Exploration of a "Closed System": the Galileo Thermometer
- Students will observe and try to interpret a "closed system", a system which allows the transfer of energy in and out, but not the exchange of material.
- Students will realize that work can be done (i.e. action can occur) within the closed system as a result of the transfer of energy into the system--even though nothing within the closed system was physically touched.
- Students will exercise their understanding of density, and the relationships among density, volume and temperature .
Students will exercise their ability to make observations, to develop a tentative interpretation (hypothesis) from those observations, and to develop a plan (proposal) to test that interpretation.
I use this activity with undergraduate non-science majors in a course called Planet Earth, in the second week of the course in an unit called "The Earth System." I think it would work fine with high school and even middle school students although the level of interpretation might be more rudimentary.
- Galileo Thermometer (easiest to see and discuss what's happening if floats have different colored liquids and main liquid in cylinder is clear). Several sources for these thermometers on line are:
- Space heater or hair dryer (and, if necessary, extension cord)
- Student worksheets
- pdf - to download Adobe Acrobat to read this pdf file please click on the Adobe logo
Logistics and Organization:
Before the class, you will need to cool down the Galileo thermometer to significantly below 60°F. You can do this by putting it into a refrigerator or leaving it outside in your car in the winter. (CAUTION: Don't let the thermometer freeze!)
Plan to set up the equipment in a place where student groups can easily come up and observe it, but where there is minimum risk that they will knock over the thermometer.
After the activity has been launched, students will need to observe the setup several times over the course of an hour. You should plan another activity for them to be doing in between observations.
My class runs for 1 hour and 15 minutes. Using the space heater allows enough time for the Galileo thermometer to warm up through its full range of temperature during the class period, allowing time for discussion at the end.
- "Observation," "Interpretation," "Hypothesis," "Proposal": As this is the first hands-on activity in my class, I lead a class discussion/review of the meaning of these terms before we begin. It's especially important that students have a clear distinction in mind between Observation and Interpretation.
- "System," "Closed System," "Open System": I do this activity after students have been exposed to these concepts by reading the "Earth System" chapter in The Blue Planet: An Introduction to Earth System Science (Skinner et al, 1999). I think this activity could also be used to have students "discover" the phenomenon of a closed system.
- Heat, temperature, density, volume: To completely and correctly interpret their observations, students need to know that: (a) when heat is added to a material, its temperature goes up, (b) for most materials (including liquid water), when temperature goes up, volume goes up and density goes down. Among my college students, at least someone in any group of three of four has this knowledge, although often not in a very well-organized or accessible form. Therefore, I don't review this knowledge before the activity, but instead let students struggle to recall and use this half-remembered knowledge.
- Buoyancy : When things are free to move, things of low density tend to float up and things of high density tend to sink down. Again, I don't review this knowledge in advance, but instead let students bring forth this half-buried knowledge in the course of the activity.
(1) Organize students (or have them organize themselves) into groups of 2-4. Give out one worksheet to each group.
(2) Put the Galileo thermometer in a central location where students can see it from their seats and can easily come up in groups to see it close up. Because you have pre-cooled the thermometer, all of the floats are initially at the top of the fluid-filled cylinder.
(3) Invite students up in groups to make their first set of observations.
(4) Turn on the space heater or hair dryer and set it up so that it heats the Galileo thermometer.
(5) Over the course of the class period, student groups come up to observe the thermometer. As time goes by, and the thermometer heats up, the floats will, one after another, gradually sink to the bottom of the cylinder. (CAUTION: Make sure that you don't overheat the thermometer! When the last float has sunk turn off the heater.)
(6) As the last float sinks, encourage students to get on with generating their interpretation/hypothesis, and formulating a plan to test their hypothesis. Circulate among the small groups listening to discussions, listening for logical flaws, asking questions to get discussions back on track.
(7) Collect worksheets. Make sure each student's name is on the sheet.
(8) Bring students back together as full class, and lead a discussion of what they observed, how they interpreted/explained what they saw, and how they might test their interpretation/explanation (see below and worksheet).
Discussion/ Assessment of Worksheets:
The following notes include questions you may wish to ask and points you may wish to bring out in the small group discussions and/or the whole class discussion at the end of the class. These are the same points that you will probably want to look for in assessing students' written answers.
"What did you see?" "What did you see the second time you looked that was different from the first time you looked?" "Did you see anything else?"
- Students should describe the basic setup, including glass cylinder, clear liquid inside cylinder, several floats made of glass with different colored liquid inside them, metal tags with numbers dangling down from the floats. They use various terms for the floats - "Spheres, bubbles, balls" all OK. Additional important points that students often overlook: the cylinder is sealed at the top; there is a space at the top of the cylinder with no liquid in it.
- Students should have noted that over time, as the heater cast heat upon the liquid-filled cylinder, the floats within the liquid gradually sank, one by one, from the top of the cylinder to the bottom. Careful students will have noted the time that each float sank, and described each sinking float (according to the color of its enclosed liquid or the number on its dangling tag).
- EMPHASIZE: Emphasize that action, movement, happened within the cylinder even though no one and no thing touched the floats to make them move.
- Note : The students may want to call the clear liquid "water." It's not actually water; it's another fluid whose volume varies more rapidly with temperature. It probably doesn't do much if any harm to have them call the liquid "water," but you might want to encourage them to say "clear liquid" instead, or query the accuracy of their observation by saying "do you know it's water?"
Interpretation: "How can you explain what you saw?"
Here is the "official" answer:
- At the beginning, the floats were all floating at the top of the cylinder because the weight of each float (weight of float glass plus dangling metal tag plus colored fluid inside float plus gas inside float) was less than the weight of the clear liquid displaced by the float. In other words, the density of each float (taken as a whole) was less than the density of the surrounding liquid. Or conversely, the surrounding liquid was more dense than the floats.
