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TXT
Question:ID Number: 1233659297.Ph
"In a dark room with ambient temperature T1, a blackbody is kept at
temperature
T2. Keeping the temperature of blackbody constant at T2, sunrays are
allowed to
fall on the blackbody through a hole in the roof of dark room.
Assuming there
is no change in the ambient temperature of the room will the quantity
of
radiation absorbed by the blackbody in unit time increase? Will the
reflected energy in unit time by blackbody remain the same?
If the temperature is constant of both room and blackbody why will
amount of radiation per unit time increase with time? Does blackbody
reflect energy? I have read that it only absorbs and radiates."
ANSWER
Greetings Aman:
References:
1.Black Body Radiation, Wikipedia article
http://en.wikiped
ia.org/wiki/Black_body_radiation
2.Kirchhoff�s Law of Thermal Radiation, Wikipedia article
http://en.wikipedia.org/wiki/Kirchhoff%
27s_law_of_thermal_radiation
Thermal radiation incident on a perfect black body has a spectral
absorption factor equal
to one and a spectral emission factor equal to one. Therefore a
perfect black body has no
reflection. This is known as Kirchhoff�s Law of Thermal
Radiation as discussed in
References 1 and 2. Thus a perfect black body will change it�s
temperature to match the
temperature of it�s environment where it will absorb and emit an
equal amount of thermal
radiation. The rate at which a black body changes it�s temperature
depends on the size of
the black body and the thermal heat capacity of the material inside
of the black body. For
example a black body filled with air has a much lower heat capacity
than a black body
filled with water. A small black body, or one with low heat capacity
will change temperature
rapidly and a large black body, or one with large heat capacity, will
take much longer to
change temperature.
In your question you are some how internally heating the black body,
with an undetermined
size and undetermined heat capacity to temperature T2 and maintaining
that temperature in a
room of undetermined size and with the low heat capacity of air at a
temperature of T1. Thus
the black body will radiate heat into T1 until the dark room reaches
T2. How fast this occurs
depends on the relative sizes and thermal heat capacity of the room
and the black body. You
assume that the dark room does not change temperature, thus the time
of the experiment must be
relatively short because the black body is constantly heating the
room toward T2. The added
thermal energy of the sunlight on the black body will be absorbed and
emitted back into the
room, aiding the black body in heating the room temperature to try to
reach temperature T2.
Thank you for your interesting question.
Best regards, Your Mad Scientist
Adrian Popa
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