Unless noted otherwise, the Experiment #2
reports are due next Monday. Try to return your materials before
the end of the week (in class or come by my office in PAS 588) so that
you can pick up the Supplementary Information handout.
You'll find a link on the
class homepage that keeps you up to date on progress being made on
grading the 1S1P and Experiment reports.
We started class again with the propane torch. On Monday I
mentioned and showed
how you could safely put your finger alongside the flame and not feel
any heat. This is because conduction is the only process able to
transport energy sideways away from the flame and air is a very poor
conductor of heat.
You would want to be very careful about putting your hand above
the torch however. Hot air and combustion gases from the torch
will rise. You can begin to feel this heat a foot or so above the
flame.
If you put the torch in front of one of the projector screens in
the classroom you could see shimmering that is produced by this rising
air. This is visual proof of energy being transported upward by
convection. You can sometimes see motions like this outdoors on a
hot day above dark colored cars or above the black asphalt road surface.
We didn't quite have time to cover the following information about
wind
chill in class on Monday. Wind chill is a 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. It 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 anyway (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 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 (9 arrows 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 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.
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.
I showed a page from National Geographic Magazine that listed 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 I did find all of the same
information online in the form of a quiz.
We spent most of the remainder of the period looking at latent
heat energy transport. This is the 3rd energy transport process
we have talked
about as we make our way from least important (in air that is
conduction) to most important (electromagnetic radiation).
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.
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) or the phase change can occur
without you playing any role. In that case the needed energy will
be
taken from the surroundings.
Here's the simplest example I can think of.
You put an ice cube
in a glass of warm water.
Energy will naturally flow from warm (the 70 F water) to cold (the
32 F ice). This would energy
transport would occur via conduction..
Once you've added enough energy to the ice it will
melt. Energy taken from the water will cause the water to
cool. The energy that needed to be added to the ice was taken
from the surroundings (the water) and caused the surroundings to cool.
Here's another 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 does so whether you want it to or not.
Evaporation requires energy and it gets that energy from your
body. Because your body is losing energy your body feels
cold.
One last example, involving dry ice.
The very cold dry ice is surrounded by warmer air.
Energy will flow from a thin layer of air (surrounding
the dry ice) into the dry ice. It's a thin layer because this is
energy transport by conduction and that doesn't get very far in
air. This energy flow will cool the layer of air.
The cold air sinks and gets out of the way. This is
convection. Warm air moves in to take the place of the cold air
and the whole process repeats itself. The piece of dry ice has
gotten smaller, some of the dry ice sublimated. We'll come back
to this example and add an additional energy transport process once
we've learned more about latent heat.
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
usually use Tedy
Bruschi as an example) 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.
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 water freezes to make a single
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"? That never happens to you (i.e. your body
doesn't steam up) because 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.
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 and
also to keep water vapor in the air from condensing onto the can.
Back to the dry ice. Energy flows from the warm air to the
cold dry ice by conduction. Cold air in contact with the dry ice
sinks and is replaced by warmer air. That's convection. If
you watch closely you'll see a faint cloud form when air comes into
contact with the dry ice. That's condensation and latent heat
energy transport.
So all three energy transport processes (conduction, convection,
and latent heat) are transporting energy to the dry ice.
This figure shows how energy can be
transported from one
location to another in the form of latent heat. The story starts
at left in the
tropics where there is often an abundance or 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.
We're
ready
to
tackle electromagnetic radiation, the most
important of the four energy transport processes (it's the most
important because it can carry energy through empty space).
First we need to review a
couple of rules concerning static electricity
and learn something about electric
field
arrows.
That's
all
we'll have time for today.
The static electricity rules are found at the top of p. 59 in the
photocopied ClassNotes
Two electrical charges with the
same polarity push each other
apart. Opposite charges are attracted to each other.
There's a demonstration of these static electricity rules that I
would like to be able to show you. I haven't been able to get it
to work very well however. The demonstration involves a Van de
Graaff generator, something that produces a lot of electric charge and
high voltage. A wire connects the dome of the generator to a
small wand used to blow bubbles. Because of the connection to the
generator the bubbles are positively charged. As they drift
toward the dome of the generator the positive charge repels them and
they move away.
While I haven't been able to get the demonstration working very well I
did find a video
that you can watch and see how things should work.
Now the concept that we be using, electric field arrows. Electric
field arrows (or just the E field) show you the direction and
give you an idea of
the strength of the electrical force that would be exerted on a
positive charge
located at that point.
In this figure (p. 59 in the
ClassNotes) a positive charge has been placed at 3 locations around a
center charge. The electric field arrow shows the direction of
the force that would be exerted on each of the charges. The force
arrow is shown in blue. The forces range from weak to strong
depending on the distance between the two charges.
The E field arrows tell you what will happen to a + charge. but you can use the
arrows to determine what will happen to a - charge also.
For a negative charge the force will point in a direction opposite the
E field arrow.
Here's a figure to test your understanding. This figure wasn't
shown in class.
The direction and strength of the E
field near the ground during fair weather and under a thunderstorm are
shown. Show the directions of the forces that would be exerted on
the charges shown in the figure. Click here when you think you
have the answer.
