 |
 |
Heat Management vs. Color
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
You are in a room, windows open but no sunlight coming in the
windows. The air temperature out and inside is say 103F, you are concerned
about feeling too warm. Using some physics, you decide that the color of you
t-shirt will help you "cool" off which is the correct answer? A) White
because it will reflect IR from your warm surroundingskeeping you cooler or B)
Black because you would then be a black body radiator able to lose heat at the
fastest rate I opt for B, but another physics teacher in my county says A?
Which is right?
Replies:
Why do Arabs were black robes in the desert? A white-colored
object reaches a higher equilibrium temperature than a black one. On the
other hand, why are all astronomical observatory domes painted flat white? I
assume because flat white is almost as good as flat black for being a good
black-body radiator AND it absorbs heat much more slowly. So, if you are
going to be in the Sahara all day, do you wear black robes or white ones?
John Hawley
With no sun coming in the windows, your ability to reflect
radiant heat is immaterial. Black body radiation does not have anything to do
with the color of the object. The answer is that the color of your shirt will
not have any affect on your temperature. Take the shirt off and fan yourself
with it, because evaporation of your sweat is the only way your 98 degree body
is going to cool down in a 103 degree room.
Unknown
First of all, heat is generally transferred by 3 mechanisms:
convection, conduction, and radiation. Convection is the actual motion of hot
things (air etc.) into cooler places, conduction is a microscopic transfer of
heat from particle to particle so it gradually diffuses to cold areas (not so
gradually for solid materials) and finally radiation, which seems to be the
main topic of this question, is the generation of electromagnetic waves by a
hot body, to be absorbed by cooler bodies. Now, there is an additional
complication when you talk about a living person - the person is actually
generating heat (otherwise the person is dead). To put the question in its
simplest form then, imagine a corpse in a vacuum-filled container (to prevent
conduction and convection effects) with a window to the outside world. The
container and world are at 103 degrees, the corpse at something cooler,
initially (the vacuum does not have any meaningful temperature in this
example). We know, thermodynamically, that eventually the corpse will reach
the surrounding 103 degrees (two objects in equilibrium are at the same
temperature). The only meaningful question to pose is, what color T-shirt
should the corpse be wearing, to stay cool for as long as possible? Now, the
spectrum of radiation from a body is always peaked at an energy that is
roughly the same as the temperature multiplied by Boltzmann's constant. The
sun, at 5000 degrees, has a peak around 0.5 eV, or slightly below the visible
region. That is why the color of a shirt makes a difference when you are
absorbing light from the sun - a black shirt does absorb in this region, a
white shirt presumably does not absorb as much. But when we talk about
radiation from bodies at 103 degrees Fahrenheit, or about 300 absolute degrees
(Kelvin), the peak in the spectrum of radiation is down by a factor of 10-20.
You cannot tell by looking at an object just how well it will absorb radiation
in that region - it is way outside the visible. Shiny metallic things
generally do not absorb anything very much at long wavelengths though, so you
can at least count on those being good reflectors. So the answer to the
question as rephrased in the preceding response is that neither the black T-
shirt nor the white T-shirt can be counted on to keep the corpse cool for
long, but a shiny metal T- shirt would probably do the job as well as it could
be done.
A. Smith
Click here to return to the Physics Archives
| |
Update: June 2012
|
|
