Friday Feb. 19, 2016

Roy Orbison from the Black and White Night Concert: "Claudette" (3:11), "Uptown" (3:20), "Candy Man" (3:26), "Pretty Woman" (6:18)
 
We were able to get the Sect. 1 quizzes graded in time to return in class today but not the quizzes from the 1 pm class.  We'll have them by Monday.  Next time we'll grade your quizzes first and the other class will have to wait until Monday (though next time will be the Friday before Spring Break so there may not be too many people in class).

All of the Experiment #1 reports (and 1 Scientific Paper report) have been graded, including the ones turned in earlier this week.  You can revise your experiment (and Scientific Paper) reports if you want to (a revision is not required).  Unless noted otherwise (on your report) revised reports are due by Wed., Mar. 2.  Please return the original report with your revised report.

There are a couple of Assignments due next week.  See the class homepage for more details.

There were network problems somewhere on campus so the class webpage wasn't available during class.  That made things a little more chaotic and confusing than normal, you'll want to be sure to read through the online notes that follow.  Everything was working fine by the time I got back to my office of course, that's how events like this work.

It's a lucky thing we don't use a textbook in this class.  If we did we would just be moving into Chapter 2.  During the next couple of weeks we will be concerned with energy, temperature, heat, and energy transport.  We'll look at the flow of energy back and forth between the earth's surface and it's atmosphere; the greenhouse effect is a big part of that.

It is easy to lose sight of the main concepts because all the details.  The following is an introduction to this new section of material and most of the figures are found on pages 43 & 44 in the photocopied ClassNotes.

It might be helpful also to keep a list of the various topics as we cover them in class today.  I've included an example list at the end of today's notes.

1. Types of energy


Kinetic energy is energy of motion.  Some examples (both large and microscopic scale) are mentioned and sketched above.  This is a relatively easy to visualize and understand form of energy.




Radiant energy is a very important form of energy that was somehow left off the original list in the ClassNotes .  Electromagnetic radiation is another name for radiant energy.  Sunlight is an example of radiant energy.  It's something that we can see and feel (you feel warm when you stand in sunlight).  Something that is not quite so obvious is that everyone in the classroom is emitting radiant energy.  This is infrared light, an invisible form of radiant energy.  And actually it's not just the people; the walls, ceiling, floor and even the air in the classroom are also emitting infrared light.  We can't see it.  Because it's there all the time I'm not sure whether we can feel it or not.

Latent heat energy is an, important, under-appreciated, and rather confusing type of energy.  The word latent refers to energy that is hidden.  That's part of the problem.  Another part of what makes latent heat energy hard to visualize and appreciate is that the energy is hidden or stored in water vapor or water - that seems an unlikely place to find energy.

It might be helpful to think of latent heat energy as being a form of potential energy.

Gravitation potential energy is something I suspect you're familiar with.





It would take a lot of energy to push a rock up a hill.
 Once at the top of the hill the rock has a lot of stored, potential energy (the energy that it took to get it there). 
This energy would reappear as kinetic energy if you were to push the rock and start it rolling down hill.

Energy is being added in the left figure, stored energy is shown in the middle figure, and the energy reemerges or is released in the final picture.

Latent heat energy





Some of the sunlight energy
hitting water warms the water. 
The rest is used to evaporate water.

 The water vapor contains a lot of stored, "latent heat", energy (sunlight energy that was added during evaporation).
 The stored energy is released when the water vapor condenses and turns back into water.

The same kind of scenario is shown here except that it involves water, water vapor, and sunlight.  Energy is added in the left figure and is used to evaporate some water.  The added energy is stored or hidden in the water vapor, and the energy is released when the water vapor condenses and turns back into water.  It takes a lot of energy to evaporate water. 








Here are three examples showing energy originally hidden in water vapor reemerging in a tornado, water rushing down a mountain or wash, and a hurricane.  A big part of the energy in the tornado, flash flood, and hurricane was initially hidden in the water vapor.
2. Energy units
Next just brief mention of units of energy





Joules are the units of energy that you would probably encounter in a physics class.  Your electric bill shows the amount of energy that you have used in a month's time, the units are kilowatt-hours.  We'll usually be using calories as units of energy.  1 calorie is the energy need to warm 1 gram of water 1 C (there are about 5 grams of water in a teaspoon). 

Here's a little miscellaneous information that you don't need to worry about remembering.  You've probably seen the caloric content of food on food packages or on menus in restaurants.  1 "food calorie" is actually 1000 of the calories mentioned above (food is probably a form of chemical energy, the energy is released when the food is consumed). 




A 150 pound person would burn almost 500 food calories while sleeping during the night (8 hours x 60 minutes per hour x 1 food calorie per minute).  This is about the energy contained in one donut.