- The space heater blew hot air on the outside of the cylinder. The heat was conducted through the glass of the cylinder into the clear liquid within the cylinder.
- As the clear liquid in the main body of the cylinder heated up, its volume increased and its density decreased.
- As the density of the clear liquid decreased, it was no longer denser than the lowest float (the float with the dangling "62" tag). As soon as the density of the lowest float got to be greater than the density of the surrounding liquid, that float sank.
- Each of the floats must be of slightly different density, with the bottom float (with the dangling "62") having the highest density and the top float (with the dangling "76") having the lowest density. As the liquid continued to heat up, it gradually got to be less dense than each of the other floats in turn; each of the floats, in turn, sank to the bottom of the cylinder.
If students aren't making progress towards an interpretation something like this, here are some questions you can ask to move their discussion forward:
Why were the floats floating at the top of the cylinder at the start of your observations?
A common error in the written answers is that students say "change of temperature" caused the floats to sink" and then stop there.
- OK, let's go back a step: why does something, anything, float?
- What do you mean by "buoyancy"? What do you mean by "density"?
- Why does something, anything, sink?
- What changed in the system when I turned the space heater on?
- OK, so you have told me that you think that the liquid inside the cylinder was getting warmer as time went by; if that is true, then what else about the liquid would have been changing? If you change the temperature of a liquid, what else changes about it? (If no one in the group, or the class, comes up with the idea that increasing temperature of most materials leads to increase of volume, decrease of density, you may have to tell them this as a clue.)
- What do you think is the reason that the floats didn't all sink at the same time?
Proposal to test explanation/hypothesis:
Students will come up with a variety of more or less practical ways to test their explanations/hypotheses. You might want to emphasize that they don't need to test every single link in their chain of logic. A test is valuable if it tests a single step in the chain of logic. Here are some possible answers.
- To test the idea that temperature change (rather than some other factor) is causing the floats to move, reverse the process you have already observed. Turn off the heaters, let the cylinder cool down, and our hypothesis predicts that the floats will gradually float back up to the top, as the clear fluid in the cylinder gets denser.
- To test the idea that the volume of liquid in the cylinder is increasing, do this experiment over again, but this time use a marker or piece of tape to mark the location of the top of the liquid in the cylinder before you begin. Our hypothesis predicts that at the end of the experiment the top of the liquid will be above this mark.
- To test the idea that the floats drop one by one because they are of different densities, break open the cylinder and take out the floats. Weigh each one on a balance. Measure its volume by displacement of water within a graduated cylinder. Calculate the density of each one by dividing its mass by its volume. Our hypothesis predicts that the floats will all be of different densities, and the bottom float will be the densest.
Note: If your discussion yields up this range of possible testing strategies, you might want to take the opportunity to talk about destructive/invasive testing versus non-invasive testing; does the test hurt or destroy the thing you are observing?
Note: In guiding the discussion of how to test their hypotheses, students have a tendency to just say what their plan is and then stop. You need to insist that they go on to explain how their plan, if carried out, would lead to a new observation which would either agree with or disagree with some prediction of their explanation/hypothesis. One way to do this is to ask: what would you expect to see if your explanation is correct? What might you see if your explanation/hypothesis is incorrect?
Closure: "What have we learned here that pertains to the Earth System?"
- The glass cylinder in our experiment is a "closed system." Energy can go into (or out of the system) but matter cannot.
- Sending energy into the closed system of the glass cylinder caused action (motion) inside the system, even though nothing and no one touched the components inside the closed system.
- In this same way, sending energy from the sun into the (almost) closed system of the Earth causes action/motion/work to be done within the system. What are some examples? Heating Earth in the tropics causes air mass to rise, which is one of the drivers of atmospheric circulation.
- In the Galileo thermometer, movement up and down movement was triggered by changes in density, which in turn were caused by changes in temperature. High density (cooler) stuff moved down relative to low density (warmer) stuff. Again and again we will meet this same phenomenon on the Earth: in the oceans, cold, high density water masses sink beneath warmer waters; in the atmosphere, warm (low density) air rises; in the Earth's mantle, cold, high density rock sinks at subduction zones.
Here are some additional though-provoking questions. A particularly-inquisitive student may ask you these questions, or you might ask students these questions to stimulate additional thought and/or discussion.
Q: Heat comes into this system from the outside. But eventually heat from the clear fluid in the main part of the cylinder will get conducted deeper inward through the glass of the float into the interior of the float. Why does the thermometer continue to work accurately, once the main cylinder's clear fluid and the float and the material inside the float are all at the same temperature?
A: The glass and metal that make up the exterior of each float experience only a tiny amount of volume change over the temperature range of our experiment. So, to a first approximation, the density of each float remains unchanged as its temperature rises, whereas the density of the clear liquid changes significantly across the same temperature rise. (It doesn't matter that the fluid inside the float may increase or decrease in volume; the air plus liquid inside each float still have the same mass and still occupy the same volume.)
Q: Up at the top of the cylinder, there is a small section of the cylinder with no liquid in it. Would the outcome of the experiment have been different if the cylinder had been completely filled with the liquid?
A: Probably. The liquid could not have expanded in volume, and thus would not have undergone a decrease in density, and thus the thermometer would probably not have worked properly. As temperature increased without the expected increase in volume, pressure would build up inside the cylinder. It is possible that the cylinder would have cracked.
Created by Kim Kastens, Lamont-Doherty Earth Observatory (firstname.lastname@example.org).
May be freely used for educational purposes provided appropriate credit is given.