Wednesday Oct. 3, 2012
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"Dragonfly" from Ziggy Marley and "Dar-Es-Salaam" by Ras Nas before class this afternoon.

The Surface Weather Map Analysis that was turned in on Monday has been graded and was returned in class today.  Many of you will find a little piece of paper attached to your work asking whether you would prefer to earn extra credit or 1S1P points.  Please return those slips of paper to me.  There were some very accurately drawn maps turned in and also some that weren't quite as good.  You'll find a completed analysis here that you can compare with your work.

Quiz #2 is one week from today and the Quiz #2 Study Guide is now available online.  Note the two links to sample questions (just questions, not answers) embedded  in the Study Guide.

I have finally set a due date for the 1S1P Assignment #2 reports - Wednesday Oct. 17. 

The Upper Level Charts Optional Assignment was collected today.  You'll most likely get the assignment back next Monday with a Green Card attached if you scored 85% or above on the assignment.

One of my favorite graphics is a page from National Geographic Magazine that lists some of the limits of human survival.  I can't just scan the original and add it to the notes without violating copyright laws.  But if you click on the link above you'll find all of the same information online in the form of a quiz.

I also showed a photograph of a nuclearn explosion which is a very dramatic example of free convection.  You'll find lots of additional and much better photographs here.  You'll also find lots of videos on YouTube.

We spent a portion of the class period 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 hot to cold; in this case from the water (room temperature would be about 70 F) to the ice (32 F).  This transport of energy would occur via conduction..

Once the ice had absorbed enough energy it would melt.  Energy taken from the water would cause the water to cool.  The energy that needed to be added to the ice would be taken from the surroundings (the water) and would cause  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.

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.

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.

One of the big problems I've been having is with the bubbles.  They don't last very long and sink to quickly.  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.

Finally a couple of questions to test your understanding.  The first question comes from the Quiz #2 Study Guide.

What is the direction of the electric field arrow at Point X halfway between a + and a - charge? 

The second question has two parts.  First you need to determine what polarity of charge must be on ground to cause the charges in the figure below to move as they are doing.  Then what direction does the electric field arrow point at a location just above the ground where the two charges are found.

Click here when you think you know the answers to these questions.