3. Energy transport processes



By far the most important process is at the bottom of the list above.  Energy transport in the form of electromagnetic radiation (sunlight for example) is the only process that can transport energy through empty space.  Electromagnetic radiation travels both to the earth (from the sun) and away from the earth back into space.  Electromagnetic radiation is also responsible for about 80% of the energy transported between the ground and atmosphere.

You might be surprised to learn that latent heat is the second most important transport process.  This term latent heat can refer to both a type of energy and an energy transport process (the energy is hidden in the water vapor, the water vapor can move around and carry that energy with it).

Rising parcels of warm air and sinking parcels of cold air are examples of free convection.  Because of convection you feel colder or a cold windy day than on a cold calm day (the wind chill effect).  Ocean currents are also an example of convection. 

Convection is also one of the ways of rising air motions in the atmosphere (convergence into centers of low pressure and fronts are two other ways we've encountered so far) 

Conduction is the least important energy transport at least in the atmosphere.  Air is such a poor conductor of energy that it is generally considered to be an insulator.

4. Energy balance
The next picture (the figure in the ClassNotes has been split into three parts for improved clarity) shows energy being transported from the sun to the earth in the form of electromagnetic radiation.  On average about half of this sunlight passes through the atmosphere and is absorbed at the ground.  This causes the ground to warm (sunlight energy striking the ocean warms the oceans but is also used to evaporate ocean water).  Measuring the energy in sunlight arriving at the surface of the earth is the object of Experiment #3.





We are aware of this energy because we can see it (sunlight also contains invisible forms of light) and feel it.  With all of this energy arriving at and being absorbed by the earth, what keeps the earth from getting hotter and hotter?  If you park your car in the sun it will heat up.  But there is a limit to how hot it will get.  Why is that? 

It might be helpful when talking about energy balance to think of a bank account.  You open a bank account and start depositing money.  The bank account balance starts to grow.  But it doesn't just grow without limit.  Why not?  The answer is that once you find money in the bank you start to spend it.  The same is true of energy and the earth.  Once the earth starts to warm it also starts to emit energy back into space (the orange arrows in the figure below).  Radiant energy is emitted by the ground, the oceans, and the atmosphere. 




Energy is emitted by the earth in the form of infrared light, an invisible form of energy (to human eyes anyways).  A balance between incoming and outgoing energy is achieved and the earth's annual average temperature remains constant.

We will also look closely at energy transport between the earth's surface and the atmosphere (see the figure below). This is where latent heat energy transport, convection and conduction operate (they can't transport energy beyond the atmosphere and into outer space).



This is also where we will find the atmospheric greenhouse.  That will be a important goal - to better understand how the atmospheric greenhouse effect works.

5. The atmospheric greenhouse effect



The greenhouse effect is getting a lot of "bad press".  If the earth's atmosphere didn't contain greenhouse gases and if there weren't a greenhouse effect, the global annual average surface temperature would be about 0 F (scratch out -4 F in the figure above and put 0 F, it's easier to remember).  Greenhouse gases raise this average to about 60 F and make the earth a much more habitable place.  That is the beneficial side of the greenhouse effect.  That's mostly what we'll be concentrating on - how can the greenhouse effect cause this warming, how can it produce this much warming.

The detrimental side is that atmospheric greenhouse gas concentrations are increasing (no real debate about that).  This might enhance or strengthen the greenhouse effect and cause the earth to warm (some debate here particularly about how much warming there might be).  While that doesn't necessarily sound bad it could have many unpleasant side effects (lots of debate and uncertainty about this).  That's a subject we'll explore at various times during the semester.


6. Energy, temperature, and specific heat
(p. 45 in the ClassNotes)

When you add energy to an object, the object will usually warm up (or if 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 (this is another equation I'll try to remember to write on the board before  the next quiz - try to understand it, you don't have to memorize it).



The temperature change, ΔT,  will first depend on how much energy was added, ΔE.  This is a direct proportionality, so ΔE is in the numerator of the equation (ΔE and ΔT are both positive when energy is added, negative when energy is removed)

When you add equal amounts of energy to large and small pans of water, the water in the small pan will get hotter.  The temperature change, ΔT, will depend on the amount of water, the mass.  A small mass will mean a large ΔT, so mass should go in the denominator of the equation. 

Specific heat is what we use to account for the fact that 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 ΔT as mass.  Specific heat is sometimes called "thermal mass" or "thermal capacity."  You can think of specific heat as being thermal inertia - a substance with high specific heat, lots of thermal inertia, will be reluctant to change temperature.

