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Joseph P. Straley
Department of Physics & Astronomy
University of Kentucky
temp
equil
texp
lc
cond
convection
convert
heat
phase
irrev
Here are collected my discussions of the questions in the Virtual Workshop on Temperature and Heat, as well as some of the discussions from participating groups.
Please see the Map showing the location of all the participants. And check out the Question Board!
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Joseph P. Straley
Department of Physics & Astronomy
University of Kentucky
Temperature
1.The thermometer that we have been studying is of a standard (somewhat old-
fashioned) design. But there
are other ways to measure or sense temperature. Make a list of these, noting such
features as temperature
range for which they are useful, precision of the measurement (this means, how
much the temperature can
change without the thermometer noticing it -- it is the difference between the
second hand and the minute
hand on a clock), accuracy of the measurement (this means, how believable is the
answer? A very precise
clock might not be accurate, if it had not been set carefully). In addition to
the usual devices designed to be
thermometers, you could also consider things that depend on temperature in a
definite way, such as whether
the bass are biting.
I did want to make the point that there are informal
thermometers -- the trees lose their leaves and the lake freezes in winter;
crickets chirp in the summer (and there is a claim that you can tell the
temperature by how often they chirp). Almost every property of
anything changes with the temperature (even the mass of an object
depends on temperature -- in about the 18th decimal place),
and so anything could be a thermometer -- the problem is in getting
the effect big enough to see, and then calibrating the thermometer
reproducibly.
My wife tells the temperature outside by looking at
the kitchen door -- if it is fogged up, it's cold outside.
This depends slightly on the fact that I beat her to
the kitchen by 15 minutes, and have already boiled
water for the coffee and made my oatmeal in the
microwave, which add water vapor to the air. We
will find out later that the temperature at which the
glass fogs is dependent on the amount of water vapor
in the air inside, but is a very sharply defined temperature:
either the glass fogs or it doesn't, depending on the
temperature outside.
Ice melts at 0 C (32 F), but
other things melt at other temperatures. Salad oil and
butter freeze in the refrigerator; tar melts on hot days;
and one way a potter tells whether the kiln is hot
involves observing a small piece of stuff that changes
shape (due to partial melting) when it gets hot enough.
The melting temperature of pure materials is very well
defined, and is used to calibrate thermometers.
One group said:
One group said:
2.Why do we care what the temperature is? Discuss some of the things that go wrong
(or go right) when it is
hot or cold.
Because the properties of materials depend on temperature, the function
of a device also is temperature dependent. With simple devices we don't
notice this, but as the devices get more complicated it is a more serious
problem. Transistors are very temperature sensitive, and I guess it is
a miracle of engineering that computers are not accidentally thermometers
(2 + 2 = 5 on really hot days). Which is to say: devices are designed to work
in a range of temperature, and at other temperatures they may stop working
altogether. Living organisms are really complicated, and so we have to
maintain an environment that is about the right temperature, and use
environments in which the temperature is all wrong to discourage bacteria --
we keep food the food cold for a while and then roast it.
One group said:
One group said:
Comments from Joe Straley:
One group said:
Equilibrium
1.The wind certainly makes you feel cold. How does it affect the thermometer
reading? Consider a
thermometer in your room that is reading 22o C (72o F). If
you turn a small electric fan on and point it at the
thermometer - or even fan it vigorously with a piece of paper folded into a fan
shape - how will it affect the
reading and why? Discuss the respects in which you differ from a thermometer that
are relevant to your
answer (after all, you both have red noses!).
It is important to realize that our sensation of hot and cold
differs from what a thermometer reads. People maintain a
constant temperature, and what we mean by "cold" is that
we are losing energy rapidly. This can happen because the
temperature is low, but it can also happen because we are
touching a material that absorbs heat better than air, or
because the air is rapidly moving past us (which helps the
air carry away energy). So the wind makes us cold, metal
is cold, and stone is cold -- even though the temperature
of these is not different from the temperature of the air
(as you found out in the first activity in this section).
The weatherman calculates a "wind chill" factor because
the thermometer reading doesn't fully explain what we feel.
Some people are suprised by the results of the first
activity, in which it was found that a thermometer
reads the same temperature in a glove, wrapped in aluminum
foil, or stored in a plastic bag full of water. It is
measuring the actual temperature of these things. To
us the metal and the bag of water would feel cooler
than the air, and the glove would feel warmer, because
they are more effective or less effective at removing
energy from us.
Determining the temperature of something by touching
it can be tricky. You can compare things of the same
kind, to tell warm rocks from cool ones or hot foreheads from
normal ones, but you cannot reliably compare different
things.
One group said:
Another group said:
Comments by Joe Straley:
2.Here are some ways to deliver energy, and some estimates of what they cost. Use
this data to determine
what a Joule costs, each way. If energy was completely easy to convert from one
form to another, the cost
for 1,000,000 J would be the same for each way. Think about and comment on why
the cost varies the way it
does.
