Global Warming - Energy flow

Subject Area

Physics

Age or Grade

9-12

Estimated Length

60-70 minutes (can be split into two lessons to fit in a 50 minute period), also could be re-arranged to form a lab rather than a demo/discussion.

Prerequisite knowledge/skills

None

Description of New Content

Phase changes, heat and energy transfer.

Goals

Specific

1. To observe energy flow in two systems that model systems relevant to climate change. 

2.  To test two claims made in the documentary An Inconvenient Truth.

3.  To introduce concepts of phase change, heat/energy transfer.

4.  To discuss some of the limitations of a model system.

General

From the Massachusetts Frameworks for Science and Technology/Engineering (2006)

Physics: Standards 3.3.1, 3.3.2, 3.3.3, 3.3.4

Materials Needed

An Inconvenient Truth (video) -- the dvd takes two class periods to view.  We watched it with a guided questionnaire over two days, then did the following exercises the next day.

Large beakers, preferably 500 mL or bigger, ideally 5 of them, though 3 would do.  If beakers are not available, any large, tall, clear container that can handle a direct heat source (pyrex)

Bunsen burner, propane torch, or butane micro-torch.  Make sure there is sufficient gas to run non-stop for at least 10 minutes.

Ice.  I made "icebergs" that completely filled the beakers, then stored them in a cooler until I needed them, and also had a supply of ice cubes.  I ended up using both.

Food coloring -- blue seems to work best for contrast.  Food "gel" (cake decoration) also works, but dissolves much more slowly.

Thermometer -- more than one would be nice, but one will do. 

Procedure

Opener

5 min.  Start-up:  "An Inconvenient Truth" made a number of claims about climate change -- today we are going to test two of them using a model of the systems involved.  The first claim was that melting sea ice causes an accelerating warming of the planet because when the ice is there "the sun's light energy bounces off of it" while the water "absorbs the sun's energy."  The second claim was that global warming will lead to stronger hurricanes, like Hurricane Katrina (note: Gore did not claim that there would be MORE hurricanes, just that the ones we have would more often be stronger).  Spend a few minutes writing down whether or not you think those two claims are correct, and if they are, what physical mechanism would make them correct.

Development

Claim 1: That ocean warming will accelerate as more of the ice melts, and that the reason for this is that the ice serves as a "reflector" of the sun's energy.  Discuss with students that this is really two separate claims -- one about what happens, and another about why that happens.  So to fully test this claim, we would have to test both parts of it.  Generate suggestions to test the first part, the bare fact of whether or not the warming accelerates.

At this point, a teacher could allow the class to follow a self-guided lab experiment, or they could follow the directions of the students in using the demo equipment, or the teacher could lead the demo.  In any case, the directions below for the demo give the outline of what you want the students to see or do, however it is arranged:

1.  Take a beaker, fill it part-way with water, then float one of your "icebergs" in it, or use a handful of ice-cubes sufficient to cover the top surface of the water.

2.  Take out your burner/torch, and describe how this will model the "sun".  Light the torch, and point it directly at the ice, moving it around somewhat to distribute the heat.

3.  Take periodic temperature readings of the water, using a thermometer stuck through the edge of the ice, or a heat-sensitive strip taped to the outside of the beaker.  Record these temperatures along with the time of their recording.  You should note that the water temperature does not seem to increase very much

4.  Continue heating as the ice melts, and continue recording temperatures.  You should note that the temperature increase seems to accelerate as time goes on. 

5.  Stop when the ice is completely melted and record the final water temperature.  Stick your hands in the water and note that it still feels cool-to-warm, but is not hot.

6.  Stop to discuss that this seems to prove the point already, as the rate of temperature increase seemed to increase as the ice melted.  Now propose trying it again with no ice in the water at all.

7.  Repeat the process with torch and temperature readings for the same amount of time as it took to melt the ice in the first part.  You should note a faster increase in temperature than before. 

