Black Body Cavity (1 Viewer)

[study-freak]

Black bodies absorb all radiation that falls on them. That energy is spread throughout the object. The cavity walls within the black body also get hotter.

How and why do the walls get hotter when a black body reached its thermal equilibrium?


Also, how do atoms of a black body really absorb and emit radiation? I know that it works like a radio antenna, but what are the things that are oscillating? Nucleus or electrons or both or what?

 

[k02033]

Does the 1st q refer to the cavity with a hole apparatus that has properties of a blackbody?
if so, then let say we heat up the cavity and then the very few incident radiation that managed to pass through the hole, will very likely be absorbed by the walls of the cavity, causing its temperature to go up, then the cavity will make sure that its rate of emission of radiation excesses its rate of absorption of incident radiation ie to cool off. This continues till equilibrium..

Also, how do atoms of a black body really absorb and emit radiation? I know that it works like a radio antenna, but what are the things that are oscillating? Nucleus or electrons or both or what?

i know the answer is that electrons are responsible, when the incident radiation's electric field and magnetic fields across the electron, they exert forces on the electron, causes it to accelerate. And accelerated charges will emit EMR of their own.

However Protons in the nucleus contribute very little to this.... i am guessing is because they experience strong nuclear forces?

 

[study-freak]

Does the 1st q refer to the cavity with a hole apparatus that has properties of a blackbody?

Yep, like a black body box with one small hole

i know the answer is that electrons are responsible, when the incident radiation's electric field and magnetic fields across the electron, they exert forces on the electron, causes it to accelerate. And accelerated charges will emit EMR of their own.

However Protons in the nucleus contribute very little to this.... i am guessing is because they experience strong nuclear forces?

Hmm.. So it's mainly the oscillation of elections but protons do contribute a minute amount?

Thanks anyway! You are the first one to either be able to answer or to be bothered to answer my question.

 

[study-freak]

Does the 1st q refer to the cavity with a hole apparatus that has properties of a blackbody?
if so, then let say we heat up the cavity and then the very few incident radiation that managed to pass through the hole, will very likely be absorbed by the walls of the cavity, causing its temperature to go up, then the cavity will make sure that its rate of emission of radiation excesses its rate of absorption of incident radiation ie to cool off. This continues till equilibrium..

So after equilibrium has been reached, the temperature of the black body will not increase?
And does the fact that radiation absorption leads to higher temperature mean that black body produces IR radiation when atoms oscillate?

 

[k02033]

haha but that answer was a huge guess... but makes some sense to me, since strong nucleus force provides restoring force, stopping any oscillations. so yea... >.>

 

[k02033]

So after equilibrium has been reached, the temperature of the black body will not increase?

well lets just say, it reached equilibrium and then something caused it to change its temperature slightly (energy exchanges are always happening) the body will then alter its rate of emission and rate of absorption again in attempt to reach that equilibrium with its surroundings again.


in another words the blackbody is constantly doing whatever it takes to reach that equilibrium

 

[k02033]

And does the fact that radiation absorption leads to higher temperature mean that black body produces IR radiation when atoms oscillate?

The blackbody emission covers the entire EMR spectrum regardless of the its temperature.

 

[k02033]

here try this thread..
http://community.boredofstudies.org...ty-vs-wavelength-graph-hertzs-experiment.html

 

[study-freak]

Hmm I see.

Thanks for help!!

3. For oscaillators oscillating with high frequency the quantum states are far apart on the energy diagram(charge has to experience a huge energy change, and statistically, this happens rarely) hence changes in quantum states are less likely to occur to oscillators with high frequency, so high frenquency photon emitted are rare(explains low energy density observed. Visa versa for oscillators with lower frenquency.

The oscillators of the electromagnetic field could be excited only when they are supplied with an energy hν. When this value is large, which is the case with high frequency oscillators or low wavelength oscillators, such oscillators are not excited because the particles of the body are not able to supply that much energy. By restricting the energy to discrete values the contribution from the high frequency oscillators is restricted.

second one is from wikipedia. Your and wiki's explanations for near absence of higher frequency seems quite different. Is wiki's also correct?

 

[k02033]

i would trust wiki, oh what do you mean by "near absence of higher frequency"?

 

[study-freak]

maybe wrong terminology

I mean the shape of the intensity vs wavelength curve.
Very small wavelength - very low intensity

Whoops, the quote on above post was actually from wikiEducator, not wikipedia

 

[k02033]

maybe wrong terminology

I mean the shape of the intensity vs wavelength curve.
Very small wavelength - very low intensity

ok here is the thing.. right now i dont believe that y axis on the blackbody curve
is simply intensity (just think about the definition of intensity and you will see why). i think its something called the energy density and is dependent on a combination of intensity and probability, something like that. i dont understand too well either

 

[study-freak]

Ah.. would that be why textbooks usually actually call it radiance, rather than intensity?

 

[k02033]

Ah.. would that be why textbooks usually actually call it radiance, rather than intensity?

exactly

 

[study-freak]

I think I get it now. Most of this black body stuff makes sense to me. (at least, that's what I think, although complex mathematics associated with this is just well beyond my brain's current capacity.)
Thanks

 

[k02033]

Oh and "Visa versa for oscillators with lower frenquency." meaning frequencies near the peak of the curve, not the lowest frequencies. that pharsing was confusing, i should fix that.

 

[k02033]

I think I get it now. Most of this black body stuff makes sense to me. (at least, that's what I think, although complex mathematics associated with this is just well beyond my brain's current capacity.
Thanks

exaclty how i feel too ha ha