One group said:
Comment by Joe Straley:
Food has to meet much higher standards of purity,
and the energy has to be in very specific forms: complicated
molecules, assembled in special ways. From this point of
view it is surprising that sugar costs "only" twenty times as
much as oil (let's pause to notice that fuel alcohol is
made from sugar, basically -- the gasohol industry is
going nowhere until the price of gasoline rises to meet the
price of sugar).
We don't eat steak for the energy alone. So what if it
costs 30 times as much as sugar, or 1000 times as much as
coal?
The manual laborer does more than lift the bricks --
he can find the bricks, sort out the broken ones, and neatly
stack them in the process. However, it costs 50,000 times
as much as electrical energy, so we will always use a
machine when we can.
The cost of 1,000,000 J would be the same, no matter what the form, if it was
completely
easy to change it from one form to another, and if we didn't care about side effects
like
smoke, dirt, radiation, or what the energy tasted like.
3.Room temperature inside your house is probably about 72o F. Now
consider other locations, such as on the
porch, or in the attic. What does "thermal equilibrium" mean in these other
environments? And how will
an object taken from inside your house be affected when it is moved into one of
those other environments?
One group said:
Another group said:
Still another group said:
Comments by Joe Straley:
A summary comment by Joe Straley:
Thermal Expansion
1.Give five examples of thermal expansion in everyday life. Hint: thermal expansion
effects are almost too
small to see, but sometimes there are audible effects.
One group said:
Another group added:
Comment by Joe Straley:
2.Compare the volume of liquid that a flower tube can hold, to the volume of
liquid that fills the little straw
that we used to make the air thermometer: approximately how many strawfuls
would it take to fill the tube?
This ratio gives you an idea of how much the volume of the air changes in the
air thermometer.
One group said:
Comment by Joe Straley:
3.Scientific theories are useful when they account for all observed facts (including
facts that were not being
considered when the theory was being constructed), and are able to predict the
outcomes of experiments that
haven't been done yet. One theory of thermal expansion that is occasionally given
(by students who have
heard too much about atoms without gaining much understanding of them), is that
materials expand with
temperature because the atoms get bigger. (The same students probably also
believe that cheese is made of
tangy orange atoms).
Please discuss this theory of thermally expanding atoms. Can it describe the behavior
of an iron rod? Does it
apply to air? Can you use it to explain the behavior of water? What does the
theory predict for sodium
chloride? And finally, do we need this theory at all?
One group said:
Comment by Joe Straley:
Liquid Crystals
1.Place a warm object -- a cup of coffee, your hand, a stone that has been sitting
in the sunlight for a while --
on a table top for a few seconds. Remove it and then place the thermal sensing
sheet where the object was.
What do you observe? Does it matter which temperature-range thermal sensing sheet
you use? How could
you modify this activity to get an effect with a cool object?
One group said:
We also need to discuss what happens to the thermal sensing
sheet when the object is cool. If the thermal sensing sheet is
already black, it probably means it only responds to temperatures
above room temperature, and the cool object will give no effect.
If the sensing sheet is tan or green or blue, then the cool object
will give a brown or black spot.
2.What is the smallest temperature difference that you are able to distinguish by
looking at the thermal
sensing sheet? What is the temperature range where the color changes a lot with a
few degrees?
One group said:
One group added:
Conduction
1.Explain why wet clothes are very dangerous to people in very cold places, such as
Polar explorers.
Comment by Joe Straley:
1. Give some examples from around the house where we choose materials
because they are poor thermal conductors. Give examples where we choose
materials because they are good thermal conductors.
One group said:
Comments by Joe Straley
It would seem that there could be an application of a good conductor
to remove energy from an object -- as a way to rapidly cool it --
but I can't think of very many good examples.
When I was a boy, we used to make ice cream in a gizmo that
you cranked (more accurately, the most enthusiastic 12-year old
cranked). The ice cream mix was inside a stainless steel can, and
cooled by immersing it in a salt-ice mixture. That would be an
example of a system to remove energy rapidly, but I think
I have seen this device with a plastic can, too. The freezer compartment
of a refrigerator tends to have a lot of metal surfaces, and
ice cube trays are frequently made of aluminum, but it isn't so clear
that this choice of materials was made with thermal conduction
in mind. There are plastic ice trays, but not styrofoam ones.
Some of the discussions of this question have revealed two
more lists we could construct: examples where we use a
good thermal conductor even though we would be better off
with an insulator, and vice versa. For example,
I think they must make pipes out of copper because it is easy to work and
doesn't
turn the water orange. Certainly we intend to keep the energy inside the pipes
instead of heating up the crawl space -- so it is unfortunate that copper is a good
conductor.
Likewise windows conduct altogether too well. We use double and triple windows
and drapes to try to minimize this unfortunate aspect of windows.
