# IIT JEE past question Thermodynamics 7

A spherical body of area A and emissivity 0.6 is kept inside a perfectly black body. total heat radiated by the body at temperature T is ...

21
tapanmast Vora ·

0.6AT4

1
skygirl ·

62
Lokesh Verma ·

This i remember was a strange question... even i thought it wud be what you have written tapan..

but fiitjee gave 1AT4. There was some logic which I din understand fully but felt that that logic was correct.

This is exactly what they wrote in their solution which i am copying from http://www.fiitjee.com/down/sol/sphysice05.pdf

When a non black body is placed inside a hollow enclosure the total radiation from the body is the sum of what
it would emit in the open ( with e<1 ) and the part (1-a) of the incident radiation from the walls reflected by it.
The two add up to a black body radiation . Hence the total radiation emitted by the body is 1.0ÏƒAT4.

Someone who can explain this ?!! :(

Even I dont know how!

62
Lokesh Verma ·

@sky that is given to be 0.6

1
Â°áƒ¦â€¢à¹“ÑÏ…Î ·

accordin to d buk wich i hav
d q is incomplete

1
Â°áƒ¦â€¢à¹“ÑÏ…Î ·

bt i had read this q in sum othr buk also n they said itz 1 cuz
wen u hav d blck body outside it dnt mattr wat is d emissitviy of d inner body ......

so e=1

62
Lokesh Verma ·

yes integration..

even I am not sure how this happened... This is one of the few question of past IIT JEE that i remember from maths and physics where I still have no clue of what is happening :(

11
Subash ·

brilliant's version :P

As per the given choices, no option is correct, but if we treat e as the symbol of Stefans constant then according to Stefan's Law rate of heat radiation =0.6eAT4

bhaiyya im sure you wont believe in this, but sky has a very good point

11
Subash ·

It would be a crime to see the explanation posted without the options

a)0.4eAT4
b)0.6eAT4
c)0.8eAT4
d)1.0eAT4

11
Subash ·

what do you say? is there any official solution bhaiyya?

11
Subash ·

bhaiyya?

62
Lokesh Verma ·

no there was no official solution forthis one..

11
Subash ·

any links for the official solution?

62
Lokesh Verma ·

http://www.iitkgp.ernet.in/jee/jee2008.htm

11
Subash ·

thanks bhaiyya :)

1
Deepak Shukla ·

Radiations emitted by a body is sum of self radiations of the body and radiations reflected by the body. One can never differentiate between the two. Now, no body can radiate all radiations except a black body. Therefore e is less then one. If we keep a body inside a black body, the black body will also be radiating all wavelenths a part of which will be reflected by our body. Hence in the radiation of the body kept inside black body, all radiations will be present. So it will also behave like a black body.