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




Equal amounts of energy (500 calories) are added to equal masses (100 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. Before we do the calculation, try to guess which material will warm up the most.  Everything is the same except for the specific heats.  Will water with its 4 times larger specific heat warm up more or less than the soil?

The details of the calculation are shown below.

With its higher specific heat, the water doesn't heat up nearly as much as the soil.  If we had been removing energy the wouldn't cool off as much as the soil would.

7. Water moderates climate
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.  Water's ΔT is smaller than land's because water has a  higher specific heat.

The yearly high and low monthly average temperatures are shown at two locations above.  The city 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 average temperature. 

Water moderates climates - it reduces the difference between summertime high and wintertime low temperatures.

Growing tomatoes in the desert - practical application
 Here's another situation where you can take advantage of water's high specific heat to moderate climate on a smaller scale.




You need to start tomatoes early in Tucson (mid February) before it gets too hot, so that they can produce fruit before it gets to hot.  In you need to protect the plants from frost.



Here's one way of doing that.  You moderate the climate and surround each plant with a "wall o water"  -  a teepee like arrangement that surrounds each plant.  The cylinders are filled with water and they take advantage of the high specific heat of water and won't cool as much as the air or soil would during a cold night.  The walls of water produce a warm moist micro climate that the tomato seedlings love.  The plastic is transparent so plenty of sunlight can get through.


Adding energy to something will usually cause its temperature to change.  But not always.  What else could happen? 

You put a pan of water on the stove and turn on the burner.  The water will warm.  It will only warm to a certain point however.  Then what happens?

8. Phase changes


Sometimes when you add energy to a material it will stop warming and will change phase.  Water will warm to 212 F (100 C) and then it will start to boil.  Adding energy to ice will first warm the ice to 0 C.  But then it will stop warming and will start to turn to melt.  The dry ice above is sublimating (changing directly from solid to gas).
It is very easy to calculate how much energy is needed to cause a phase change.


The energy needed depends on the amount of material present (the mass) and on the material itself (that's the Latent Heat term above).  It also depends on the specific phase change.  I.e. there are different Latent Heat values depending on whether the material changes from solid to liquid, liquid to gas, or solid to gas.

Measuring the latent heat of vaporization (evaporation) of liquid nitrogen
I'm hoping we'll have enough time to conduct an experiment.

Here's the object of the experiment:

Students working on Experiment #2 are doing something like this, they are measuring the latent heat of fusion of ice (the energy needed to melt ice).

The source of energy in our experiment will be the energy contained in a cup of room temperature water.  We'll pour some liquid nitrogen into the cup of water.





Energy will naturally flow from hot (the water) to cold (the liquid nitrogen).  As energy is taken from the water it will cool.  We'll assume that all of the energy taken from the water is used to evaporate nitrogen, no energy flows from the cup into the surrounding air (that's part of the reason we conduct the experiment in a Styrofoam cup.









Our earlier equation is shown above at left.  If you know how much energy is added to something you could determine the temperature change that would result.  We can turn the equation around so that is we measure the temperature change that any object undergoes we can calculate the amount of energy added or removed (the equation at right).

Here we put everything together.  We'll determine how much energy is taken from the water.  Then we'll assume all of that energy is used to evaporate nitrogen.  That's the right hand equation above.

The numbers below should be the same ones that were collected in class today.

We start with a medium size Styrofoam cup filled about 1/3 full with room temperature water.





The cup and the water together weighed 168.0 g of room temperature water.  The cup weighed 3.9 g, so we really had 164.1 g of water.  The water's temperature was 21.0 C.

 Next we poured some liquid nitrogen into a second, smaller styrofoam cup.



We're going to evaporate 32.3 grams of liquid nitrogen.  The total amount of energy needed to do that, ΔE, is the mass of the liquid nitrogen times the Latent Heat of
vaporization of Nitrogen (LHvap). 

ΔE = mass x LHvap

LHvap is the energy needed per gram to vaporize (evaporate) liquid nitrogen.  That's the quantity we are trying to measure.


We poured the 30 grams of liquid nitrogen into the cup containing 148.2 g of water.  Energy flows naturally from hot to cold.  We assume that any energy lost by the water is used to evaporate nitrogen.



Once the liquid nitrogen was gone (it had evaporated) we remeasured the water temperature.  It had dropped to 8.5 C.  Now we're ready to calculate the latent heat of vaporization


We'll plug in all our measurements and solve for LHvap. 

We set up an energy balance equation (energy lost by the water = energy used to evaporate nitrogen) and plugged in all our measured values.  We obtained a measured value of LHvap = 55.9 calories/gram (53 cal/g later in the day in the 11 am class).  A student in the class informed us that the known value is 48 cal/g.  Not too a bad result at all.

Here's a list of the topics that were covered in class today