Metal spoons might be used to transfer energy rapidly, but I don't think this is
the usual reason for them. They are strong, light, and easy to clean. (I
remember
once that a family was visiting us, and my wife made hot cocoa for the children.
She served it with the wedding silver! And got in trouble, because the spoons were
too hot to hold. Stainless steel does not conduct as well, and would have been a
better choice). One example of making use of metal spoons because they are good
conductors might be dipping the spoon in hot water just before you try to scoop
out ice cream straight from the freezer.
2. If you cover your home thermostat with a blanket, will it make the
house hotter or colder, or have some other effect?
One group said:
Another group said:
Comment by Joe Straley
However, if we assume the temperature under the blanket is too warm,
the AC
will turn on and stay on and the African Violets will be shivering in
their pots
when finally enough heat leaks through the blanket that the thermostat
realizes
that the house is now cold enough. The AC turns off, and the house
will begin to
return to normal ... and then get warmer and warmer until it is nearly
the same as
the temperature outside, and the dog is about to collapse when the AC
turns back
on again. Something similar will happen in heating season, and the
temperature
will again seesaw between much too hot and much too cold.
We first asked this question just as a way to check that
people had gotten over the idea that it is always warm inside
a blanket (some people are very surprised by the first activity
in the equilibrium section). But then I realized
that the effect would be far more undesirable: your house would
hardly ever be the right temperature -- it would almost always
be much too hot or cold. "
Convection and Radiation
1. The wind can be used as a source of energy. But where does the wind get
this energy from? Please give a discussion tracing the flow to energy.
Comment by Joe Straley:
One group said:
2. Describe three different heat transfer processes that occur inside your
oven. Explain how these affect how you use your oven.
One group said:
A comment on this discussion:
Conversion of Other Forms of Energy into Thermal Energy
1. Give five examples of energy of other forms being converted into thermal energy
that can be
detected or sensed as a rise in
temperature. For at least two of these, verify experimentally that the energy
transferred is indeed
large enough to be detected with the
thermal sensing sheets.
One group said:
B. Take a container half full of sand and shake it up for a minute. The container
will feel warm
and it does transfer energy to the
thermal sensing sheets causing a color change. The transformation of energy in
this example
involves nuclear energy from sun
transformed into chemical energy by photosynthesis, chemical energy transformed
into kinetic
energy, transformed into thermal
energy.
C. Nuclear energy from sun, chemical energy from coal, transformed into electrical
energy
transformed into light energy, transformed
into heat energy in an overhead projector. The thermal sensing sheets detect the
warmth in this
situation.
D. Drop a book from a meter distance above the ground. Transformations include
nuclear energy
from sun, chemical energy from
food to lift book up, transformed into potential energy, transformed into kinetic
energy,
transformed into thermal energy.
E. Rub hands together. We are transforming chemical energy
(from the food we eat) into thermal energy. The
thermal sensing sheets definitely detect
the thermal energy.
"
Phase Changes
Under what conditions would you expect to encounter fog? Give both some examples and
a
general description of the necessary circumstances in terms of what you have
learned in this
unit.
One group said:
An excellent discussion.
Early one sunny spring morning Carol walked outside in the grass and got her shoes
wet
because dew had collected on the grass. A few days later however, she walked
outside at the
same time and her shoes stayed dry -- there was no dew on the grass that morning.
Explain
some environmental factors that led to these two different occurrences.
One group said:
An excellent discussion!
Air itself doesn't radiate very well. It is transparent, and there is a rule
that
things that don't absorb light also don't radiate light (there is a tiny asterisk
here,
because air does absorb infrared light to a certain extent -- eventually this is what
the greenhouse effect is all about. But on clear nights a reasonable fraction of
the
infrared light emitted by the ground escapes completely through the atmosphere).
So it is the ground and stuff on the ground that does most of the radiating. A
grass blade is radiating, but is insulated from the ground below it by the layer of
air beneath it. The grass blade gets really cold. The air beneath the grass is
damp, because it is in contact with the ground. Condensing water out of the
air releases energy and helps warm up the grass blade; it causes more water to
evaporate from ground, which cools down the ground.
Dew does not form as readily on a sidewalk as it does on the grass, because
the sidewalk is a relatively good thermal conductor -- you have to cool down
the whole slab, which involves removing a lot of energy. The sidewalk doesn't
get as cold as the grass.
"Some satellites can detect infrared rays which appear different colors depending
on whether it is warmer or cooler. Liquid crystals will
turn different colors in the presence or absence of more heat. Precise
temperatures can be taken using a device called a
thermocouple. The human body also a built in thermometer to gauge approximate
temperatures.
Satellite - usefulness:variations in earth and space temperatures;precision:very
precise; accuracy:accurate.
Liquid crystals - usefulness:body temperature;precision: some;accuracy:within 1-2
degrees body temperature.
Thermocouple - usefulness:lab or factory; precision:very precise;accuracy:very
accurate.
