# Talk:Kirchhoff's law of thermal radiation

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## Query

In line 2 of the article, it says that Kirchoff's law was proved in 1861. Laws of nature are not something that one can prove or disprove. My understanding is that what Kirchoff formulated with regard to black body radiation was actually a theorem and not a physical law. So shouldn't this article be entitled Kirchoff's theorem instead of its current title? -- Metacomet 00:54, 16 December 2005 (UTC)

There isn't as sharp a distinction between "law" and "theorem" as you seem to think. For example, Kirchhoff's voltage "law" is really just a consequence of Maxwell's equations, as is Lenz's "law" and Snell's "law". There are lots of things called "laws" that are really just theorems proved from more basic descriptions of physical law, which in turn can be derived from other more basic derivations, etcetera. (Partly, the naming is a historical question of which came first, but I don't think there is a general rule.) Regarding the article title, you have to stick with common usage as it is. —Steven G. Johnson 01:03, 16 December 2005 (UTC)
That said, the concept of "proof" is probably one to be avoided in science, where the general idea is that hypotheses are either supported or refuted through evidence. In this case, yes - the mathematical proof rests on the assumption of the underlying physical theory of thermodynamics being correct. So it's not really a proven theory, as much as a logical deduction from the best current model of physics. --JB Gnome (talk) 04:29, 24 November 2010 (UTC)

The last line of this article, about the reflectivity of emergency blankets sounds plausible,except the reflectivity/emissivity of a substance is wavelength dependent, so just because something is reflective in the visible portion of the spectrum does not mean it will be the same in the long wave thermal region. Perhaps it is true in this case, but I don't believe that it logically follows.

Metals generally become better reflectors at longer wavelengths, so a metal that is a good reflector at optical wavelengths is also almost certainly going to be reflective at infrared wavelengths. In practice, I seem to recall that these blankets use Aluminum, a graph of whose reflectivity is given at Optical coating. True, the text is a bit vague (it just says "reflective material" and doesn't specify a wavelength or a metal), and could be improved. —Steven G. Johnson 19:47, 5 January 2006 (UTC)

## The Kirchoff's "Equation" ?

I have tried to search for this equation (approximation for liquid-gas transition):

${\displaystyle {({\frac {\partial {{\mathcal {H}}_{fg}}}{\partial {T}}})}={\mathcal {C}}_{P_{g}}-{\mathcal {C}}_{P_{f}}}$

and the equation (approximation for solid-liquid transition):

${\displaystyle {({\frac {\partial {\frac {{\mathcal {H}}_{sf}}{T}}}{\partial T}})}={\frac {{\mathcal {C}}_{P_{f}}-{\mathcal {C}}_{P_{s}}}{T}}}$

They are called Kirchoff's equations. I think there should be a disambiguation page for these.

--Musically ut 06:42, 27 November 2006 (UTC)

## Thermal blankets

It is why, for example, lightweight emergency thermal blankets are based on reflective metallic coatings: they lose little heat by radiation. I'm not convinced that this is a good example. I think they work mainly because of their reflectivity, bouncing radiation back to the body inside, and absorbing little to be conducted to the outer surface where the losses will be mainly down to convection and conduction rather than radiation? Derek Andrews (talk) 11:00, 4 May 2009 (UTC)

But that's exactly why it's a good example! Since their reflectivity is high, their absorptivity is low, which means their emissitivity is low too.--JB Gnome (talk) 04:32, 24 November 2010 (UTC)