Tuesday Oct. 6, 2009


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Tuesday Oct. 6, 2009

A couple of songs ( " "My Song," and "The Story" ) from Brandi Carlile before class today.  She'll be in Tucson at the Rialto Theatre on Fri., Oct. 16.

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

The first part of the Quiz #2 Study Guide is now available online.  By the time today's notes are online, the complete study guide (in preliminary form) should be available.

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 Tuesday (Oct. 13).  Please return your materials this week and pick up the supplementary information sheet.  You can bring them to class or drop them off in my office (PAS 588).

In the next week or so we will be learning about several different forms of energy, energy transport, and the atmospheric greenhouse effect.  Class started with a little bit of an overview before getting into the details.

When you add energy to an object, the object will usually warm up (conversely when you take energy from an object the object will cool).  It is relatively easy to come up with an equation that allows you to figure out what the temperature change will be.

The temperature change will first depend on how much energy was added.  This is a direct proportionality, so delta E is in the numerator of the equation (delta E and delta T are both positive when energy is added, negative when energy is taken from something)/

When you add equal amounts of energy to large and small  pans of water, the small pan will heat up more quickly.  The temperature change, delta T, will depend on the mass.  A small mass will mean a large delta T, so mass should go in the denominator of the equation.  

Different materials react differently when energy is added to them.  A material with a large specific heat will warm more slowly than a material with a small specific heat.  Specific heat has the same kind of effect on delta T as mass.  Specific heat is sometimes called "thermal mass" or "thermal capacity."

Here's an important example that will show the effect of specific heat (middle of p. 45)

Equal amounts of energy (1000 calories, note that calories are units of energy) are added to equal masses (200 grams) of water and soil.  We use water and soil in the example because most of the earth's surface is either ocean or land. Water has a higher specific heat than soil, it only warms up 5o C.  The soil has a lower specific heat and warms up 25o C, 5 times more than the water.

These different rates of warming of water and soil have important effects on regional climate.

Oceans moderate the climate.  Cities near a large body of water won't warm as much in the summer and won't cool as much during the winter compared to a city that is surrounded by land.

The city above on the coast has a 30o F annual range of temperature (range is the difference between the summer and winter temperatures).  The city further inland (assumed to be at the same latitude and altitude) has an annual range of 60o F.  Note that both cities have the same 60o F annual mean temperature.

Adding energy to an object will usually cause it to warm.  But there is another possibility (bottom p. 45),  the object could change phase (change from solid to liquid or gas).  Adding energy to ice might cause the ice to melt.  Adding energy to water could cause it to evaporate.

The equation at the bottom of the figure above allows you to calculate how much energy is required to melt ice or evaporate water or sublimate dry ice.  You multiply the mass by the latent heat, a variable that depends on the particular material that is changing phase. 

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. 

We will be using the equation in a slightly different way in a class experiment/demonstration. We will measure the temperature change and use that to determine the amount of energy lost by an object.

The object of the experiment was to measure the latent heat of vaporization of liquid nitrogen.  That just means measuring the amount of energy needed to evaporate a gram of liquid nitrogen.  The students that are doing Experiment #2 are measuring the latent heat of fusion of ice, the energy needed to melt one gram of ice.  You'll find the following figure on p. 45a in the photocopied Classnotes.

Some room temperature water poured into a styrofoam cup weighed 155 g.  The cup itself weighed 3.8 g, so we had 151.2 g of water.

The water's temperature was 20.8 C  (room temperature).

  44.0 g of liquid nitrogen was poured into the cup of water.

It takes energy to turn liquid nitrogen into nitrogen gas.  The needed energy came from the water.  This flow of energy is shown in the middle figure above.  We assumed that because the experiment is performed in a styrofoam cup that there is no energy flowing between the water in the cup and the surounding air.


After the liquid nitrogen had evaporated we remeasured the water's temperature.  It had dropped to 6.2 C.  That is a temperature drop of 20.8 - 6.2 = 14.6 C.

Because we knew how much water we started with, its temperature drop, and water's specific heat we can calculate how much energy was taken from the water.  That is the 2207.5 calorie figure above.  This was used to evaporate 44 grams of liquid nitrogen.  So we divided 2207.5 calories by 44 grams to get 50.2 calories needed per gram.  That is our measured value of the latent heat of vaporization of nitrogen.  A trustworthy student in the class informed us that the known value is 48 cal/g, so our measurement was pretty darn close.

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.  We really didn't have time to get into this very far in class on Tuesday.  Have a quick look through this material; we'll review it quickly in class on Thursday.

Convection is energy transport by organized motion of the atoms or molecules in a material.  Rather than moving about randomly, the atoms or molecules move as a group.  Convection works in liquids and gases but not solids (the atoms or molecules are bound to each other and aren't able to move around freely). 

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.


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