Human body - usefulness:to determine "safe" temperatures; precision:some;
accuracy:cannot determine exact degrees but can determine
degree of safety."
"We care about temperature for many reasons. We care if it gets to freezing or
below because of the
effect it has on our plants and animals outside. We all like to be comfortable, so
that is another reason we
don't want it to be too hot or too cold. We also use more energy during temperature
extremes. Farmers
need certain temperatures to grow crops. People who build houses, roads, or
generally work outside are
effected by temperature and care a lot about it. No one wants to swim when it is
cold outside."
"Temperature effects our daily lives in that it helps us know what type of
activities we can plan, how to dress for the day, and the type of
precipitation we could expect (if any).
For example, if the temperature is 90 degrees F. it may be a better idea to go
swimming than it would be to mow the grass. If the temperature
is 32 degrees or below and the sky is cloudy it may snow and we may need to stay
home."
...and worry about whether the pipes are going to freeze. Right now it is
hot, and we have sweet corn and
real tomatoes
and watermelon ... and mosquitos and chiggers and ragweed pollen. Biological
activity is very sensitive
to temperature --
somewhere I read that increasing the temperature 10 degrees makes everything happen
twice as fast .
The amount of
water in the air is also very sensitive to the temperature, and in the summer my
office door swells and
sticks. At
low temperatures not only does water freeze, but so do oils (think of butter), and
flexible things become
stiff --
so that anything that is designed to work at one temperature will not work as well
at another.
"In daily life, we experience a very narrow range of temperatures which
determine, in part, our daily plans and activities. When the weather
is too hot, people who are sensitive to heat experience more breathing
difficulties, tend to dehydrate rapidly, have lower blood pressure,
experience syncope, which could lead to death. If the temperature is too cold,
a person can experience hypothermia, frostbite, breathing
difficulties, and possible death. For this answer, we focused on temperature
pertaining to the human body but realize there are MANY
other things that can go right or wrong (i.e. machinery, other organisms, the
earth's crust, stationary objects) depending on extreme hot
or cold temperatures."
"Wind doesn't directly change the temperature but it actually effects how it
makes you feel. If you put a
thermometer in front of a fan it doesn't actually change the temperature of the
thermometer. The only
thing the fan is doing is stirring the air around making it seem cooler. The wind
causes whatever the
temperature is to stir the air around making it feel warmer or cooler to you."
"Wind does not effect the thermometer reading. Our bodies maintain a constant
temperature of approximately 98.6
degrees F regardless of the external temperature. A thermometer's temperature
changes depending on what it comes
in contact with until it reaches thermal equilibrium. However, a fan will
make a person feel cooler because the wind
from the fan combined with air temperature causes a person to lose body heat
by evaporating the moisture on our skin
and by blowing away warm air around us. Wind chill would only effect people
and animals, not non-living objects (i.e.
a thermometer.)
If you are going to mention evaporation in the context of people, you will have
to
qualify your last sentence more -- a lake is nonliving, but it will be cooled by
evaporation,
too.
Other than that, a very nice explanation. If you just brought the thermometer into
the room, blowing a fan on it
would get it to the temperature of the room as fast as possible -- so it would be
more
accurate.
The result of the calculation is that there is a very
wide range of prices for energy. To make sense out of these,
a
order them by cost. I find it useful to draw a bar graph
showing the results. Here is what it looks like:
(This is
http://www.pa.uky.edu/~sciworks/public_html/images/buckperJ.gif )
The cheapest sources of energy are the raw materials. We use coal and gas
the way they come out of the ground, and oil is hardly more processed.
Gasoline is a purified component of oil (and there are all those taxes),
so it costs more; electricity is also purified in important respects --
all the smoke and dirt is left back at the power plant (and this is
the cost delivered to your living room).
"Thermal equilibrium means that heat energy will be transferred from one medium
to another. Some
environments have a hotter "normal" temp. or colder "normal" temp. than others.
The attic under normal circumstances will be hotter
than the rest of the house. The basement would be colder and the middle layer,
usually the living quarters is in between. The living
quarters are usually regulated by the person living there using the thermostat.
Most objects that are moved from one location to another
initially will have the temperature of the original location, but
then by transferring energy come into equilibrium with the new
environment."
"Thermal equilibration will mean different readings in the attic or on a porch.
For example, if on a very warm day you took a
glass of ice water to the porch it would not take as long for it to reach room
temperature as it would in a house at 72 degrees. But,
it would take longer than it would if you placed the ice water in the attic. In
each of these environments thermal equilibration would
still take place. The only difference would be the readings and the time factor.
This experiment could take place anywhere that 2
or 3 different "room temperatures" could be made available.
"
"Thermal equilibration is the exchange of heat between objects of different
temperatures in the environment.
Thermal equilibrium is result of the exchange of heat between objects of different
temperatures in the environment.
Objects that are at
room temperature are object that have been in the room for a period of time and
have been allowed to reach thermal equilibration.
