Wednesday Feb. 23, 2011
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Three songs before class today ["Rich Woman", "Sister Rosetta Goes Before Us", and "Gone, Gone, Gone (Done Moved On)"] from the Robert Plant & Alison Krauss "Raising Sand" CD.

The Optional Surface Weather Map Analysis was due today.

The Optional Assignment on Upper Level Charts is due on Friday.

 The Experiment #2 reports are due next Monday.  Please return your materials this week and allow me to have the pleasure of handing you a copy of the Supplementary Information handout.  You can drop off the materials at my office (PAS 588) if that's more convenient between about 8:30 am and 5 pm Thursday or Friday (there'll be a box just inside my office door and a stack of Supplementary Info. sheets nearby).

 Today's pictures of the day come from a fairly recent issue of National Geographic magazine.  I can't really scan what I displayed in class because that would probably violate copyright laws.  I'll try to sketch the figures and include the information on human limits under extreme conditions.

An in-class Optional Assignment was handed out in class.  If you would like to download the assignment, answer the questions, and turn it in at the beginning of class on Friday you can earn at least partial (and maybe full) credit.

Latent heat energy transport was the first topic of the day.  This is the second most important energy transport process (second only to electromagnetic radiation). 

If you had an object that you wanted to cool off quickly you could blow on it.  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. 

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 sublimates when placed in a warm room, it turns directly from solid carbon dioxide to gaseous carbon dioxide). 

In each case 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.  When you step out of the shower in the morning, the water takes energy from your body and evaporates.  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 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 water. 

When you freeze water and make an ice cube energy is released into the surroundings.  You can picture the released energy as being equivalent to a 240 lb person running at full speed.




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 never happens (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.  This would warm the air.

Energy arriving in sunlight in the tropics has effectively been transported to the atmosphere in Tucson.

I brought some dry ice to class and dumped it out so that you could see it.  Here's a sketch


If you were close enough and looked carefully you could see a thin faint cloud hugging the dry ice.  And the cloud was sinking.

Several questions were posted for you to think about.
1.  Is energy being transported from the air in the room to the dry ice  or  from the dry ice  to the room air?


2. How is this energy transport being accomplished?  (there are at least two processes that you can see)  The sinking air motions occur when warm air in contact with the dry ice loses energy (that would be conduction).  The now cold high density air sinks and is replaced with warmer air.  This organized motion of air is convection.  This is shown with the big red arrows in the figure below.  The formation of a cloud indicates that water vapor is condensing and forming small droplets of liquid water (maybe ice crystals).  This means latent heat energy is being released and is going into the dry ice (the smaller orange arrows in the figure below).

3.  Depending on how you answer the 1st question, the dry ice is either losing or gaining energy.  What is happening to the dry ice as a result of this energy transport?  The dry ice is sublimating, turning from solid CO2 to gaseous CO2.



We're almost ready to tackle electromagnetic radiation, the most important of the four energy transport processes.

First we need to review a couple of rules concerning static electricity and learn something about electric field arrows.

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.

Electric field arrows 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.



The blue arrows show the electric field at the locations of two positive charges.
The figure below shows the directions of the forces.  The sizes of the arrows show the force. 




You'll find the following on p. 60 in the photocopied ClassNotes.

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.  The following figure wasn't shown in class on Wednesday.  This is about where we'll start on Friday.


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.

We spent the last few minutes of class wathcing the last of the Piccard video segments.  This one featured Bertrand Piccard (Jacques son, Auguste's grandson).  He was one member of the two-man team that first circled the globe non-stop in a balloon. 

There were actually balloons in the air at the same time.  The first was
the Cable and Wireless Balloon piloted by Andy Elsen & Colin Prescot.  They launched Feb. 17, 1999 from Almeria, Spain and built up a 10 day lead.

The second balloon was the Breitling Orbiter 3 with Brian Jones & Bertrand Piccard.  They launched launched Mar. 1, 1999 from Chateau D'Oex, Switzerland.

The Cable and Wireless balloon crashed into the ocean just off the coast of Japan.  Piccard and Jones became, on Mar. 20, the first people to complete a non stop trip around the globe in a balloon.