# Homework Help: Black body radiation question?

1. Jan 2, 2012

### vkash

black body radiation question??

1. The problem statement, all variables and given/known data

Ina dark room ambient temperature T0, black body is kept at a temperature T. keeping the temperature of the black body constant(at T), sun rays are allowed to fall on the black body through the hole in the roof of the room. assuming that there is no change in the ambient temperature of the room, which of the following statements are correct?

(a) the quantity of the radiation absorbed by the black body in unit time will increase.
(b)since emissivity = absorptivity, hence the quantity of the radiation emitted by the black body in unit time will increase
(c) Black body radiates more energy in unit times in the visible spectrum
(d) The reflected energy in unit time by the black body remains constant

2. Relevant equations

emissivity=absorptivity

3. The attempt at a solution

I think all the options are correct;
Now after opening the hole in the roof greater amount of heat is falling on the black body so it will absorbe greater amount of heat; since emissivity=absorptivity so it will reflect greater quantity of heat energy;
But answer is different from my htought;
can u please explain the correct answer for this question.

2. Jan 2, 2012

### LawrenceC

Re: black body radiation question??

The problem states that the body is kept at constant temperature, T, even with the hole in the roof admitting the sun's rays that are absorbed. If that is the case, radiation being emitted from the black body remains constant because radiation is only a function of temperature at constant emissivity. The additional heat is being removed by another mechanism.

A black body does not reflect. It absorbs 100% of incident thermal radiation.

Seems to me that only 'a' is true.

3. Jan 7, 2012

### vkash

Re: black body radiation question??

No that's not correct.
I also think as u think but i was incorrect. and u are also incorrect. soon i am going to ask it to my physics teacher. In last class the i forget to ask but i will try to ask on upcoming class on Monday.

4. Jan 7, 2012

### LawrenceC

Re: black body radiation question??

I would be interested in hearing the teacher's explanation. I am skeptical because the object's temperature remains constant so heat is leaving by another mechanism in order to maintain a steady state condition. This is entirely possible with constant ambient temperature (as the problem states) if the air movement is increased, thereby increasing heat transfer by convection.