Some examples are tables, chairs, and books. Object at are not at room temperature
would be things newly introduced into the
environment. Examples of this would include a cup of hot coffee and a glass of ice
water. These things would eventually reach
"room temperature" but time would have to pass before this occurred. (Electronic
devices, as long as they stayed on, would not
reach room temperature because they generate their own heat)."
These are all nice discussions. When we talk about equilibrium, we are assuming
that the system is isolated from "the rest of the
world," so that the exchange of energy and material is small enough to be
neglected. Then the system exchanges within itself
until ... ...until it stops doing so! In the last section "Irreversibility" we
will discuss why it stops. Finally we should remember
that we don't live in an equilibrium environment: the sun shines, bringing in
energy, which then escapes somehow. Equilibrium is
dead; life is a consequence of a system that is constantly being disturbed
This was a rather general section. It establishes an important starting point
-- we have to understand
that equilibrium happens before we can talk about temperature in a meaningful way,
and in this
context has the same importance as the number zero in arithmetic, or what we mean by
a point in
plane geometry. It's about as exciting to discuss, too.
The windows in my house expand when the sun shines directly on them in the morning
and make a popping sound. A hot
water heater will expand slightly when it is filling with water and makes a
clicking sound. A toaster will made a clicking sound as
well both when it is heating and when it is cooling. Sometimes a dresser drawer
will stick when the indoor temperature is warm
enough and this is due to thermal expansion. Highways, especially in cold
climates, have spacers to allow for thermal
expansion. If they did not have these spacers the roads would buckle much sooner
than they do.
a. Baking sheet will bend when in the oven but will go back to it's original shape
when it is cooled. The
increased temperature of the baking sheet made the molecules move faster (hitting
against each other
more and harder) therefore expanding. Again, it is not the atoms or molecules
getting bigger. When it
cools the sheet goes back to it's original shape and size (even though we can not
detect the size
difference). The atoms or molecules are not moving as fast therefore shrinking.
b. Car engines make noises when it is cooling. I hear ticks as the engine
cools. The ticking
is metal going back (bending) to its original shape and size. We have also heard
ticking from water
heaters cooling, toasters cooling,
c. Guitar strings tighten when in the heat. Because room temperatures are
different musicians will
have to retune their metal strings before playing. If it is cold then they would
have to tighten them. If it is
hot then they would have to loosen them.
Good examples. Thermal expansion is not a big effect, but it is rather
inexorable. Raising the temperature of an object
makes only a tiny increase in its dimensions, but it could take an enormous force to
prevent the change. In fact, if you put your
thermometer in water that is much above 50o C, it may break!
When the temperature of the guitar changes,
the strings change length and the the guitar expands, too. Because
the materials are different, the result is that the strings are
tighter or not as tight. My violin goes flat when it gets
warmer (so I have to tighten the strings) which is the
opposite of the behavior described for the guitar -- but maybe
this was a fiberglass guitar, instead of a wooden violin.
My favorite example is what happens when you drop an
icecube out of the freezer into a liquid. The outside of the ice
cube expands, while the inside is still cold. The ice cube cracks, and makes
crackling noises. House pipes change length
when you run hot water through them, and if the pipe is going through a tight fit
somewhere there will be little noises
pop.......pop........pop.........pop........ as it returns to its original length.
It sounds like the bogeyman is coming up the stairs but
actually its just a reminder that you took a shower.
Warm weather is also humid weather; this can be a
separate reason for the dresser drawer sticking. If it's thermal
expansion, the drawer should start working on the first cool day; if it is humidity,
you will have to wait until the furnace starts
working. (We will sudy humidity later, too)
2. The flower tube will hold approximately 7 straws of liquid. The ratio would
be 7:1. This experiment demonstrates on a
small scale the importance of taking into account thermal expansion in larger
objects. Examples of this would be on road
and bridge construction. The experiments in this sections could easily be done in
the classroom to demonstrate thermal
expansion. As an extension to this learning after the experiment students could
compile their own list of thermal expansion
around their home.
OK. When you move the air thermometer from ice water to pretty hot water, the
liquid level in the straw changes by a lot
-- most of a strawful. This means the air in the flower tube expanded by 1/7 = 14%.
This would not be a very big change, if
we had a balloonful of air -- it would get just a little bit bigger. So the
smalll volume of the straw relative to the tube is a way
to make the small change look larger.
Combining the 1/7 (which is 14%) change in volume with the
40o C temperature change that causes it gives a
measurement of the thermal expansion of air: 0.35 % per celsius
degree. It's not a big effect, yet much bigger than the thermal
expansion of solids and liquids.
Here's another interesting thing about the measurement.
