Today was the first of the 1S1P Assignment #1 due dates. At least
one of your reports was due today. You can turn in a 2nd report
(and the surface weather map analysis) next Wednesday, Mar. 4.
The Experiment #2 reports are due next Monday (Mar. 2). The
revised Expt. #1 reports are due next Wednesday (Mar. 4).
There was
an In Class Optional Assignment during class today. If you
weren't in class but are reading the online notes, you can turn in
answers to the questions at the start of class next Monday for partial
credit. In class Optional
Assignment Question #1
There are at least two phase changes occurring in the picture
above. What are they?
Is energy being transported from the air INTO the dry ice or AWAY FROM
the dry ice & into the air?
(hint: the visible cloud is not carbon dioxide gas)
We'll finish up the material on latent heat energy transport today
and just start the section on electromagnetic radiation, the most
important of the energy transport processes.
You can consciously remove energy from water vapor to make
it
condense
or from water to cause it to free (you could put water in a
freezer; energy would flow from the relatively warm water to the
colder surroundings). Or if one of these phase
changes occurs 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.
A can of cold drink will warm more quickly in warm moist surroundings
than in warm dry surroundings. 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. The condensation may actually be the dominant process.
You feel cold when you step out of a shower and water on
your body
evaporates. The opposite situation, stepping outdoors on a humid
day
and actually having water vapor condense onto your body (it can happen
to your sunglasses but not to you, your body is too warm). If it
did
happen it would warm you up.
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.
Energy arriving in sunlight in the tropics has effectively been
transported to the atmosphere in Tucson.
Note: two additional processes that cause rising air motions are listed
in the figure above. The 4 processes are: winds converging into
centers of low pressure, fronts, convection, and topographic lifting.
The following figure wasn't shown
in class.
The formation of a cloud means that
latent heat is being released into the air. Two energy transport
processes are at work in this picture: convection and latent heat
(conduction is also present, that is how is energy is transported from
the hot ground into the thin layer of air in contact with the
ground. In this case energy is being transported vertically in
the form of latent heat.
Question #2
The person is trying to cool the hot steaming bowl of soup by blowing
on it.
What two energy transport processes are at work here?
(hint: you've seen that head before that should suggest at least one
answer)
Question #3
A person is standing outside on a cold windy day in A,
has fallen into cold water in B, and is perspiring heavily in C.
Match each energy transport process below with the most
appropriate situation in the drawing.
Conduction_______ Convection _______ Latent
heat _______
We'll
spend the next three or four class periods on electromagnetic
(EM) radiation. It is the most important energy transport process
because it can travel through empty space.
To really understand EM radiation you need to understand
electric
fields. To understand electric fields we need to quickly review a
basic rule concerning static electricity. Here's a little more
detailed version of what we did in class.
Static electricity was demonstrated (one
of the poorer demonstrations of the semester) using a sweater
(a gift from my Aunt Ethel and Uncle Nelson made of acrylic
fiber and wool) and two balloons.
Each balloon was rubbed with the sweater. The
balloons (and the sweater) became electrically charged (the balloons
had one polarity of charge, the sweater had the other).
We didn't know what charge
the balloons carried just that they both had the same charge.
If you bring the balloons close to each other they are
pushed apart by
a repulsive electrical force.
The sweater and the balloon carry opposite
charges. IF
they are
brought together they experience an attractive electrical force.
Question #4
Would the formation of a cloud WARM or COOL the
surrounding air?
Does the formation of frost WARM or COOL the
air?
Question #5
Who or what do you think showed up at my back door yesterday evening
while I was trying to watch American Idol, record Survivor, and talk on
the phone?
(hint: click here)
Our next step in understanding EM is to learn something about electric
field arrows. Imagine placing a + charge at the three
positions shown in the figure
below.
Then choose one of the three arrows
at the bottom of the picture to show both the direction and the force
that would be exerted on each charge.
Here's the answer. The closer
the charge is to the center, the greater the strength of the outward
force. With just a little thought you can see that if you were to
place + charges at other
positions you would quickly end up with a
figure that looks like the pattern at the bottom of p. 59 in the
photocopied ClassNotes.
The electric field arrows in this
picture show the direction and give an idea of the strength that would
be exerted on a positive placed at any position in the figure.
The figures on p. 60 in the photocopied class
notes have been redrawn
below for clarity.
We imagine turning on a source of EM radiation and then
a
short time
later we take a snapshot. The EM radiation is a wavy pattern of
electric and magnetic field arrows. We'll ignore the magnetic
field lines. The E field lines sometimes point up, sometimes
down. The pattern of electric field arrows repeats itself.
Note the + charge near the right
side of the picture. At the time
this
picture was taken the EM radiation exerts a fairly strong upward force
on the + charge.
Textbooks often represent EM radiation with a wavy line like shown
above. But what does that represent?
The wavy line just connects the
tips of a bunch of electric
field
arrows.
This picture was taken a short time after the first snapshot when
the radiation
had
traveled a little further to the right. The EM radiation now
exerts a somewhat weaker downward force on the + charge.
The + charge is now being
pushed upward again. A
movie
of
the +
charge, rather than just a series of snapshots, would show the
charge
bobbing up and down much like a swimmer in the
ocean would do as waves passed by.
The wavy pattern used to
depict EM radiation can be described spatially in terms of its
wavelength,
the distance between identical points on the pattern. By
spatially we mean you look at different parts of the radiation at one
particular instant frozen in time.
Or you can
describe the radiation temporally
using the frequency of oscillation
(number of up and down cycles completed by an oscillating charge per
second). By temporally we mean you at one particular point for a
certain period of time.
Question #6
Would the electric field at a point midway between the + and a - charge
point UP DOWN RIGHT LEFT or would the electric
field be ZERO?
