Friday, Oct. 1, 2010
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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 Bonus 1S1P Assignment was collected in class today.  I'll try to get those graded quickly (next Wednesday?).  The Assignment #1 reports on the Ozone Hole and Radon are still being graded but should also be done soon.

A student in the class asked me whether I had any more sample questions that she could use to prepare for the quizzes.  To be fair I feel like I should give everyone in class access to the same materials so here is what I came up with.  The questions cover material in the this week's class notes up to, but not including, electromagnetic radiation.  There'll probably be another set later next week.

3 Pictures of the Day in class this afternoon.  The first was from the March 2005 issue of National Geographic. A Buddhist monk was standing in a frigid waterfall.  The caption for the photograph read "To focus the mind and increase awareness of self, Shingon Buddhists like Souei Sakamoto practice takigyo,chanting for hours while standing in frigid waterfalls at the Oiwasan Nissekiji Temple in Toyama, Japan."  (I can't really scan the photograph and include it in the classnotes because of copyright laws)

A second photograph from the December 2005 issue showed a monk hanging from a tree by his feet.  The caption there read "To see life as it truly is - that's the goal of a student in China who strengthens mind and body under the rigorous tutelage of a Shaolin kung fu master."

You'll have to come to my office to see the 3rd Picture of the Day.  It is a drawing from a former NATS 101 student.

Perhaps the most amazing example of a physical and mental task (not mentioned in class) is the 1000-day challenge undertaken by the "marathon monks" of Mount Hiei, Japan.

Why am I interested in Buddhist Monks?  I spend a lot of time riding my bicycle uphills.  It's not really painful but can definitely be uncomfortable.  I've noticed that you can sometimes be distracted by a thought and ride a mile or so and completely blank out the discomfort.  With some "Buddhist monk like" training I wonder if maybe I couldn't ride uphill more or less indefinitely and not feel any discomfort at all.  In the winter it is sometimes cold in the morning when I'm out riding my bike.  With some mental training I hope be to be able to block out the feeling of cold fingers and toes.  I'm not there yet but will continue to work on it.

Latent heat energy transport was the next topic of the day.
Energy transport in the form of latent heat 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. 

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

We'll spend the next two or three class periods on electromagnetic (EM) radiation.  It is the most important energy transport process because it can travel through empty space.  The notes that follow are quite a bit more detailed than those written down in class.

To really understand EM radiation you need to know something about electric fields.  To understand electric fields we need to quickly review a basic rule concerning static electricity. 


We did a short not very impressive static electricity demonstration using a sweater and two balloons.


Each balloon was rubbed with the sweater (acrylic fiber and wool).  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.  The strengths of the attraction and repulsion are exagerated a little bit in the figures above.

Our next step in understanding EM is to learn something about electric field arrows.  Imagine placing a + charge at the three positions around a center charge as 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. 

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 Friday.  This is probably where we will start on Monday.


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.