Going from 40 degrees Celsius (pretty hot water) to 0 degrees
Celsius, the volume of the air changes by 1/7. What would happen if we went to -40
degrees Celsius, and then to -80 degrees
Celsius, and then to -120 degrees Celsius, and so on? One possibility is that the
volume decreases by the same amount in
each temperature interval. Then we can only lower the temperature by 7 x 40 = 280
degrees: suppose we had 7 cubic
centimeters to begin with, at zero degrees C. Then at -40 we would have 6 cc, and
-80 we would have 5 cc, ... at -280 we
would have 0 cc. Something has to happen, because we can't have a negative volume!
Of course, there are many
possibilities: the volume change might be different at other temperatures, or the
air could freeze solid and stop changing. But
the interesting possiblity is that there is a limit to the temperature scale, that
it just comes to an end about -280 C, and it
doesn't have any meaning to talk about a temperature below this. Which turns out
to be what happens -- the Right Answer,
in this case. So your observation is a determination of the absolute zero of
temperature.
During thermal expansion every linear dimension increases by the same percentage
with an increase in temperature (assuming
that the expanding material is uniform). The atoms (molecules) that make up an
iron rod would expand-move farther apart from
each other-when heated. This also applies to air molecules. When air is heated
the molecules move faster, expand their area,
and rise. On the other hand, water contracts with increasing temperature (0-4
degrees C). The theory predicts that sodium
chloride, being a solid, would increase in temperature as its atoms and molecules
vibrate with greater speed and amplitude.
Yes, we need the theory of thermal expansion for safety reasons and for correct
manufacture of devices which would be afftected
by themal expansion. Since this is an occurence that can not readily be observed,
it is most important to be understood.
This is a good summary of what thermal expansion means, in terms of the atoms.
But I was asking a different question, too: how
can we tell the difference between the theory that says there is more
space between the atoms, and the theory that says the atoms get bigger? I'd like to
avoid
explanations that describe everything in terms of the properties of atoms, because I
have never
seen one, and to say "This is a magnet because the atoms are magnetic" isn't really
an
explanation at all. So is there anything wrong with the claim that thermal
expansion is due to
the atoms and molecules getting bigger?
The theory that atoms change size with temperature
would be a way to explain why objects change size with temperature.
But we have to consider the other implications of the
theory.
The actual theory of matter assumes that most of the properties of
the atoms are quite unaffected by temperature. Thermal expansion
occurs because the atoms are farther apart. Then liquids and solids
are hard to compress, because their atoms are almost touching, while
gases are easier to compress, because all we are doing is removing
some of the space between the atoms.
"
1.Both sensing sheets absorbed the heat. The brown sensing sheet is more
sensitive to the warmth from Donna's hands.
This is what we predicted since it's temperature range is higher degrees.
When we used a cup of ice water, the blue sheet
went to green, light brown, dark brown and black. The brown sheet went to
black and stayed there.
"
Joe Straley commented:
Maybe this is just a matter of language,
but eventually it becomes a conceptual difference:
The sensing sheet is a kind of thermometer.
It tells you what its temperature is, by changing
color. When you touch the sensing sheet, you change
its temperature, and I think what you intended to
say in your first sentence was "Both sensing sheets
responded to warm objects."
The language point is that when I say "heat" I
am talking about energy. Causing the thermal
sensing sheet to change color actually does involve
a small amount of energy transferred, but it isn't
much; to say "it absorbs the heat" means something
very different to me -- this would be an object that
feels cool to your fingers, because it is that loss of
energy that you sense.
Your observation about sensitivity is quite right.
The temperature range from a rich chocolate to
pea green is not very big, but then it stays blue for
a larger range. I can tell from your comments that
it was a warm day, and that the AC was not cranked
up to full. When you try this again in February, you
will find that the "brown" sheet is flat black and won't
talk to you at all, while the "blue" sheet may be brown,
or maybe it, too, thinks the room is entirely too cool.
That's why we gave you two different pieces!
A question that was discussed with some groups was
what to do if the sheet was black and you were trying to find a cold
spot -- for example, to determine where a can of cold soda had been
placed on a table. My idea was pretty simple -- turn up the thermostat --
but one group suggested warming the table top a little bit with a heating
pad, and then testing it, and another group came up with an even better
idea: place the thermal sensing sheet on the table top where you think
the cold spot is, and then blow on the sheet! This works quite well!
We quickly warm up the surface layer of the table with our warm breath,
and the cold spot lags behind, and is displayed as a dark area on the
now-colored thermal sensing sheet.
If you touch the thermal sensing sheet, how long does it take to change color?
How long does it take to
return to its original color (note that the answer is rather different if it
started out black, than if it started
out with some color)
-- if it is lying on the table top?
-- if you wave it in the air?
"
We were unable to detect a difference at ½ degree C but could see a change of
color at one degree intervals. For 4-5
degrees, it stays in one family of colors (green, blue, violet) before
changing to brown or black. The sheet changes color
immediately when you touch it but takes longer to change back. If the sheet
is already black because the temperature is
too cold and out of range, then it will not change color when it comes in
contact with something even colder. That would
also be true if the temperature is too hot and out of range and you touch the
sheet with something hotter.
