Tuesday Mar. 3, 2015
Music selected from the following: Chvrches "Do I Wanna
Know" (5:13), Au Revoir Simone "The Lead is
Galloping" (3:11), "Just Like a
Tree" (4:49), "Fade Into You"
(5:10), [Mazzy Star "Fade Into You"
(4:16)], "Crazy"
(3:25), "Gravitron"
(4:55)
A cold front rolled through town yesterday
afternoon. See if you can tell when it was on Yesterday's time lapse cloud
video
The Experiment
#2 reports were collected today. If you didn't return
your experiment materials please try to bring them in on
Thursday. We need the thermometers for Experiment #3; I'm planning on having
those materials in class on Thursday.
The Surface Weather Map Analysis was also collected today.
The Upper
Level Charts Assignment is due this coming Thursday.
Quick review: temperature & heat, temperature scales
I quickly reviewed some material on the relationship between
the kinetic energy of the atoms or molecules in a material and
the object's temperature. That was something that I
stuck onto the end of the class notes
from last Thursday's class. There was also some
material on temperature scales. You should
remember the temperatures at which water boils and
freezes on the Celcius and Fahrenheit scales. The Kelvin
scale never goes below zero. The global annual average
temperature of the earth's surface is about 300 K.
Energy transport by conduction
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. Imagine heating
the end of a piece of copper tubing just so you can visualize
a hot object. If you held the object in air it would
slowly lose energy by conduction and cool off.
How does that happen? 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 orange).
In the middle picture the initial layer of energetic
molecules have collided with some of their neighbors and
shared energy with them (these are pink). 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 (yellow) have
now gained some energy. The random motions and
collisions between molecules is carrying energy from the hot
object out into the colder surrounding air.
Conduction transports energy from hot to cold. The
rate
of
energy
transport
depends
first
on
the
temperature
gradient
or
temperature
difference
between
the hot object and the cooler surroundings. If the
object in the picture had been warm rather than hot, less
energy would flow and energy would flow at a slower into the
surrounding air. If there were no temperature difference
there wouldn't be any energy transport at all.
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 and 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 (apparently the highest of all
known solids). 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.
Here's another sketch of conduction shown in class.
I brought a propane torch (2 of them actually, one to serve as
a backup) to class to demonstrate the behavior of materials
with different thermal conductivities. Unfortunately my
matches were damp and wouldn't start (someone came by at the
end of the period and was able to get a couple to ignite).
Here's what I wanted to illustrate
A piece of copper tubing is held in the
flame in the picture at left. Copper is a good
conductor. Energy is transported from the flame by the
copper and you must grab the tubing several inches from the end to
keep from burning your fingers. Part of a glass graduated
cylinder is held in the flame in the center picture. You
could comfortably hold onto the cylinder just a couple of inches
from the end because glass is a relatively poor conductor.
The end of the glass tubing got so hot that it began to glow (its
is emitting radiant energy, the 4th of the energy transport
processes we will discuss). Air is such a poor conductor
that it is safe to hold your finger just half an inch from the hot
flame and still not feel any heat coming from the flame (but be
careful putting your hand or fingers above the flame)
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 a demonstration you need
something that has a strong smell but is safe to breathe.
I tried curry. The classroom is too large and the
ventilation system too efficient so the smell doesn't get very
far. Though the demonstration is
still instructive, I think, because you can visualize what
should happen.
Also we added a new element
to the demonstration that might help you to understand
the difference between conduction and
convection.
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 momentarily).
Here are some examples of insulators that use air:
Foam is often used as an insulator. Foam is
filled with lots of small air bubbles, that's what provides
the insulation.
You can safely hold
onto a foam cup filled with liquid
nitrogen (-320 F) because the foam does
such a good job insulating your fingers from the cold
liquid inside.
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 my
cats can get in and out of the house (that's not
particularly energy efficient).
