Thursday Sept. 27, 2007

A new Optional Assignment was handed out.  It will be due at the beginning of class next Monday, Oct. 1.

A couple of new Reading Assignments have also been posted.
We spent quite a bit of time at the start of class looking at some material (energy transport by conduction and convection and some practical applications) that was not discussed in class on Tuesday but was nonetheless added to the end of the Tuesday notes.  You might have a look at that material if you haven't already done so.

Then we had a look at energy transport in the form of latent heat.  This is the second most important energy transport process (second only to electromagnetic radiation).  This process 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).

A 240 pound man (or woman) running at 20 MPH has just enough kinetic energy (if you could somehow capture it) to be able to melt an ordinary ice cube.  It would take 8 people 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.

The small figure at the bottom of the picture above shows that when a teaspoon or two of water freezes and makes a single ice cube energy is given off.  It's not just a little bit of energy, it is the kinetic energy that a 240 pound football player running 20 MPH would have.

The story starts at left in the tropics where there is often an abundance or surplus of energy; 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.
A piece of dry ice was passed around in class.  Dry ice is solid carbon dioxide and is very cold.  Dry ice sublimates, it changes directly from solid carbon dioxide to carbon dioxide gas.  Energy must be added to the dry ice in order for it to sublimate.  The figure below (not shown in class) shows that all three energy transport processes play a part.

In the top figure, conduction transports energy from a thin layer of warm air in contact with the dry ice to the dry ice.

In the middle figure warm air coming into contact with the dry ice looses energy and cools.  This cool air sinks and is replaced by warmer air.  This process, free convection, also transports energy to the dry ice.

Finally in the bottom figure, air that comes into contact with the dry ice is cooled to the dew point and a cloud forms.  Condensation releases energy that flows into the dry ice.
A bottle containing solid iodine crystals was also passed around class together with a bottle filled only with air.  Iodine sublimates.  What is unusual however is that the iodine gas is visible.  The bottle containing the iodine has a just barely visible pink or purple color.  You can see a picture of iodine and iodine gas at the webelements.com website.
We'll spend the next couple of class periods on electromagnetic radiation.  It is the most important energy transport process because it can travel through empty space. 

To really understand EM radiation you need to understand electric fields.  To understand electric fields we need to quickly review static electricity.

An electrical force is produced when two charged objects are placed next to each other.  The force can be repulsive or attractive.
We used a sweater (acrylic fiber and wool) and two balloons to demonstrate the rules above.

We rubbed two balloons with a sweater containing 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 next figure is from the bottom of p. 59 in the photocopied class notes.

The balloons can help you understand the picture above.  Imagine placing one of the balloons at the center of the picture and assume that it is positively charged.  The second balloon is placed at various positions (1, 2, and 3) around the central balloon.  The arrows in the picture are the electric field.  They give the direction and strength of the force that would be exerted on the second positive charge.  At Position 1, for example a positively charged balloon would be pushed upward (by the + charge on the center balloon) with  moderate force.  At Position 2 the force would point toward the left but the force is stronger than at Point 1 because Position 2 is closer to  the center charge.  At Position 3 the charge is pushed toward the lower right  with a weak force.

You can also use the electric field arrows to figure out what would happen to a negative charge.  The direction of the force is reversed.  A negative charge at Point 4 would be pulled in toward the center positive charge with moderate force.

The figures on p. 60 in the photocopied class notes have been redrawn below for clarity.


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 picture of the swimmer wasn't shown in class)

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 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 period of time.