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Black Room and Temperature
Name: Sara
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 3/10/2004
Question:
It is true darker colour absorbs more heat. But if we are in a one
black painted room, is the temperature in the room will be increasing or decreasing
(since black colour absorbs heat)?
Replies:
Sara,
Black is the best absorber of heat -- however, it is also the best at heat radiation.
Assuming that is all that is going on in this "hypothetical" situation, the person in
the room would radiate heat to the walls which, in turn, would re-radiate that heat back
to the person. If that heat could not escape, the room would gradually warm up because
the person present represents a source of heat.
Regards,
ProfHoff 821
The zeroth law of thermodynamics says that objects in thermal equilibrium come to the same
temperature, Einstein pointed out, this means that a good absorber must also be a good
emitter.
If you are in a black room with no energy coming in or leaving, the temperature will, of
course, be constant. If you are in a perfectly
insulated black room in outer space with a single window facing the sun, the sunlight will
come in, the black walls will absorb the sunlight and the room will get warmer. As the
room gets warmer, it emits more heat through the window that the sunlight is coming in.
This will continue until the room reaches the temperature of the sun at which point it
will emit as much heat as it is absorbing.
In a real room on earth, of course the room never gets that hot due to attenuation of the
sunlight, window shades, loss of heat through the walls, etc. It can, however, get very
hot. Spaceships are carefully designed to emit heat on the dark side of the spaceship to
make up for heat absorbed on the bright side.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Remember that heat is associated with electromagnetic energy in the
infrared region, wave lengths longer than about 700 nanometers. A colored
object will absorb visible light, and that energy must be converted to
infrared radiation to be "heat". That process depends on the nature of the
material and is a complicated process, so two bodies having the same color
can differ in the ease with which that process occurs. A "black" room means
that there is no detectable visible light. However, there is a lot of
infrared radiation, we just cannot see it with the unaided eye. Since heat
will flow from a higher temperature to a lower temperature as required by
the second law of thermodynamics, the room will tend to come to some
equilibrium temperature, regardless of the color of the walls.
Vince Calder
Sara,
Black colour absorbs AND emits heat faster. If the air in the room is warmer than the
temperature of the wall, heat is absorbed into the wall faster than if the wall were
white. If it is the wall that is warmer, heat is emitted from the wall faster than if
the wall were white. This is why the inside of a thermos is either very shiny or very
white. Hot things stay hot and cold things stay cold.
Dr. Ken Mellendorf
Physics Professor
Illinois Central College
I am afraid your question leaves a lot of uncertainty.
What is the source of the heat energy that is being absorbed? Will it be the sun? If so,
how will the sunlight enter the room to be absorbed? Is there a window?
In addition to being a good absorber of radiation a "perfect black" object is also an
emitter of radiation. This has the effect of cooling the object.
In the final analysis, whether the temperature will increase or not will depend on the
rate that heat (energy) is absorbed relative to the rate that heat (energy) is lost.
That is, is more energy being lost through the windows and "black body" radiation than
is being absorbed?
Greg Bradburn
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