Klyf Brown on fri 2 nov 01
John.
Thanks for the in depth analysis of "heat" Most all points of your
explanations make sense, but I am having a bit of trouble with this
quote;
Heat energy is always being transfered in one way or another,
wherever
there is any difference in temperature. Just as water will run downhill.
always flowing to the lowest possible level, so heat, if left to itself,
flows down the temperature hill, always warming the cold objects at
the
expense of the warmer ones. The rate at which heat flows depends
on the
steepness of the temperature hill as well as the properties of the
materials through which it has to flow. The difference of temperature
per
unit distance is called the temperature gradient in analogy to the idea
of
steepness of grade, which determines the rate of flow of water."
Physics for Science and Engineering
Webber, White and Manning
McGraw Hill
-----------------------------
My problem here is the ocean. This is a body large enough to
accurately see the temp gradient. If heat (applied at the surface by the
sun) is flowing to the cold (and downhill according to this quote), why
is it that it gets colder as you go deeper. Does the bouyancy of the
warmed water negate this effect? Or is it that the air above it is colder
and the heat transfers in that direction?
Actually, I only use the pad on the floor method when I have forgotten
to bring the proper stuff with me. As warming epoxies in the colder
season is a necessity not a wimp factor, I long ago made bucket
"cozys" for my material. I use the "radiant barrier" blankets. They are
two layers of bubble pack, face to face and coated on both outer
sides with virgin aluminum. The cozys have bottom and walls in one
piece with a fitted lid. The bucket fits inside and a heating pad is
inserted on a side. I am relying on what I have heard termed "gravity
syphon" to heat the liquid within. As the liquid near the heat source is
heated it rises and is replaced by cold flowing in from underneath, sort
of a circular motion.
For large projects or winter storage I built some hot boxes. A 3X3'
palette (skid) for the base, 1/2" plywood over that, two layers of
radiant barrier on the floor and walls, two layers on top folded down
3" and tied. Then I throw a few discarded cloth blankets on top of all
that for good measure. This box contains nine five gallon buckest and
can be kept at 80 degrees by three heat pads on low.
I DO NOT advise placing buckets on heating pads because of the
potential for an electrical short and fire. Sometimes I just do dumb
things cause I'm lazy.
Klyf Brown in New Mexico, still trying to come to terms with "heat"
after all these years of using it.
John Baymore on sat 3 nov 01
My problem here is the ocean. This is a body large enough to
accurately see the temp gradient. If heat (applied at the surface by the
sun) is flowing to the cold (and downhill according to this quote), why
is it that it gets colder as you go deeper. Does the bouyancy of the
warmed water negate this effect? =
Klyf,
Said I wouldn't post again on this stuff.... but you asked a direct
question . Not standing by my former binding statements....terrible
. Appologies to all. Interesting you chose that example.......
started out college in marine sciences before I switched to being a BFA
ceramics major .
It gets colder as you go deeper for a few reasons. It does not negate th=
e
"law". True equilibrium is never established... where the ocean is the
same temperature, surface to floor. There is always a gradient if heat
energy is being applied in one area and not another. So heat energy is
flowing. As/if it approches equilibrium..... the driving force for heat
transfer decreases.... and the rate of transfer approaches, but never
reaches, zero. So there will always be a slight gradient.
One factor is that the heat energy being picked up at the surface takes
TIME to dissipate downward. The ocean is DEEP. Another factor is how mu=
ch
heat energy it takes to raise water in temperature...... a lot of energy
for a small change in T. So the T change over a unit of distance relativ=
e
to an amount of heat energy applied is not large. Net result....gets col=
d
with depth fast . The bigger factor though is that (as a
generality.......outside the discussion here is what happens at the
freezing point of water...way O.T.) water being a fluid, the warmer porti=
on
is free to move...and it moves to the top.....like hot air does in colder=
air. Or you could say the colder water sinks . Density issues again.=
Then there is the whole fluid dynamics issue that deals with the formatio=
n
of thermoclines and the like which can affect the way things transpir=
e
a bit. Strata within the water.......temperature layering. Complex
subject. If you've done any open water diving you likely know a bit abou=
t
thermoclines and water density. If your weightbelt is adjusted for neutr=
al
buoyancy at one temperature of seawater...... you can sometimes find a
"layer" of water that has formed below the surface that you can sort of
"float" upon. Relative to the density of that stratified water....you ar=
e
"lighter". Oops... I digress .
The ocean acts kinda' same way that you mention about the "circulation" i=
n
your buckets I'm quoting below: =
"The bucket fits inside and a heating pad is inserted on a side. I am
relying on what I have heard termed "gravity
syphon" to heat the liquid within. As the liquid near the heat source is
heated it rises and is replaced by cold flowing in from underneath, sort =
of
a circular motion."
Good basic description. You might rephrase "...is replaced by cold flowi=
ng
in....." to "...is replaced by colder liquid flowing in...." to make it
just a tad more accurate.
Or is it that the air above it is colder and the heat transfers in that
direction?
That can happen too....but is not the reason for the "cold ocean bottom"
phenomona. =
You'll often hear the weather forcasters mention "radiational cooling" on=
those crystal clear fall and winter nights. That is heat energy stored i=
n
the planet's surface (water, rocks, soil, plants, buildings, etc.) being
transfered to the black coldness of space. The top layer of the ocean is=
some of the warmest water..... so it certainly can give off heat energy. =
And the ocean is a LARGE portion of the surface of the planet . =
Radiation doesn't care too much about the air temperature directly above
it. (See a definition of radiation as a method of heat energy transfer.)
Then there can be some conductive (and then convective) warming of the ai=
r
above the water happening too.
OK....... end of subject. Everyone else is probably getting bored to tea=
rs
.
Best,
..............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"Earth, Water, and Fire Noborigama Woodfiring Workshop 2002 Dates TBA"=
| |
|