8.  When the time is up, record the final temperature, and ask whether anyone would be willing to stick their hand in the water now -- the temp will be quite high, and the water will be steaming noticeably.

9.  Discuss with the students whether they have proved that the first part of Gore's claim was correct (they have), and also whether they have helped to answer the second part of the claim (they haven't).  Tell them to leave the why question for now...

Claim 2: That global warming will lead to greater strength in the hurricanes that occur.

This claim is harder to show directly, unless we have a way of generating hurricane-like storms in the classroom!  But we can establish whether or not the claim is plausible.  Discuss with the class that stronger storms must mean there is more energy in the system, and that, on its face at least, there is obviously more energy in the global system if temperatures increase.  But how does temperature connect to energy? Is there a way that we can visualize how higher temperatures imply higher energy?

Demo:

1.  Before the class arrives, set out 3 stirring/hot plates and turn the controls of the plates away from the class so they cannot see what the settings are.  Put chilled ice water into one large beaker, regular tap water in one beaker, and tap water in the third, but turn on the hot plate to a medium setting, so the water will be hot by the time you get to this demo in class.  Put stirrer bars in all three beakers, so they all look identical to the students.  Turn on the stirrer for the middle beaker, the one with regular tap water and no heating -- fast enough that if you were to increase the speed anymore, a noticeable vortex would form.

2.   Put a couple of drops of food coloring into the chilled water.  The coloring will form spidery trails pointing mostly downward.  Watch the coloring disperse for a few seconds, and point out how it is spreading out, but mostly headed slowly toward the bottom of the beaker.

3.  Put a couple of drops of food coloring in the hot water.  The coloring will form trails that point in all directions, even some headed upward, and will spread out noticeably faster than in the cold water.  Point out that this coloring isn't just headed downward -- help them remind you that this must mean that there is an additional force acting on the coloring droplets than just gravity.  They will probably mention on their own that this must be the individual water molecules bouncing off the particles of the food coloring.

4.  Ask for suggestions on what must be different about the two beakers you have used so far -- they will pretty quickly suggest temperature difference, which you can confirm using the thermometer.

5.  Discussion:  Why does the hot one spread faster?  Ans: More energy in the water molecules, so they bang into the coloring particles with more energy and more often, spreading it farther, faster.  While you are talking, the coloring will have spread farther -- pretty much filling the hot beaker uniformly and the cold beaker will still have tendrils pointed downward.

6.  Now drop some coloring into the beaker with the stirrer on -- the coloring will spread uniformly through the water almost instantly.  Some students will suggest that this water must be VERY hot, which you can dispel with the thermometer.  After some discussion of other ways one could make the water molecules move very fast, turn the stirrer up a little bit to form a nice vortex, and explain that the water had been spinning already.  Drop in a few more bits of coloring, and watch them diffuse instantly.  (It is also satisfying here to use a different color of the food gel, and drop it very carefully into the center of the vortex -- the gel drop will be trapped for a moment in the center and will be clearly visible as a solid drop, and then it will break apart very quickly, changing the color of the water slightly.)

Closure

10 min.  Debriefing: 

How do we know that more temperature means more energy?

How is temperature indicative of the motion of the atoms/molecules of the material?

Can you explain how higher temperatures might lead to stronger hurricanes?

Can you offer another explanation for the acceleration of the warming after the ice melts?

2.  Discuss some of the mechanism of hurricane formation, particularly evaporation of warm seawater which provides the energy to drive the storm.  Many online pictures show changes in the strengths of storms as the move over different bodies of water and over land -- gaining strength when over warm water, losing it when over cold water or over the land.

References

http://www.doe.mass.edu/frameworks/scitech/1006.pdf  Massachusetts Science Frameworks

Authors

Mark Betnel, Boston University GK-12 fellow

Erica Wilson, The Engineering School, 12th grade Physics and Engineering teacher

Douglas Dagan, The Engineering School, 12^th grade Physics and Engineering teacher