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What is the emissivity of a surface as compared to a black body at the same temperature?
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How does a silver surface work in radiation?
The concept involved here is Emissivity - the ability to emit energy. e is the usual symbol, and a perfect emitter would have an e of 1. Your perfect black body will have this emissivity of 1.A highly polished surface (polished at all the wavelengths concerned) will have an emissivity much smaller. Polished silver has an emissivity of about 0.02.In general the emissivity of a surface is inversely related to its reflectance, but this does vary with the surface finish, colour, and the mass of the object.It is worth noting, that low emissivity glass is used with effect in the double glazing of windows.
What is the definition of 'emissivity'?
The term "emissivity" refers to the relative power of a surface to emit heat by radiation. It could also be defined as the ratio of energy radiated by a particular material to energy radiated by a black body.
Why heat sinks are black anodized?
Radiative heat transfer (heat transfer by electromagnetic radiation) is proportional to e*(T1^4 - T2^4) where T1 is the absolute temperature of the material, T2 is the absolute temperature of the surroundings, and e is the emissivity coefficient. A black material has a high emissivity coefficient, while a silvery material has a low emissivity coefficient. However, the emissivity coefficient cuts both ways, so to speak. A black material in thermodynamic equilibrium with its environment absorbs more radiation, true. But it also emits more radiation (this is necessary for equilibrium to hold). Likewise, a silvery material absorbs less radiation, and also emits less radiation. Conductive heat transfer ensures that the black material on the surface of the heat sink remains hot. The surroundings are at a lower temperature. Therefore T1 and T2 are set, and the heat transferred from the heat sink to the surroundings is simply proportional to e, the emissivity coefficient.
What classification of stars has the lowest surface temperature?
Black dwarfs have the lowest surface temperature.
What is the emissivity of black body?
Emissivity of the black body is 1. And this is probably the reason that the condenser of refrigerator is painted black to loose heat efficiently by radiation ,though there are other modes of heat transfer like conduction and convection which is not affected by colour of the body
Find the approximate temperature of the blackbody radiator?
On first acquaintance with the question one might think that the black body may be perfectly cold as the heat emitted is equal to the heat absorbed and emissivity being a perfect 1, and I think you would be close to the truth. However emissivity is a function of temperature of the body so heat emitted at 0ºK is 0 Joules... So in space that is more or less true, however, consider a black body hovering between Earth and moon - at least at Earths orbital distance. Every 'body' as such has to have at least two sides even if it is infinitely thin. Suppose then that the black body is infinitley thin and that it had an infinitley shiny surface on the reverse. I think the temperature of the surface of the black body facing the sun would then approach and then equal the radiation temperature of the suns surface. (No losses). However if the reverse of the surface was as black as the the front then the losses out back would equal the incoming gains...but to do that it would have to have some temperature. So, I propose that a a perfect black body would have a temperature in perfect proportion to its area facing the sun and the suns area and temperature as seen by each other, and inversly proportional to its distance from the hot body. So, hang on, the area of the black body does not matter, if it emits the same out back or it doesn't. If it does not- (perfect absorbtion, no losses) it approaches and attains the suns surface radiation temperature. If losses equal gain then gain is proportional to inverse of square of distance of heat source temperature wise. OK sun is 20 million ºK at surface. Distance at 0ºK is ∞, mmm.. what does that make Earths orbit theoretical temp?... I am lost. My maths has deserted me! Any way the main thing is that a perfect black body's temperature in deep space is essentially 0ºK or at the background temperature of space( (maybe a few millionths of a degree Kelvin above Zero. But the closer it gets to a heat source, according to the square of its distance from that heat source, so its temperature increases until at the surface of that heat source it attains the same temperature of the heat source. Seems reasonable. Though I would like to Know how to calculate the theoretical temp at a fixed distance from a heat source!
What is hotter blacktop or sidewalk?
Generally blacktop will get hotter because the color black is a better absorber of heat and energy than a lighter colored material. Black is a higher emissivity or rate at which it absorbs energy due to heat. This is why you feel warmer when you wear a black shirt on a hot day compared to a white shirt.
What are dark spots called on the photosphere called?
The black spots on the photosphere of the sun are called sunspots.
Why car radiators are painted black?
Car radiators are painted black because it emits the most heat through radiation (highest emissivity). This improves the heat transfer out of the radiator when air isn't moving through the radiator."black color = high emissivity/absorptivity" - http://en.wikipedia.org/wiki/Thermal_radiationThe paint acts to protect the radiator and its components from rust, scale, and minor impacts (bugs); there is some argument over the claim that the paint offers a smooth uniform surface for 'faster airflow' and better cooling of the radiator. Also note that many cars are intended for the area behind the grille to be black (a backdrop to the grille).
Why do sunspots appeared black when observed from earth?
The relative temperature of sunspots is lower than the surrounding temperature of the sun's surface. Since we adjust our camera's to the high temperature of the sun the relatively cooler sunspots appear black to us.
Dark marks on the sun?
This is called a black spot and is where the sun is hottest on the surface.
If you know that a surface is a good absorber of energy what can you infer about its ability to radiate energy?
Yes. As for the reasons, one of them is that if this were not so, the Second Law of Thermodynamics would be violated, because two side-by-side objects, one black, one white, would develop a temperature difference.
