You certainly don't need to try to
remember all these
numbers. The world high temperature record was set in Libya, the
US
record in
Death Valley. The continental US cold temperature record of -70 F
was set in Montana and the -80 F value in Alaska. The world
record -129 F was measured at Vostok station in Antarctica. This
unusually cold reading was the result of three factors: high latitude,
high altitude, and location in the middle of land rather than being
near or
surrounded by ocean (water moderates climate).
Liquid
nitrogen is cold but it is still quite a bit warmer than absolute
zero. Liquid helium gets within a few degrees of absolute zero,
but it's expensive and there's only a limited amount of helium
available. So I would feel guilty bringing some to class.
This next figure might make clearer the difference between
temperature (average kinetic energy) and heat (total kinetic energy).
A cup of water and a pool of water
both have the same
temperature. The average kinetic energy of the water molecules in
the pool and in the cup are the same. There are a lot more
molecules in the pool than in the cup. So if you add together all
the kinetic
energies of all the molecules in the pool you are going to get a much
bigger number than if you sum the kinetic energies of the molecules in
the cup. There is
a lot more stored energy in the pool than in the cup. It would be
a lot harder to cool (or warm) all the water in the pool than it would
be the cup.
In the same way the two groups of people shown have the same
average
amount
of money per person (that's analogous to temperature). The $100
held by the larger group at the
left is
greater than the $20 total possessed by the smaller group of people on
the right (total amount of money is analogous to heat).
Conduction
is the first of four energy transport processes
that we
will cover (and the least important transport process in the
atmosphere). The figure below illustrates this process. A
hot object is stuck in the middle of some air.
In the top picture some of the
atoms or molecules near the
hot object have collided with the object and picked up energy from the
object. This is reflected by the increased speed
of motion or increased kinetic energy of these molecules or
atoms (these guys are colored pink).
In the middle picture the
initial bunch of
energetic molecules have
collided with some of their neighbors and shared energy with
them (these are orange). The neighbor molecules have gained
energy though they don't
have as much energy as the molecules next to the hot object.
In
the third picture molecules further out have now (the yellow ones)
gained
some energy. The random motions and collisions
between molecules
is carrying energy from the hot object out into the colder material.
Conduction transports energy from hot to cold. The
rate
of
energy
transport depends first on
the temperature
gradient of temperature difference. If the object in the picture
had been warm rather
than hot, less energy would flow or energy would flow at a slower into
the surrounding air.
The rate of
energy transport also depends on the material transporting energy (air
in the example
above). Thermal
conductivities of some common materials are listed. Air is a very
poor conductor of energy. Air is generally regarded as an
insulator. Water is a little bit better conductor. Metals
are generally very good conductors (cooking pans are often made of
stainless steel but have aluminum or copper bottoms to evenly spread
out heat when placed on a stove). Diamond has a very high
thermal conductivity. Diamonds are sometimes called "ice."
They feel cold when you touch them. The cold feeling is due to
the fact that they conduct energy very quickly away from your warm
fingers when you touch them.
Transport of energy by conduction is similar to the
transport of a strong smell throughout a classroom by diffusion.
Small eddies of wind in the classroom blow in random directions and
move smells throughout the room.. For our demonstration we used
curry powder.
With time the smell should have
spread
throughout the room. It didn't seem to though. Next time
maybe I'll trying frying some smelly fish of something like that.
Because
air has such a low thermal conductivity it is often used as an
insulator. It is important, however, to keep the air trapped in
small pockets or small volumes so that it isn't able to move and
transport energy by convection (we'll look at convection
shortly). Here are some examples of
insulators that use air:
Foam is
filled with lots of small air bubbles, they're what provides the
insulation.
Thin
insulating layer of air in a double
pane window. I don't have double pane
windows in my house. As a matter of fact
I leave a window open so the cats can get in and
out of the house (that's not particularly energy
efficient). And the stray cats have found out about it and come
in to eat my cat's food (and pee on the furniture). Maybe
sprinkling curry powder on the carpet will keep the stray cats out.
Here's another example
that I didn't
mention in class. Hollow fibers
(Hollofil) filled with air used in
sleeping
bags and
winter coats. Goose feathers
(goosedown) work in a similar way.
Convection
was the next energy transport process we had a look at. Rather
than moving about randomly, the atoms or molecules move together as a
group (organized motion). Convection works in liquids and gases
but not
solids (the atoms or molecules in a solid can't move freely).
At Point 1 in the picture above a
thin layer of air
surrounding a hot object has
been
heated by conduction. Then at Point 2 a person (yes, that is a drawing
of a
person's head) is blowing the blob of warm air
off to the right. The warm air molecules are moving away at Point
3 from the
hot object together as a group (that's the organized part of the
motion). At Point 4 cooler air moves in and surrounds the hot
object and the whole process can repeat itself.
