Friday Feb. 29!, 2008

The 1S1P Topic #3 reports were returned in class today.  All of the 1S1P Assignment #1 reports have now been graded.  Copies of the 1S1P Assignment #2 worksheets were distributed in class.

An in-class Optional Assignment was also distributed in class.  The assignment was turned in at the end of class.

Here's a little story to start out NATS 101 today.
Chapter 1
About 20 years ago the instructor of this class lived and worked in France for a short time (you might remember hearing about his Peugeot 404 auttomobile earlier in the semester).  One of the people he met there was a big solidly constructed guy named Jean-Emanual (perhaps Jean-Emmanuel).  If you met Manu, as he was more commonly called, at a party and if he had had a little bit to drink he would sometimes greet you by shaking your hand and saying "I could crush you" (he'd say it in English to me just to be sure I understood him).  Now so far as I know Manu never really did crush anyone, but something like that makes quite an impression.

Why on earth is Mr. Weidman telling this story, has he been drinking?  Did he suddenly get the idea that he could crush us (the students in NATS 101)?

Click here to read Chapter 2.
The next topic was Archimedes Law of Bouyancy. You'll find this discussed on pps 53a and 53b in the photocopied Classnotes.

A gallon of water weighs about 8 pounds (lbs).  Density is mass divided by volume; if someone were to ask you what the density of water was you could respond "8 pounds per gallon."  Strictly speaking pounds aren't units of mass, but we aren't going to worry about that technicality here.

If you submerge a 1 gallon jug of water in a swimming pool, the jug becomes, for all intents and purposes, weightless.  Archimedes' Law (see figure below) explains why this is true.

The jug will displace 1 gallon of pool water.  One gallon of pool water weighs 8 pounds.  The upward bouyant force will be 8 pounds, the same as the downward force on the jug due to gravity.  The two forces are equal and opposite.

Now we imagine pouring out all the water and filling the 1 gallon jug with air.  Air is about 1000 times less dense than water; the jug will weigh practically nothing.

If you submerge the jug in a pool it will displace 1 gallon of water and experience an 8 pound upward bouyant force again.  Since there is no downward force the jug will float.

One gallon of sand (which is about 1.5 times denser than water) jug will weigh 12 pounds.

The jug of sand will sink because the downward force is greater than the upward force. 

You can sum all of this up by saying anything that is less dense than water will float in water, anything that is more dense than water will float in water.

The same reasoning applies to air in the atmosphere.

Air that is less dense (warmer) than the air around it will rise.  Air that is more dense (colder) than the air around it will sink.

 Here's a little more information about Archimedes which wasn't covered in class.

It was time for a colorful demonstration involving water and objects that either float or sink in water.

A can of regular Pepsi was placed in a beaker of water.  The can sank. 

Both cans are made of aluminum which has a density almost three times higher than water.  The drink itself is largely water.  The regular Pepsi also has a lot of sugar or corn syrup, the diet Pepsi doesn't.  The mixture has a density greater than plain water.  Both cans contain a little air (or perhaps carbon dioxide gas).  This is much less dense than water.

The average density of the can of regular Pepsi (water&sugar + aluminum + air) ends up being slightly greater than the density of water.  The average density of the can of diet Pepsi (water + aluminum + air) is slightly less than the density of water.

In some respects people in swimming pools are like cans of regular and diet Pepsi.  Some people float (they're a little less dense than water), other people sink (slightly more dense than water).  The next two figures weren't shown in class.

Many people can fill their lungs with air and make themselves float, or they can empty their lungs and make themselves sink.

People must have a density that is about the same as water. 

The course instructor has gained some weight over the winter and weighs 160 pounds.  If you assume he has a density about equal to water, 8 pounds per gallon, you find that the course instructor has a volume of about 20 gallons.
Latent heat energy transport was the final 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).  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).

Some dry ice was displayed on the projector screen.  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.  You were asked (on the in-class Optional Assignment) to look closely at the dry ice and identify two energy transport processes that you could actually see in operation.


In the top figure, conduction transports energy from a thin layer of warm air in contact with the dry ice to the dry ice.  You can't really see this process.

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, is transporting energy to the dry ice.  Because you could actually see these sinking air motions, this is one of the answers to the question.

Finally in the bottom figure, air that comes into contact with the dry ice is cooled to the dew point and a cloud forms.  You could actually see the cloud.  The cloud is produced by moisture in the air, it isn't carbon dioxide gas.  As we will see the formation of the cloud, a phase change, is an additional source of energy.  This would be an example of latent heat energy transport.  This is the second energy transport process you should have identified. 

A bottle containing solid iodine crystals was also passed around class together with a bottle filled only with air.  The crystals of iodine were sublimating, changing from solid to gas.  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.



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 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.