"
Joe Straley commented
It depends on whether you are trying to tell the temperature or
detect a temperature difference. Identifying a particular color
well enough to say "This means 26 C" is hard, but you can see rather
small differences in color, where one brown is slightly greener than
another. In this latter regard, the temperature differences we can
detect are probably rather smaller. That's one of the neat things
about this stuff.
"
We had a good laugh over the fact that Donna's fingertips (and anyone else's who
came along while we were working)
registered a change of temperature on the sheets but Susan's would not!!! Anyone
else have that problem?????
"
Joe Straley replied
Oh, yes, it is quite common. Your fingers can be quite a bit
cooler than the rest of you. Though I would expect that with
the blue sheet there would be some effect, and if the other
sheet is showing brown, Susan only has to be 1 degree warmer than
the room air
to give
a detectable difference. The problem will be more severe
when it is cooler in the room -- now it can happen that the sheet
is black and will stay black until you warm it into its active range.
People with fingers cooler than this will have no effect at all.
Some days it happens to me, too. "
One group said:
"
1.Wet clothes are very dangerous to people in very cold places because
heat moves from a warm object to a colder one. This can easily cause
hypothermia because ones body heat is being transferred to the colder
surroundings. The wet clothes add to this problem because although
the moisture from the clothes is evaporating slowly, this process is
conducting a lot of heat energy away from one's body. This is
especially dangerous if it is the socks that are wet, because the feet
are generally cooler than the other body parts. Also, as thermal
equilibration takes place, one's ody temperature is lowered to a
dangerous level.
"
"
Insulation generally is mostly air (using vacuum would be
far better, but then requires a strong stiff container); water is a
much better conductor.
Your mention of evaporation is a little confusing to me.
Evaporation would make the clothes even colder on the
outside, but they already are too cold, and evaporation is
not a very fast process at low temperatures. However,
evaporation is relevant because if the polar explorer gets
overheated, he or she will try to cool by sweating -- expecting
evaporation to do the job -- and now we have water and water
vapor to get rid of.
"
"
Poor thermal conductors--insulated walls, oven mitt to pick up a pan, hot
pads on the table, styrofoam cups, pan handles make of wood or plastic,
plastic cooking utensils, houseslippers.
Good thermal conductors--aluminum foil to cover a baking potato, reflecting
pans on the stove, metal or glass pans to cook in, teakettle to boil water,
curling iron, metal cooking utensils, toaster, iron.
"
Among the uses of poor conductors we could add blankets
and coats and clothes in general! This list goes on and on -- keeping hot
things hot, warm things warm, and cold things cold dictates the
form and construction of a lot of everyday objects.
The list of uses of good conductors is shorter -- perhaps because
we use metal a lot anyway for other reasons, so that examples
where it is clear that nothing else would do are harder to
think of. The contexts in which we want a
good thermal conductor are when we want to move energy
from a source into something. Sticking the bacon directly into the gas
flame isn't going to work well; a frying pan delivers the energy
more uniformly. A baking nail helps move energy into the center of a
potato.
*copper hot water pipes
*windows
*metal spoons used for cooking
"
Since the blanket is a poor conductor the cool or warm air in the room will
not reach the thermostat coil as quickly. Therefore, the room will get
cooler or hotter more quickly because the thermostat will not kick
on.
"
"
3. If you cover your home thermostat with a blanket the house will
get cooler. The blanket insulates the thermostat and will keep it at
a constant temperature. In the summer the air surrounding the
thermostat would likely be warmer than the air outside the blanket.
Therefore, the air conditioning system would run trying to cool the
air under the blanket to the temperature setting on the thermostat.
In the winter the opposite would be true. The thermostat would
"think" the air temperature outside the blanket was the same as the
temperature under the blanket. Therefore, the heating system would
not come on to warm and the house would get colder.
"
"
Your analysis is correct, except perhaps for the part about "the air
surrounding the
thermostat would likely be warmer" -- it's warmer under the blanket
when we are
there, because we are constantly converting chemical energy in to
thermal energy, but
there's no reason for the temperature under the blanket to be warmer or
colder than the air in the living room (unless the thermostat has a
lightbulb in it that
is making heat). The temperature under the blanket will tend to stay
wherever it is,
and will change only very slowly as heat leaks in or out through the
blanket.
One group said:"
The wind gets its energy from the sun since the sun is the major source of
energy for everything on earth. Since the sun is nearer the equator, air is
hotter there. As the hot air rises toward the poles, it cools down and sinks
back down causing a huge convection current of air flow.
"
"
This contains the main ideas. The sun is delivering power, and does so
unevenly. Thermal expansion in the hot places, and the tendency of hot
air to float over cold, drives the wind. You have described the large scale
flow of air, but the wind in Kentucky is more associated with fronts --
temperature changes over a few hundred miles, rather than tens of thousands.
