Monday Oct. 5, 2009
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A little "late night at the end of a hard day" music ("Pueblo Nuevo", "Dos Gardenias", and "Veinte Anos") from the Buena Vista Social Club before class today.

The 1S1P Assignment #1 Topic #2 reports were collected today.  I expect to return the Radon reports in class on Wednesday.

The first part of the Quiz #2 Study Guide is now available online.  The complete study guide in preliminary form should be available by Wednesday.

Optional Assignment #1 was returned in class today.  If you don't see a grade marked on your paper it means you earned full credit (0.5 extra credit points).

The Experiment #2 reports are due next Monday (Oct. 12).  Please return your materials this week and pick up the supplementary information sheet.

Now back to the subject at hand.  Here's a little review of some of what we covered last Friday.

If you add energy to or remove energy from an object, the object will usually change temperature.  You can calculate the temperature change if you know the object's mass and its specific heat. 

When you add energy to an object and the object warms, what exactly is happening inside the object?

The figure above is on p. 46 in the photocopied Class Notes.  Temperature provides a measure of the average kinetic of the atoms or molecules in a material.  The atoms or molecules in a cold material will be moving more slowly than the atoms or molecules in a warmer object.

You need to be careful what temperature scale you use when using temperature as a measure of average kinetic energy.  You must use the Kelvin temperature scale because it does not go below zero (0 K is known as absolute zero). The smallest kinetic energy you can have is zero kinetic energy.  There is no such thing as negative kinetic energy.

You can think of heat as being the total kinetic energy of all the molecules or atoms in a material.  The next figure might make the distinction between temperature (average kinetic energy) and heat (total kinetic energy) clearer.


A cup of water and a pool of water both have the same temperature.  The average kinetic energy of the water molecules in the pool and in the cup are the same.  There are a lot more molecules in the pool than in the cup.  So if you add together all the kinetic energies of all the molecules in the pool you are going to get a much bigger number than if you sum the kinetic energies of the molecules in the cup.  There is a lot more stored energy in the pool than in the cup.  It would be a lot harder to cool (or warm) all the water in the pool than it would be the cup.

In the same way the two groups of people shown have the same average amount of money per person (that's analogous to temperature).  The $100 held by the larger group at the left is greater than the $20 total possessed by the smaller group of people on the right (total amount of money is analogous to heat). 

Speaking of temperature scales.


You should remember the temperatures of the boiling point and freezing point of water on the Fahrenheit, Celsius, and perhaps the Kelvin scales.  300 K is a good easy-to-remember value for the global annual average surface temperature of the earth.


You certainly don't need to try to remember all these numbers.  The world high temperature record was set in Libya, the US record in Death Valley.  The continental US cold temperature record of -70 F was set in Montana and the -80 F value in Alaska.  The world record -129 F was measured at Vostok station in Antarctica.  This unusually cold reading was the result of three factors: high latitude, high altitude, and location in the middle of land rather than being near or surrounded by ocean.     Liquid nitrogen is cold but it is still quite a bit warmer than absolute zero.

Conduction is the first of four energy transport processes that we will cover (the least important transport process in the atmosphere).  The figure below illustrates this process.  A hot object is stuck in the middle of some air.


In the top picture some of the atoms or molecules near the hot object have collided with the object and picked up energy from the object.  This is reflected by the increased speed of motion or increased kinetic energy of these molecules or atoms (these guys are colored pink). 

In the middle picture the initial bunch of energetic molecules have collided with some of their neighbors and shared energy with them (these are orange).  The neighbor molecules have gained energy though they don't have as much energy as the molecules next to the hot object.

In the third picture molecules further out have now (the yellow ones) gained some energy.  The random motions and collisions between molecules is carrying energy from the hot object out into the colder material.

Conduction transports energy from hot to cold.  The rate of energy transport depends first on the material (air in the example above).  Thermal conductivities of some common materials are listed.  Air is a very poor conductor of energy.  Air is generally regarded as an insulator.  Water is a little bit better conductor.  Metals are generally very good conductors (cooking pans are often made of stainless steel but have aluminum or copper bottoms to evenly spread out heat when placed on a stove).  Diamond has a very high thermal conductivity.  Diamonds are sometimes called "ice."  They feel cold when you touch them.  The cold feeling is due to the fact that they conduct energy very quickly away from your warm fingers when you touch them.

