Zempel David wrote:
> Ken Streeter suggests that I keep the paint on the thin
> side to prevent over insulating the drums. Something I
> wouldn't have thought of. Besides, I was under the
> impression that black objects were the best radiators
> and that painting the drums black would improve their
> ability to shed heat. I think I learned this in physics.
> None the less you can bet I'll be checking on this with
> my dad, who taught physics until he retired. Let's see
> where this thread goes.
I'm not a physics professor, but my understanding of the
issue is that black objects *absorb* the most heat from
radiation, which won't make the slightest difference in
this application on brake drums. Being painted black has
nothing to do with thermal transfer characteristics when
the black object is the one radiating energy. In any case,
a radiator doesn't give off most of its heat by radiation,
but rather by convection! I'm curious to see what your dad
says.
(If you want to avoid a technical discussion, you better
delete this article before reading further...)
Basically, what am I claiming? Simply that painting
your brake drums (or radiator) black, rather than white,
won't make the slightest difference as to how well they
work, assuming the same type and thickness of paint is
used. Actually, if one wants to worry about heat
absorption from sunlight, it would be best to paint the
radiator and brake drums *white*, but that is a non-issue
for they are practically always shaded.
Now, on to physics: Why does a black object absorb more
heat from radiation (particularly near the visible spectrum)
than a white object? Well, a black object *looks black*
because it doesn't reflect light. That is, any light
hitting the black object is absorbed, rather than
reflected. The energy of this absorbed light is largely
converted to heat, and warms up the object. A white object
reflects nearly all of the incoming light, and thus stays
cooler. For this reason, you'll stay a lot cooler
on a sunny day in Phoenix if you wear white clothes,
rather than dark clothes. (Heat from sunlight is in
the form of radiation.)
With brake drums and radiators, we're dealing with trying
to transfer heat primarily through convection (from the
radiator or brake drum to the air). Why a radiator
is called a "radiator" rather than a "convector" is beyond
me, as most of the heat transfer isn't even by radiation,
but is by convection! (If the heat transfer was all by
radiation, rather than by convection, blowing more air
over the radiator wouldn't make much of a difference.)
What matters most in the case of heat transfer by
convection is that the materials involved have good
thermal conductivity. (Copper and aluminum would be good,
styrofoam and fiberglass would be bad.) The color of
the material is completely irrelevant. I am assuming
that paint is a relatively poor thermal conductor, and
so you would want to minimize the thickness of the paint
on the radiator or brake drum, in the same way that you
wouldn't want to wrap the brake drum with household
fiberglass insulation.
Just my two cents, from an engineering perspective,
not a physics professors.
--ken
PS: Nearly all of our "on-earth" heating/cooling systems
use convection, rather than radiation. (If you go up to
a satellite, it's a whole different problem, as there is
no air to convect with!)
--
Kenneth B. Streeter | EMAIL: streeter@sanders.com
Sanders, PTP2-A001 |
PO Box 868 | Voice: (603) 885-9604
Nashua, NH 03061 | Fax: (603) 885-0631
|