Wednesday Oct. 7, 2009
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A couple of songs from an Austin TX group that I just heard about a week or two ago Grupo Fantasma

Some how or another I have finally managed to read and grade all of the 1S1P reports on radon.  They were returned in class today.

The Optional Assignments were collected today.  You should get those back next Monday at the latest.  In the meantime here are the answers to the questions on the assignment.

The complete Quiz #2 Study Guide is now available in preliminary form (mostly it is a question of how much of the material on the study guide we will cover in class before the quiz).  Quiz #2 is next Wednesday, Oct. 14.

The Expt. #2 reports are due next Monday.  Please try to return your materials this week. 

We learned a little bit about energy transport by conduction and convection in class on Monday.  We also learned that our perception of cold is a better indicator of how quickly our body is losing energy than an accurate measurement of temperature.  This basic knowledge puts us in a perfect position to understand the concept of wind chill temperature.

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 works hard to keep its core temperature around 98.6 F.  A thermometer behaves differently, it is supposed to cool to the temperature of the surroundings.  Once it reaches 40 F it won't lose any additional energy.

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).  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.  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.   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. 

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 which is also a serious outdoors risk in S. Arizona.

At this point I showed a picture 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."

Perhaps the most amazing example of a physical and mental task is the 1000-day challenge undertaken by the "marathon monks" of Mount Hiei, Japan.

I hope you don't mind an occasional digression like this.  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.  This time of the year it is often a little cool in the morning.  In another month or so it will be cold.  With some mental training I hope be to be able to blank 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.  You'd here 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 a small amount of 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 needed energy will be taken from the surroundings (from your body when you step out of a shower in the morning).


 When your body starts to lose energy, it feels cold.

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.




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.

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.

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 a little more detailed that was done in class.

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. 


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.

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. 

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.


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.

Here are the answers (in red) to the series of questions shown in class.




1.  What two phase changes are occurring in the picture?  (you might not be able to see them, also the cloud you see is not carbon dioxide gas) The dry ice is first of all sublimating (turning from solid to gas).  The sublimation is invisible. The cloud that you do see is composed of water droplets or ice crystals.  Water vapor coming into contact with the cold dry ice condenses to form water droplets (or perhaps ice crystals).
Is energy being transported from the surroundings INTO the dry ice or AWAY from the dry ice and into the surrounding air?  Energy flows from the warmer surrounding air into the much colder dry ice.  That is what cools the air enough for the cloud to form and become visible.  The condensation that occurs as the cloud forms also releases hidden latent heat energy which goes into the dry ice.

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?  (conduction is not one of the answers I'm looking for)
 Blowing on the bowl of soup is forced convection.  The hot soup is also evaporating (the soup is steaming hot).  The energy needed for water in the soup to evaporate is taken from the soup and cools the soup (just like the water that evaporates off your wet body when you step out of a shower takes energy from your body and makes you feel cold).

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___B___   Convection ___A___   Latent heat ___C____

4.  Would the formation of a cloud  WARM  or  COOL  the surrounding air?  Water vapor condenses to form the cloud.  Latent heat energy is released into the surrounding air when water vapor condenses. 
Does the formation of frost  WARM  or  COOL  the air?  This is a similar situation.  Latent heat energy is released into the surroundings as water vapor changes directly to ice (deposition).