The rate of energy transport also depends on temperature difference.  If the object in the picture had been warm rather than hot, less energy would flow or energy would flow at a slower into the surrounding material.

Transport of energy by conduction is similar to the transport of a strong smell throughout a classroom by diffusion.  Small eddies of wind in the classroom blow in random directions and move smells throughout the room..  For our demonstration we used curry powder.


The curry powder was actually placed on a hot plate.  With time the smell should have spread throughout the room.


By the end of class some students in the back of the room claimed they could detect just the faintest hint of the curry smell.

Because air has such a low thermal conductivity it is often used as an insulator.  It is important, however, to keep the air trapped in small pockets or small volumes so that it isn't able to move and transport energy by convection (we'll look at convection shortly).  Here are some examples of insulators that use air:


Foam is filled with lots of small air bubbles


Thin insulating layer of air in a double pane window



Hollow fibers (Hollofil) filled with air used in sleeping bags and winter coats.  Goose down works in a similar way.
Convection was the next energy transport process we had a look at.  Rather than moving about randomly, the atoms or molecules move as a group.  Convection works in liquids and gases but not solids. 


At Point 1 in the picture above a thin layer of air surrounding a hot object has been heated by conduction. Then at Point 2 a person (yes, that is a drawing of a person's head) is blowing the blob of warm air off to the right.  The warm air molecules are moving away at Point 3 from the hot object together as a group (that's the organized part of the motion).  At Point 4 cooler air moves in and surrounds the hot object and the whole process can repeat itself. 

This is forced convection.  If you have a hot object in your hand you could just hold onto it and let it cool by conduction.  That might take a while because air is a poor conductor.  Or you could blow on the hot object and force it to cool more quickly.  If I had put a small fan behind the curry powder demonstration y it would probably have spread the smell faster and further out into the classroom.
A thin layer of air at Point 1 in the figure above (lower left) is heated by conduction.  Then because hot air is also low density air, it actually isn't necessary to blow on the hot object, the warm air will rise by itself (Point 3).  Energy is being transported away from the hot object into the cooler surrounding air.  This is called free convection and represents another way of causing rising air motions in the atmosphere.  Cooler air moves in to take the place of the rising air at Point 4 and the cycle repeats itself.

The example at upper right is also free convection.  Room temperature air in contact with a cold object loses energy and becomes cold high density air.  The sinking air motions that would be found around a cold object have the effect of transporting energy from the room temperature surroundings to the colder object.

In both examples of free convection, energy is being transported from hot toward cold.

Now some practical applications of what we have learned about conductive and convective energy transport.  Energy transport really does show up in a lot more everyday real life situations than you might expect.


Note first of all there is a temperature difference between your hand and a 70 F object.  Energy will flow from your warm hand to the colder object.  Metals are better conductors than wood.  If you touch a piece of 70 F metal it will feel much colder than a piece of 70 F wood, even though they both have the same temperature.  A piece of 70 F diamond would feel even colder because it is an even better conductor than metal. 

Something that feels cold may not be as cold as it seems.  Our perception of cold is more an indication of how quickly our hand is losing energy than a reliable measurement of temperature.

Here's a similar situation.


It's pleasant standing outside on a nice day in 70 F air.  But if you jump into 70 F pool water you will feel cold, at least until you "get used" to the water temperature (your body might reduce blood flow to your extremeties and skin to try to reduce energy loss).

Air is a poor conductor.  If you go out in 40 F weather you will feel colder largely because there is a larger temperature difference between you and your surroundings (and temperature difference is one of the factors that affect rate of energy transport by conduction). 

If you stick your hand into a bucket of 40 F water (I would suggest you give it a try sometime), it will feel very cold (your hand will actually soon begin to hurt).  Water is a much better conductor than air.  Energy flows much more rapidly from your hand into the cold water. 


Ice feels cold even though it is not a particularly good conductor.  This is because of the large temperature difference between your hand and the water.

What about liquid nitrogen?  It has a temperature of -320F!  You can safely stick your hand in liquid nitrogen for a fraction of a second.  It doesn't feel particularly cold and doesn't feel wet.  Some of the liquid nitrogen quickly evaporates and surrounds your hand with a layer of nitrogen gas.  This gas is a poor conductor and insulates your hand from the cold for a short time.