"
"
Answer: The wind comes from the unequal heating of the earth. Due to the earth's
rotation and
tilt of the axis, some areas of the
earth are getting more direct transfer of radiation from the sun while others are
receiving
radiation at more of an angle, and therefore
receive less radiation from the sun. This sets up areas of warmer air and areas of
cooler air. The
warmer air located toward the
equator will try to transfer its energy by convection currents to the cooler air at
the poles. So you
basically have cooler air sinking and
warmer air rising toward each other all over the earth. Winds can also be set up
by various
landscape features like lakes and such.
Water tends to take longer to heat, but also it holds heat longer, so it will be
warmer at the end of
the day because less convection
will have occurred between it and the air. Land tends to heat up quickly, but it
also convicts its
heat more quickly too. So you end up
with air cooler over the land and warmer over the water and then a convection
current between
the two with the warm air flowing
toward the land. This also can occur with varying elevations where heat energy is
transferred
more quickly and lost at higher
elevations and transferred less quickly at lower elevations again causing unequal
heating and
convection currents.
"
"
OK let's say we are making some yummy peanut butter cookies. One heat transfer
process that
occurs in my oven would be
conduction. The metal rack gets hot and then conducts that heat energy to the
cookie sheet
because it is cooler. The cookie sheet
then conducts heat energy to the cookies. A second heat transfer process that
would be
convection currents set up by the air
furthest away from the source of heat cooling down and sinking and the air closest
to the source of
heat heating up by convection and
rising. So the cookies near the edge would be getting a little more heat transfer
due to air
convection currents. A final heat transfer
process would be radiation from the heat source like infrared waves of energy. If
any of the
cookies had some turned up edges then
they would be more prone to the radiation transfer.
"
Radiation is probably more important than we realize. When my wife makes an
apple pie,
she puts little aluminum foil bits on the edge of the crust half-way through the
baking, to prevent
this part from getting scorched. Aluminum is a very good conductor, so this
seems the wrong
thing to do. But it does reflect light, and so perhaps it is keeping the radiated
infrared light
away. (A different explanation would be that the hot convection currents are
blocked by
the aluminum foil. The foil gets hot, but now there is a thin layer of air between
the foil
and the pie crust that acts like a kind of insulation. I don't think this is the
right explanation,
but we really need an experiment to find out. A pie-making experiment!)
Because convection does play an important role, we have to be careful not to block
air flow
in the oven. When I bake bread (6 loaves at a time) I space them out in the oven
for this
reason.
"
A. When you start a fire you are taking the stored up chemical energy in the wood
and providing
enough energy to activate it. The
chemical energy is in turn transformed into thermal energy. This can be felt by
the warmth of the
fire.
Measuring this might be hard, but you could try the same method as
described in the very first page of this section, where you whack
a board
with a hammer and then look at the place you hit with the thermal sensing sheet.
"Fog forms when moist air near the earth's surface is chilled so that its water
vapor
condenses. The air temperature
must be at or below the dew point for the water vapor in the air to condense. Fog
appears when
wind blows warm moist air over
a cold surface. For any given temperature, there is a definite limit to the amount
of water vapor
the air can hold. The warmer
the air, the more moisture it can hold before it becomes saturated. Fog forms when
moist air is
cooled below its saturated
temperature which is the dew point. Fog usually occurs over colder surfaces such
as water (rivers,
lakes, oceans). At night
radiation can cool the air over rivers and lakes to make foggy mornings. We had
fog form here
last week as warm air moved
over the snow during the afternoon hours just as it started to rain.
"
* I see fog in the morning in river vallies. The air has cooled (by radiation)
and the water is still
warm (from yesterday, perhaps).
*
I see fog over my coffee cup. The water is hot and is evaporating; as it meets
the room air, it is
cooled below the dew point and makes fog; but then as it mixes with the room air, the
fog droplets
evaporate again, because the humidity in the air was not 100%.
*
An jet plane contrail is a kind of fog. The jet burns hydrocarbon fuel, which
produces carbon
dioxide and water vapor, that comes out in the exhaust. Up where the plane is
flying it is extremely
cold, and so the water vapor condenses immediately into tiny water drops.
*
When I open the refrigerator door I encounter fog, which is caused by the warm
moist kitchen air
meeting the cold air from the refrigerator.
"
Dew forms when moist air is cooled by direct contact with cold objects out in the
open
(condensation). Blades of grass (and
other things) receive heat from the sun during the day by radiation. These objects
lose the heat
again at night. The air then
deposits excess moisture as dew onto the grass. The first morning could have
followed a clear
night, while the second day
followed a cloudy one. Dew forms best on clear nights because objects (grass) lose
heat faster.
The air may have also
contained more moisture on the first night than on the second.
"
Perhaps they should have mentioned that
the
objects lose the heat again at night due to radiation -- this would have clarified
the mention of the role of cloudiness.