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Transfering Heat
Name: Katy M.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001
Question:
Can you please settle a dispute about the effect of color on heat
transfer IN THE DARK? I say that since dark colors absorb more light
and that's what results in their increased heat, if they only way they
are receiving heat is though means other than light, the color will
make no difference. My BOYFRIEND, on the other hand, thinks that a
baked potato will cook more quicky with the shiny side out, and that
you'll sleep more warmly in a sleeping bag with the shiny lining, and
that this is why house insulation has a shiny coating on it. (of course
not only is the "color makes a difference in the dark" logic wrong in
my opinion, but it's ALSO backwards because in the light it'd be the
dull, not the shiny, objects that would absorb more heat, right? In
addition to shiny objects in the dark, could you also address different
colored objects in the dark?
Replies:
Katy,
There are three basic methods for transferring heat from one object to
another: conduction, convection, radiation. Conduction is the method we
are most familiar with. Conduction is a hot object touching a cold object.
Convection is moving liquids and gases carrying heat from one location to
another. An example of this is air circulating through ducts and vents.
Neither of these has anything to do with color.
Radiation is radio waves, microwaves, light waves , x-rays, etc. carrying
heat energy from one surface to another. Visible light depends on color.
In the dark, there is no visible light. Infrared light is the most common
heat transfer radiation in the dark. Some material can easily absorb
infrared radiation, and some cannot. Again, it is not based on color.
What does affect it is "shininess". A shiny surface is very smooth. It is
easier for any radiation to reflect from a smooth surface. Shiny metallic
surfaces tend to be smoothest and best reflective. This is not due to
color: a smooth metallic surface has no little grooves or holes in which to
trap extra waves. Color as we define it only relates to visible light.
One comment about a baked potato: in an oven very little heat transfer is
due to radiation. The majority results from hot air coming in contact with
the potato, or with the foil around it. I do not expect the shininess of the
foil will make a big difference. It is important that foil make very good
contact with the potato. Air pockets between the foil and potato act as
insulating zones, slowing the heat transfer.
Dr. Ken Mellendorf
Katy,
We have a mixture of concepts involved in your question. Speaking of light
absorption: You are correct, darker colors absorb light (and heat) better than
lighter colors. However, sleeping inside a sleeping bag with a shiny lining
will cause more of your body heat to be reflected back to you. That is why
thermos bottles are made with a silver lining. The shiny surface reflects
incoming heat to keep the cool contents of the bottle cool just as it can
reflect outgoing heat to keep warm contents warm.
Regarding foil-wrapped baked potatoes (never cooked in a microwave oven --
unless you want to damage the oven): When heated in a conventional oven, the
foil around the potato reflects incoming heat. However, the foil itself heats
up and ultimately transfers heat to the potato, thus allowing it to cook. Once
cooked and removed from the oven, the foil reflects heat back to the potato so
that it stays warm longer than if it had been immediately unwrapped.
Try this experiment: Wrap a potato in plastic wrap and cook it in a microwave
oven. It works like a champ and is faster than cooking the potato in a
conventional oven. It is done that way in some restaurants because it's quick
and the plastic wrap keeps the moisture in the potato. However, no diner wants
to see his/her potato delivered in a messy, unattractive piece of limp plastic
that's a pain to remove. So the cook simply removes the plastic wrap and
replaces it with foil before the server delivers it to the diner. While
snug in
the foil, the potato remains warm.
BTW, you can cook corn on the cob quickly and very nicely by wrapping it in
plastic wrap and using the microwave oven.
Regards,
ProfHoff
I am still having trouble locating the shiny side of
the baked potato . :)
As for insulation, the "shiny" foil is a vapor barrier
which has purposes other than insulating; the vapor
barrier is placed towrd the warm part of the area
being insulated .
My suspicion is that the insulating value of the
materials you mention have more/most to do with the
insulating value of the materials themselves and
less/little to do with a shiny surface.
Within a laboratory setting, one could probably
measure minute differences in heat radiation from
shiny versus dull surfaces of the same materials, but
in a real environment setting, I dont personally
believe one would notice a difference in feeling of
'warmth' in a shiny vs. non-shiny sleeping bag
material, assuming the insulation materials were of
equal structure and weight.
Thanks for using NEWTON!
Ric Rupnik
In total darkness [as the term is commonly used] there is no light, so NO
color.
On the other hand, there can be infrared radiation, to which the eye in not
sensitive.
Home insulation has aluminum foil back for two reasons: to reflect infrared
[heat] radiation, and to provide a moisture barrier. The same reasoning
applies to sleeping bags.
In the case of aluminum foil around a baking potato, I suspect that the
major effect is the moisture barrier the aluminum foil provides so that the
steam cooks the potato more rapidly if it can't escape.
Vince Calder
"In the dark" means no visible light, but visible light is only a
snippet of the full spectrum of electromagnetic radiation (infrared,
microwaves, ultraviolet, etc.), and the color of an object in visible
light says nothing about whether the object will absorb or reflect
other wavelengths. Aside from this, heat transfer doesn't have to
involve radiation. In heat transfer by direct contact, of course,
color doesn't matter at all.
Tim Mooney
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