0 Members and 71 Guests are viewing this topic.
It is weird
Quote from: hamdani yusuf on 25/11/2021 03:07:35If I want to minimize heat loss from thermal radiation of a hot vessel, say 1000 °C, I must make the emissivity of its surface to near 0.If the emissivity is near zero, that means (by Kirchhoff's law) that the absorptivity is also zero. If the surface absorbs no light, then it must reflect it all.That's why they silver the insides of thermos flasks.
If I want to minimize heat loss from thermal radiation of a hot vessel, say 1000 °C, I must make the emissivity of its surface to near 0.
But it emits light quite brightly in a dark room.
Quote from: hamdani yusuf on 05/12/2021 13:24:18But it emits light quite brightly in a dark room.No.A black body at the same temperature would be much brighter.
It varies with temperature and wavelength.
Quote from: Bored chemist on 06/12/2021 08:56:01It varies with temperature and wavelength.Is it possible for a material to have emissivity higher than black body for some specific range of frequency? (presumably lower for other frequency range)
An Analysis of Universality in Blackbody RadiationPierre-Marie Robitaille, Ph.D.*Chemical Physics ProgramThe Ohio State University, Columbus, Ohio 43210Through the formulation of his law of thermal emission, Kirchhoff conferred upon blackbody radiation thequality of universality [G. Kirchhoff, Annalen der Physik 109, 275 (1860)]. Consequently, modern physics holds thatsuch radiation is independent of the nature and shape of the emitting object. Recently, Kirchhoff’s experimental workand theoretical conclusions have been reconsidered [P.M.L. Robitaille. IEEE Transactions on Plasma Science. 31(6),1263 (2003)]. In this work, Einstein’s derivation of the Planckian relation is reexamined. It is demonstrated that claimsof universality in blackbody radiation are invalid.From the onset, blackbody radiation was unique in possessing the virtue of universality [1,2]. The nature of theemitting object was irrelevant to emission. Planck [3], as a student of Kirchhoff, adopted and promoted this concept[4,5]. Nonetheless, he warned that objects sustaining convection currents should not be treated as blackbodies [5].As previously discussed in detail [6], when Kirchhoff formulated his law of thermal emission [1,2], he utilizedtwo extremes: the perfect absorber and the perfect reflector. He had initially observed that all materials in his laboratorydisplayed distinct emission spectra. Generally, these were not blackbody in appearance and were not simply related totemperature changes. Graphite, however, was an anomaly, both for the smoothness of its spectrum and for its ability tosimply disclose its temperature. Eventually, graphite’s behavior became the basis of the laws of Stefan [7], Wien [8] andPlanck [3].For completeness, the experimental basis for universality is recalled [1,2,5,6]. Kirchhoff first set forth tomanufacture a box from graphite plates. This enclosure was a near perfect absorber of light (ε =1, κ =1). The box had asmall hole, through which radiation escaped. Kirchhoff placed various objects in this device. The box would act as atransformer of light [6]. From the graphitic light emitted, Kirchhoff was able to gather the temperature of the enclosedobject once thermal equilibrium had been achieved. A powerful device had been constructed to ascertain thetemperature of any object. However, this scenario was strictly dependent on the use of graphite.Kirchhoff then sought to extend his findings [1,2,5]. He constructed a second box from metal, but this time theenclosure had perfectly reflecting walls (ε =0, κ =0). Under this second scenario, Kirchhoff was never able to reproducethe results he had obtained with the graphite box. No matter how long he waited, the emitted spectrum was alwaysdominated by the object enclosed in the metallic box. The second condition was unable to produce the desired spectrum.As a result, Kirchhoff resorted to inserting a small piece of graphite into the perfectly reflecting enclosure [5].Once the graphite particle was added, the spectrum changed to that of the classic blackbody. Kirchhoff believed he hadachieved universality. Both he, and later, Planck, viewed the piece of graphite as a "catalyst" which acted only toincrease the speed at which equilibrium was achieved [5]. If only time was being compressed, it would bemathematically appropriate to remove the graphite particle and to assume that the perfect reflector was indeed a validcondition for the generation of blackbody radiation.However, given the nature of graphite, it is clear that the graphite particle was in fact acting as a perfectabsorber. Universality was based on the validity of the experiment with the perfect reflector, yet, in retrospect, and givena modern day understanding of catalysis and of the speed of light, the position that the graphite particle acted as acatalyst is untenable. In fact, by adding a perfect absorber to his perfectly reflecting box, it was as if Kirchhoff lined theentire box with graphite. He had unknowingly returned to the first case. Consequently, universality remains without anyexperimental basis.
In principle, it depends how long you are prepared to wait.You need to give enough time for all the light bouncing round the box to hit the black object. (probably a few times because it's not actually a perfect absorber.Another way to consider it is to imagine looking into the box- if it doesn't look black, it won't emit as if it is.
You will never get a perfect black hole.But imagine shining a narrow beam of light in through the hole.Will it hit the black particle as it bounces round?It's also important to remember that no surface is a truly perfect reflector.So a small hole is quite a good black body, even if the box is polished.
QuoteKirchhoff then sought to extend his findings [1,2,5]. He constructed a second box from metal, but this time theenclosure had perfectly reflecting walls (ε =0, κ =0). Under this second scenario, Kirchhoff was never able to reproducethe results he had obtained with the graphite box. No matter how long he waited, the emitted spectrum was alwaysdominated by the object enclosed in the metallic box. The second condition was unable to produce the desired spectrum.
Kirchhoff then sought to extend his findings [1,2,5]. He constructed a second box from metal, but this time theenclosure had perfectly reflecting walls (ε =0, κ =0). Under this second scenario, Kirchhoff was never able to reproducethe results he had obtained with the graphite box. No matter how long he waited, the emitted spectrum was alwaysdominated by the object enclosed in the metallic box. The second condition was unable to produce the desired spectrum.
Eventually it will.
In principle, it depends how long you are prepared to wait.
I imagine a 1 liter plastic cube containing a 1 kg spinning bar magnet with negligible friction at 100 rotations per second.The temperature is 300K, just like the room temperature.A 1kg Aluminum cube is placed right next to the first cube. Its initial temperature is 310K.The Eddie current would increase the temperature of the aluminum cube, while reducing the rotation rate of the spinning magnet. Here we see electromagnetic energy transfer from lower temperature body to higher temperature body. Thus the radiation type can't be thermal, although it's surely electromagnetic in nature.
imagine
Energy is transferred to the aluminium, but it isn't thermal energy which the spinning magnet looses; but kinetic energy.
I imagine a 1 liter plastic cube containing a 1 kg spinning bar magnet with negligible friction at 100 rotations per second.The temperature is 300K, just like the room temperature.A 1kg Aluminum cube is placed right next to the first cube. Its initial temperature is 310K.