This is forced
convection. If you have a hot object in your hand you could just
hold onto it and let it cool by conduction. That might take a
while because air is a poor conductor. Or you could blow on the
hot object and force it to cool more quickly. I put a
small fan behind the curry powder to try to help spread the
smell faster and further out into the classroom.
And actually you don't need to force convection, it will often happen
on its own.
A thin layer of air at Point 1 in
the figure above (lower
left) is
heated by conduction. Then because hot air is also
low density air, it actually isn't necessary to blow on the hot object,
the
warm air will rise by itself (Point 3). Energy is being
transported away
from the hot object into the cooler surrounding air. This is
called free convection. Cooler air moves in to take the place of
the rising air at Point 4 and the cycle repeats itself.
The example at upper right is also
free convection. Room temperature air in contact with a cold
object loses energy and becomes cold high density air. The
sinking
air motions that would be found around a cold object have the effect of
transporting energy from the room temperature surroundings to the
colder object.
In both examples of free convection, energy is being transported
from
hot toward cold.
Now some
what I think are some fairly practical applications of what we have
learned about conductive and
convective energy transport. Energy transport really does show up
in a lot more everyday real life situations than you might expect.
Note first of all there is a temperature difference between
your hand and a 70 F object. Energy will flow from your warm
hand to the colder object. Metals are better conductors than
wood. If you touch a
piece of
70 F metal it will feel much colder than a piece of 70 F wood, even
though they both have the same temperature. A
piece
of 70 F diamond would feel even colder because it is an even better
conductor
than metal.
Something that feels cold may not be as
cold as it seems. Our perception of cold is more an
indication of how
quickly our hand is losing energy than a reliable measurement of
temperature.
Here's a similar situation.
It's pleasant
standing outside on a nice day in 70 F air. But if
you jump into 70 F pool water you
will
feel cold, at least until you "get used" to the water temperature (your
body might reduce blood flow to your extremeties and skin to try to
reduce energy loss).
Air is a poor conductor. If you go out in
40 F
weather you will feel colder largely because there is a larger
temperature difference between you and your surroundings (and
temperature difference is one of the factors that affect rate of energy
transport by conduction).
If you stick your hand
into a bucket of 40 F water (I probably shouldn't, but I will suggest
you try this), it will feel very
cold (your hand will
actually soon begin to hurt). Water is a much better conductor
than air. Energy flows much more rapidly from your hand into the
cold water. Have some warm water nearby to warm your cold hand
back up.
You can safely stick your
hand in liquid nitrogen for a fraction of a second. It doesn't
feel particularly cold and doesn't feel wet. Some of the liquid
nitrogen quickly evaporates and surrounds your hand with a layer of
nitrogen
gas. This gas is a poor conductor and insulates your
hand from the cold for a very short time (the gas is a poor conductor
but a
conductor nonetheless; if you leave your hand in the liquid nitrogen
for very long it will freeze and your hand would need to be amputated).
Our
perception of cold is a better indicator of how quickly our body is
losing energy rather than an accurate measurement of temperature.
This basic knowledge puts us in a perfect position to understand the
concept of wind
chill temperature.
Your body works hard to keep its core temperature around
98.6 F. If you go outside on a 40 F day (calm winds)
you will
feel
cool; your
body is losing energy to the colder surroundings (by conduction
mainly). Your body will be able to keep you warm for a little
while anyway (maybe indefinitely, I don't know). A thermometer
behaves differently, it is supposed to cool to the temperature of the
surroundings. Once it reaches 40 F it won't lose any additional
energy. If your body cools to 40 F you will probably die.
If you go outside on a 40 F day with 30 MPH winds your
body
will lose
energy at a more rapid rate (because convection together with
conduction are transporting energy away from your body). Note the
additional arrows drawn on the figures above indicating the greater
heat loss. This
higher rate of energy loss will make it feel colder
than a 40
F day
with calm winds.
Actually, in terms of the rate at which your
body loses energy, the windy 40 F day would feel the same as a 28
F day without any wind. Your body is losing energy at the same
rate in both
cases. The combination 40 F and 30 MPH winds results in a wind
chill temperature of 28 F.
The thermometer will again cool to the
temperature of its surroundings, it will just cool more quickly on a
windy day. Once the thermometer reaches 40 F there won't be any
additional energy flow. The
thermometer would measure 40 F on both the calm and the windy day.
Standing outside on a 40 F day is not an immediate life
threatening
situation. Falling into 40 F water is.
Energy will be conducted away from your body more quickly
than
your
body can replace it. Your core body temperature will drop and
bring on hypothermia.
Be
sure
not
to
confuse
hypothermia
with
hyperthermia
which
can bring on
heatstroke and is a serious outdoors risk in S.
Arizona.