IIT JEE , AIEEE Forum - Physics , Thermal Physics

ram_gennas

An ideal black-body at room temperature is thrown into a furnace . it is obseved that

a) initially it is the darkest body and at later times the brighest.

b)it is the darkest body at all times.

c)it cannot be distinguished at all times.

d) initially it is the darkest body and at later times it cannot be distinguished .

please help me in this question .

ram_gennas

An ideal black-body at room temperature is thrown into a furnace . it is obseved that
a) initially it is the darkest body and at later times the brighest.
b)it is the darkest body at all times.
c)it cannot be distinguished at all times.
d) initially it is the darkest body and at later times it cannot be distinguished .
please help me in this question

Aatish

A perfectly black body absorbs all the radiations and afterwards emits this radiation, so a body which absorbs more will emit more, as according to Kirchoff's Law e/a = constant

where e is emittance & ais absorbance.

so answer is (a)

Cheers.......

bhupesh

Dear

(a) answer is correct if its perfect black body

saru_jal

no guys i think the answer gotta be (d). initially the body is cold so its radiation spectra doesn't matches with tht of the furnace so it can b distinguished but as the temp of the body matches with tht of the furnace then same radiations r emitted by black body too so it cannot b distinguished...wt say???

vish0001

the answer is ' a ' as since it is a black body , firstly it will absorb all radiations and hence will be the darkest ones and when it reaches the point of saturation, emits back the light to full extent , so intially darkst and then the brightest !

saru_jal

by brightest u mean its emittin more radiations than the furnace itself which is illogical...at saturation pt it will emit radiations equal to tht by furnace so it can't b distinguised. answer is (d).

fd

do u ur self know what u r speaking saru ??

saru_jal

see man..its simple. black body put into furnace...initially darkest as it is absorbing all the radiations....after some time when saturation will reach then no. of radiations emitted per unit area of black body becomes equal to number of radiations emitted per unit area of furnace so its not brightest but cannot be distinguised...by brightest u mean it unit area is emitting more radiations than unit area of furnace which is impossible...if u still stuck in here then better ask someone else....chow..

alokmittal

for a perfect back body answer is (a)

b'coz iabsorbs as well as emitt all the wavelengths

01725074660vir

i think the ans is d at saturation pt it emit radiations equal to tht by furnace so it can't b distinguised

puja

Hey the answer should be a.

Since it is a perfect black body it first absorbs all the radiations incident on it and then emits radiations. It obeys the law - A good absorber is a good emitter.

Cheers!

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