@ChrisWhite Hm. The mods have been taking a vacation from closing stuff (to see if the community can be prompted to start doing it), but if you want I'll have a look and punt them out of the queues
user54412
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I do not agree with the statement that the heat flux has to be normal to the isothermal surface because it would create a transverse gradient otherwise. For example, one could imagine that the vector is not normal to the surface because it did not have time to generate the transverse gradient. The true explanation, in my opinion is a symmetry reason: imagine an infinite isothermal plane. The heat flux has to be normal to it because all contributions will be statistically distributed symmetrically. — fffred10 hours ago
fffred has it exactly right, you can only get to the orthogonality condition through symmetry arguments. It's not true, for example, in the case of a material with a highly anisotropic thermal diffusivity. I don't know if such materials exist, but they're certainly not ruled out by thermodynamics. — Nathaniel8 hours ago
They're basically saying that when the material is anisotropic, the flux will be normal due to symmetry
Because if it wasn't
Then there would be a "preferred direction" to the material
And that's not allowed by the symmetry of anisotropy
Before the question: I am working on numerical calculation of three dimension parabolic equation that based on Fourier's Law of which I am a little confused.
Here comes the law in modern mathematics language.
"The local heat flux is proportional to temperature gradient"
$$
\vec{q}=-k\n...
Darn, where in the world do all these people come from?! At present, the flood of popular, basic one sentence, and bad homowork questions rises again :-/... There is no point in posting anything technical or generally non-basic questions at present, since they would just be flooded down with a rate of 6 very basic posts of laypeople / hour and just disappear unnoticed down the river.
Maybe this is the revenge or attack of Quora ...?
I still suspect that it is not only correlated in time but even related to the change in the frontpage, which gives to not logged in visitors the impression we do mostly popular science and homework help.
And indeed this seems to be what is being done here now and in the future. Physics SE becomes more and more indistinguishable from Quora.
A reminder to everyone who has permissions: check the review queues! This is how we get up to five community close votes on questions. physics.stackexchange.com/review
Before the question: I am working on numerical calculation of three dimension parabolic equation that based on Fourier's Law of which I am a little confused.
Here comes the law in modern mathematics language.
"The local heat flux is proportional to temperature gradient"
$$
\vec{q}=-k\n...