We really haven't needed winter coats yet in Tucson this
semester (rain coats yes but not winter coats).

|

|
Down feathers used
often used in coasts and sleeping bags. Packing
together a bunch of the "clusters" produces very good
insulation. source
of this image |
Synthetic fibers (Primaloft -
Synergy are shown above in a microphotograph) have
some advantages over down. There is still some
insulation when wet and the material is
hypoallergenic. source
of this image |

|

|
"A quarter-inch sheet of
this aerogel
polymer would provide as much insulation
as three inches of fiberglass." I
am going to have learn more about aerogel, it is
sometimes known as frozen smoke or solid
air. The quote and the image come
from this
source.

A photograph of aerogel (image source).
I've got to get my hands on a sample of this stuff.
|
A scanning electron
microscope photograph of asbestos which was once
widely used as insulation. Asbestos
fibers can cause lung cancer and other damage to
your lungs when inhaled. The white bar at
the top left edge of the image is 50 um across.
You can find this image and read more
about asbestos here.
|
Small natural and synthetic fibers tightly packed
together create lots of pockets of air and provide good
insulation.
Energy transport by convection
If I had been able to light the propane torch I would have used
it to heat up a piece of a broken graduated cylinder again.
The glass gets so hot that you can see it starting to glow red.
How would you cool off a hot object like this? You could
just hold onto it and it would eventually cool by
conduction. If you were in a little bit more of a
hurry you could blow on it. That's forced convection,
the energy transport process we will be covering next. Or
you could stick the hot end of the cylinder into some water (the
glass would probably shatter). That would be an example of
latent heat energy transport, we'll also be discussing later in
the period.
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 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
repeats itself.
__________
Think back to the curry powder
demonstration earlier in class. Diffusion
alone wasn't able to spread the smell very far into the
classroom. To try to spread the smell somewhat further, we
could put a small fan behind the curry powder and try to blow
the smell further into the classroom. That would be more
like forced convection and would be more effective than just
diffusion.
_________
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.
I could put my finger alongside the flame from the propane
torch without any problem. There's very little energy
transported sideways through air by conduction.
Be careful if you put your finger or hand
above the torch. That's because there's a lot of very hot air
rising from the torch. This is energy transport by free
convection and its something you can sometimes see.
If I had been able to light the torch you would
have been able to see (barely) the shimmering of hot
rising air when I held the torch in front of the projector
screen. There is a technique, called Schlieren
photography, that can better catch these barely visible
air motions (it is able to see and photograph the
differences in air density). The photo at right is
an example and shows the hot rising air above a
candle. The photo was taken by Gary Settles from
Penn State University and can be found at this
site.
Now some fairly practical applications, I think, 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 room temperature (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. A piece of aluminum and a piece of
wood (oak) were passed around class so that you could check
this out for yourself.
Something that feels cold may not be as cold as it seems.
Our
perception of cold
is more an indication of how quickly our
body or hand is losing energy
than a reliable measurement of temperature.
Here's another example
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 extremities and skin to try to reduce energy
loss).
Air is a poor conductor. If you go out in 40 F
weather you will feel cold 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, it will feel very
cold (your hand will actually soon begin to hurt).
Keep some warm water nearby to warm up your hand.
Water is a much better conductor than air.
Energy flows much more rapidly from your hand into the
cold water. I mentioned in class that I thought this
might be good for you. The reason is that successive
application of hot and then cold is sometimes used to
treat arthritis
joint pain (it used to work wonders for my Dad's
knee).
You can safely stick your hand into
liquid nitrogen for a fraction of a second.
There is an enormous temperature difference between
your hand and the liquid nitrogen which would
ordinarily cause energy to leave your hand at a
dangerously high rate (which could cause your hand to
freeze solid). It doesn't feel particularly cold
though and doesn't feel wet. The reason is that
some of the liquid nitrogen evaporates and quickly
surrounds your hand with a layer of nitrogen
gas. Just like air, nitrogen is a poor conductor
(air is mostly nitrogen). The nitrogen gas
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 even a few seconds it would
freeze. That would cause irreparable damage.
