Tuesday Feb. 28, 2012
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Three Roy Orbison songs before class today.  The first was "You Got It", followed by "Mean Woman Blues" and "Pretty Woman".  I should probably have showed the last two videos in class.

The 1S1P Assignment #1 reports on ozone have been graded and were returned in class today.

The Surface Weather Map Analysis assignment was collected today.  Several people also turned in the Upper Level Charts Optional Assignment today also even though it isn't due until Thursday.

The Experiment #2 reports were collected today.  You should get the graded reports back sometime next week.  If everything goes according to plan the Experiment #3 materials will be distributed on Thursday.

There was also an In-class Optional Assignment handed out today.  If you weren't in class but would like to do the assignment, you can turn it in at the start of class on Thursday.

And here's a link to the page from National Geographic Magazine that lists some of the limits of human survival.  It's in the form of a quiz rather than all of the images on a single page.

We spent about the first half of class today 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.

We'll use this concept of electric field to begin to understand electromagnetic radiation and how it can transport energy from one place to another.
 
An electric field arrow
shows the direction and
gives an idea of the strength
of the electrical force
that would be exerted on a positive charge

You'll find most of the following on p. 60 in the photocopied ClassNotes.  What follows is a little more detailed explanation than was shown in class.


We imagine turning on a source of EM radiation and then a very short time later we take a snapshot.  In that time the EM radiation has traveled to the right (at the speed of light).  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.


Textbooks often represent EM radiation with a wavy line like shown above. They don't usually explain what the wavy line represents.

The wavy line just connects the tips of a bunch of electric field arrows.
 
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 (we use the E field arrow at the location of the + charge to determine the direction and strength of the force exerted on the + charge).



Th picture above was taken a short time after the first snapshot aftere the radiation had traveled a little further to the right.  The EM radiation now exerts a somewhat weaker downward force on the + charge.

A 3rd snapshot taken a short time later.  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 (what you would see in a snapshot) in terms of its wavelength, the distance between identical points on the pattern. 



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 look at one particular fixed point and look at how things change with time.
EM radiation can be created when you cause a charge to move up and down. If you move a charge up and down slowly (upper left in the figure above) you would produce long wavelength radiation that would propagate out to the right at the speed of light.  If you move the charge up and down more rapidly you produce short wavelength radiation that propagates at the same speed.

Once the EM radiation encounters the charges at the right side of the figure above the EM radiation causes those charges to oscillate up and down.  In the case of the long wavelength radiation the charge at right oscillates slowly.  This is low frequency and low energy motion.  The short wavelength causes the charge at right to oscillate more rapidly - high frequency and high energy.

These three characteristics: long wavelength / low frequency / low energy go together. So do short wavelength / high frequency / high energy.  Note that the two different types of radiation both propagate at the same speed.

The following figure illustrates how energy can be transported from one place to another (even through empty space) in the form of electromagnetic (EM) radiation.





You add energy when you cause an electrical charge to move up and down and create the EM radiation (top left).

In the middle figure, the EM radiation that is produced then travels out to the right (it could be through empty space or through something like the atmosphere). 

Once the EM radiation encounters an electrical charge at another location (bottom right), the energy reappears as the radiation causes the charge to move.  Energy has been transported from left to right.




This is really just a partial list of some of the different types of EM radiation.  In the top list, shortwave length and high energy forms of EM radiation are on the left (gamma rays and X-rays for example).  Microwaves and radiowaves are longer wavelength, lower energy forms of EM radiation.

We will mostly be concerned with just ultraviolet light (UV), visible light (VIS), and infrared light (IR).  Note the micrometer (millionths of a meter) units used for wavelength for these kinds of light.  The visible portion of the spectrum falls between 0.4 and 0.7 micrometers.   UV and IR light are both invisible.  All of the vivid colors shown above are just EM radiation with slightly different wavelengths.  When you see all of these colors mixed together, you see white light.

And note that indigio was added to the colors in the spectrum, I believe it falls between blue and violet (this after a question in the MWF class).  While researching this I came across the following (compact) list of colors.