sun is an "isotropic emitter" refers to how it is emitting globally, not at any given point. At any given point on the surface of the Sun, that point is certainly not emitting isotropically, it is passing a flux from the interior. But the Sun as a whole, when you consider all those tiny areas, is an isotropic emitter, just by the global spherical symmetry.