Black body problem

Question

What will happen if a black body at room temperature is dropped into a furnace?? What will it look initially and how in the steady state.? I guess initially it should be black as it is absorbing a lot of heat and heat absorbed is less than heat released. In the steady state should'nt it be not visible as heat absorbed is equal to heat radiated?

score 2 · Answer 1 · answered Feb 18 '19 at 09:28

2

Sunspots which look black even though they are at temperature of approximately $3800\,\rm K$ are a good example of a colder body being immersed in a hotter body, the photosphere of the Sun which has a temperature of approximately $5800\,\rm K$.

Eventually your black body will absorb heat from the furnace and when it reaches the temperature of the furnace it will be indistinguishable from the contents of the furnace.

answered Feb 18 '19 at 09:28

Farcher

95,680

I mean that's what I am thinking but it is given that it becomes the brightest thing in the furnace. – dawood mansoor Feb 18 '19 at 10:07
@dawoodmansoor It all depends on how close the furnace is to being a black body. – Farcher Feb 18 '19 at 11:04
Can you please take a look at the answer given by @jaromrax and point what is wrong in which answer?? – dawood mansoor Feb 19 '19 at 11:56

jaromrax · Accepted Answer · 2019-02-18T12:03:03.253

1

Your problem seems more about a wrong understanding the term black body. It doesn't mean "black hole" (in common sense, not considering Hawking radiation), something that only consumes all the light (radiation) that arrives on its surface.

The term "black body" wants to express, that such a body will not reflect the incoming radiation (yes, this sounds pretty much the same as black hole) and whatever you see from the surface of this black body (radiation, light...) has the origin in the body own temperature (here is the difference). No reflections.

So - black body's main feature is that it actually emits spectrum (consult https://en.wikipedia.org/wiki/Planck%27s_law) . At low temperatures it is infrared (and it looks black), at high temperatures, our Sun is also a black body. See the picture from How can it be that the sun emits more than a black body?:

The answer: The black body will will accumulate the heat and it will get to the temperature of the furnace. If the temperature reaches more than approx. 800 Celsius, dark red glow may start to appear. With higher temperature, the color will promote further to red and to yellow. Just imagine piece of metal color when getting heated.

Wood reaches normally 300-600 Celsius, the body would remain black for eye.

edited Feb 18 '19 at 12:03

answered Feb 18 '19 at 09:33

jaromrax

1,984

Black holes supposedly emit light according to their temperature. In fact they are the closest "thing" to a black body one can find in the universe. – untreated_paramediensis_karnik Feb 18 '19 at 09:47
I understand what a black body means. I wrote reflected by mistake. – dawood mansoor Feb 18 '19 at 09:58
I meant radiated – dawood mansoor Feb 18 '19 at 09:59
Please answer the main question – dawood mansoor Feb 18 '19 at 10:03
@thermomagneticcondensedboson - thanks for the comment. I see, BH should have the same spectrum, although not because of its temperature. Just the typical T~ 10^-8 K and less. Opinion based statement - Sun is closer. – jaromrax Feb 18 '19 at 11:47
@dawoodmansoor - I am sorry, I had a wrong impression of your understanding. I add a line and I will delete all after a while if no points arrive. – jaromrax Feb 18 '19 at 11:50
Quite logical but answer is given in terms of brightNess of the body relative to contents of furnace. – dawood mansoor Feb 19 '19 at 10:25

Black body problem

2 Answers2