A question came up in class a few semesters ago
about sticking you hand (or maybe just the tip of one
finger) into molten lead. I've never seen it
done and certainly haven't tried it myself. But
I suspected that you would first need to wet your
hand. Then once you stick it into the lead the
water would vaporize and surround your hand with a
thin layer of gas, water vapor. The water vapor
is a poor conductor just like the nitrogen and oxygen
in air, and that protects your hand, for a short time,
from the intense heat. Here's a
video (and water does play a critical role)
Wind chill
Wind chill is a really good example of energy
transport by convection. As a matter of fact I'm
hoping that if I mention energy transport by
convection that you'll first think of wind chill.
Wind chill is also a reminder that our
perception of cold is an indication of how quickly our
body is losing energy rather than an accurate measurement
of 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
(perhaps indefinitely, I don't know). The 5
arrows represent the rate at which your body is losing
energy.
A thermometer behaves differently,
it is supposed to cool to the temperature of the
surroundings. Once it reaches 40 F and has the
same temperature as the air around it the energy loss
will stop. If your body cools to 40 F you will
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 (9
arrows in both cases). The combination 40 F
and 30 MPH winds results in a wind chill temperature
of 28 F.
You would feel colder on a 40 F day with 30 MPH winds
but the actual temperature is still 40 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 or further cooling. 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, you'd last about 30 minutes (you'd
probably go unconscious before that and die by
drowning).
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 in the summer.
Talk of how long you would last in 40 F water
reminds me of a page from National Geographic Magazine
that lists some of the limits
of human survival. I can't just scan the
original and add it to the notes without violating
copyright laws. But if you click on the link
above you'll find all of the same information online
in the form of a quiz.
Latent heat energy transport
We spent the remainder of the class period looking at
latent heat energy transport. This is the 3rd and
the next to most important energy transport process that
we will cover.
If you had an object that you wanted to cool off
quickly you could blow on it. That might take a
minute or two (maybe more). Or you could stick it
into some water, that would cool it off pretty quickly
because water will conduct energy more rapidly than
air. With a really hot object immersed in water,
you'd probably hear a brief sizzling sound, the sound of
boiling water. A lot of energy would be taken
quickly from the hot object and used to boil (evaporate)
the water. The cooling in this case takes only a few
seconds.
Latent heat energy transport is sometimes a
little hard to visualize or understand because the energy
is "hidden" in water vapor or water.
You should be able to name each of
these phase changes sketched above (this is p. 55 in the
ClassNotes). You should also be able to indicate
whether energy must be added or removed in order for
each phase change to take place. I.e. do you
need to add energy to ice or take energy from a piece of
ice to cause it to melt.
Latent heat energy transport is
associated with changes of phase (solid to liquid, water
to water vapor, that sort of thing) A solid to liquid
phase change is melting, liquid to gas is evaporation,
and sublimation is a solid to gas phase change.
Dry ice is probably the best example of
sublimation. When placed in a warm room, dry ice
turns directly from solid carbon dioxide to gaseous
carbon dioxide without melting first. If you wash
clothes and stick them outside on a cold (below
freezing) day they will eventually dry. The
clothes would first freeze but then the ice would slowly
sublimate away.
In each case above energy must be added to the
material changing phase. You can consciously add
or supply the energy (such as when you put water in a
pan and put the pan on a hot stove and cause it to
boil).
That much is pretty clear. The confusing part of
this topic is when phase changes occur without you
playing any role. Energy is still required to melt
ice; in this case the needed energy will be taken
from the surroundings.
Here are a couple of examples
You put an ice cube in a glass of room temperature water.
Energy will naturally flow
from hot to cold; in this case from the water (about 70
F) to the ice (32 F). This transport of energy
would occur via conduction.
