IIT JEE , AIEEE Forum - Physics , Mechanics - no one answers in Th. Physics... so please do it here...i m sorry for asking here

biki

one week left for my board exams.... so

PLEASE ANSWER

i doubt anyone opens thermal physics forum or not... it has the least no. of replies... So i thought of asking here.... please forgive me once again for asking two questions at a time... i really dont hav the time to make two topics and ask.. i askd in thermal physics part.. but only one answer of ramyani came up and that too an extract form wikipedia.. i want someone who has read HEAT TRANSFER to answer it.... please...

READ MY QUESTIONS CAREFULLY AND ANSWER...PLEASE I M BEGGING FOR AN ANSWER PLEASE

doubt1

When we consider tranfer of heat through a body of length x area of c.s. A and having temperature diff. T₂-T₁ (which is fixed) and termally insulated side walls, then it is considered that initially the energy from the molecules present at one cross sec^n is used by the molecules in the immediate next c.sec^n in 2 ways :-

1) in increasing its temperature

2) in conducting it to the next layer...

But after sometime when steady state is reached it is said that the layers donot increase their temperature further and each layer maintains a constant temperature which decreases as we move towards the colder end. It then only conducts. But why does the process of increasing tempearature cease after sometime? what is the reason behind this?

doubt2

In kirchoff's law of heat radiations it has been said that the ratio of spectral emissive power to the spectral absorptive power ( both corresp. to a wavelength

) at same temperature is same for all bodies and is equal to the spectral emissive power of a black body corresp. to same wavelength

at that temperature.

e

/ a

= E

This proves that emissive power

absorptive power (at same temp.)

But while citing examples, books tell us the example of sodium vapour which when absorbs the light of same wavelength ( when light form some source at higher temperature is allowed to pass through it when cold ) as it would have emitted when excited.

and many more examples where the temp. of absorption is not equal to that of emission... WHY THIS CONDITION STATED IN THE LAW IS NOT OBEYED ??? OR AM I WRONG IN UNDERSTANDING THE LAW ???

biki

and ramyani.... if u hav read this chapter.... only then answer ... please.... DONT MIND

anchitsaini

1)
I'll try to explain like this-
taking an example of a rod which is heated at one of its end , we can divide the rod into several cross sections. each cross section absorbs heat(A) and transfers a part of it to the next cross section(E)(not taking radiations, etc into consideration).We can consider E to be constant but however as temperature of the cross section increases A becomes less as it absorbs less and less heat because its temperature comes closer to that of the adjacent crosssection until a stage comes when E=A. this can be called as steady state when the heat it absorbs =heat transferred resulting in constant temperature of a particular cross section.
this is my own theory so any body who finds a mistake or has a sureshot correct answer do tell me. it will add to my knowledge and i'll edit this

anchitsaini

temperature of absorption not being equal to that of emission doesn't mean anything sensible. however from what i think your doubt is that why a body doesn't emit the same energy as it absorbs. my answer would be that kirchoffs law doesn't say that emissive power=absorptive power it says its proportional to it.hence good emitters are good absorbers but not with an equality sign!!

sidsgr88

solution 2 doubt 2
---------------------
I think u were wrong in understanding kirchoff?s law. What I could gather it that from which ever book u have done kir law u u took into account a body in a uniform temp enclosure?..BUT kirchoff?s law is a general law and holds good for all conditions of pure temp radiations?
This is due to the fact that the emissive and absorptive powers depend only on the nature of the body and not on its surroundings

biki

just tell me this... nd i'll close this topic from my side..

will the body emit the same radiation that it absorbs when it is heated???.... i,e, will proportionality sign become equality sign when the body is excited ???

anchitsaini

well to this question i can say only this-
absorptive power and emissive power have different definitions. one is a ratio and other energy emitted per sec per unit area. hence i don't think it is logical to talk about the equality sign between them. we can talk in relative terms only.
about your question i think you want to ask that if a body is heated would it give away the same number of radiations that it absorbs. well i think it would not. it would just emit more radiations than other materials around heated to the same temperature thus appearing brighter.

feynmann

for doubt - 1 see

http://www.goiit.com/posts/list/heat-thermodynamics-please-answer-urgent-42409.htm

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