Once the ice had absorbed
enough energy it would melt. Energy taken from
the water would cause the water to cool. The
energy that needed to be added to the ice would be
taken from the surroundings (the water) and would
cause the surroundings to cool.
Here's another, maybe even better, example you
should be very familiar with.
When you step out of the shower in the morning you're
covered with water. Some of the water
evaporates. It doesn't ask permission, it just does it
whether you like it or not. Evaporation requires
energy and it gets that energy from your body. Because
your body is losing energy your body feels cold.
The object of this figure is to give you some
appreciation for the amount of energy involved in phase
changes. A 240 pound man (I have been using Tedy
Bruschi as an example for several years but he's now
retired so I think I'll switch to Scooby Wright) or woman
running at 20 MPH has just enough kinetic energy (if you
could capture it) to be able to melt an ordinary ice
cube. It would take 8 people running at 20 MPH to
evaporate the resulting ice water.
Phase changes can also go in the other
direction.
Try to again name the phase changes
and show whether energy flows in or out of the water
vapor or water when they change phase.
You can consciously remove energy
from water vapor to make it condense. You take
energy out of water to cause it to freeze (you could
put water in a freezer; energy would flow from
the relatively warm water to the colder
surroundings). If one of these phase changes
occurs, without you playing a role, energy will be
released into the surroundings (causing the
surroundings to warm). Note the orange energy
arrows have turned around and are pointing from the
material toward the surroundings. It's kind of
like a genie coming out of a magic lamp. One
Tedy Bruschi worth of kinetic energy is released when
enough water freezes to make an ice cube. Many
genies, many Tedy Bruschis, are released when water
vapor condenses.
This release of energy into the surroundings and
the warming of the surroundings is a little harder for
us to appreciate because it never really happens to us
in a way that we can feel. Have you
ever stepped out of an air conditioned building into
warm moist air outdoors and had your glasses or
sunglasses "steam up"? Water vapor never
condenses onto your body (your body is too
warm). However if it did you would feel
warm. It would be just the opposite of the cold
feeling when you step out of the shower or a pool and
the water on your body evaporates. You know how
cold the evaporation can make you feel, the same
amount of condensation would produce a lot of warming.
I suspect we'd be surprised at how much warming it
produces.
Here's a practical application of what we have been
learning
A can of cold drink will warm more quickly in warm
moist surroundings than in warm dry
surroundings. Equal amounts of heat will flow
from the warm air into the cold cans in both
cases. Condensation of water vapor is an
additional source of energy and will warm that can
more rapidly. I suspect that the condensation
may actually be the dominant process.
The foam "cozy", "koozie",
or whatever you want to call it, that you can put
around a can of soda or beer is designed to insulate
the can from the warmer surroundings but also to
keep water vapor in the air from condensing onto the
can.
Now a couple of figures to illustrate how latent
heat energy transport can carry energy from location
to another.
1. You've just stepped out of
the shower and are covered with water. The water
is evaporating and energy is being taken from your
body.
2. The water vapor (containing latent heat
energy, the energy taken from your body), drifts into
the kitchen where it finds a cold can sitting on a
table.
3. Water vapor comes into contact with the cold
can and condenses. The hidden latent heat energy
in the water vapor is released into the can and warms
the drink inside.
Energy has effectively
been transported from your warm body in the
bathroom to a cold can in the kitchen.
We start in this picture in the tropics where there is often a
surplus of sunlight energy. Some of the incoming sunlight
evaporates ocean water. The resulting water vapor moves
somewhere else and carries hidden latent heat energy with it. This
hidden energy reappears when something (air running into a
mountain and rising, expanding, and cooling) causes the water
vapor to condense. The condensation releases energy into the
surrounding atmosphere. This would warm the air.
Energy arriving in sunlight in the tropics has effectively been
transported to the atmosphere in a place like Tucson.