Exploration of a "Closed System": the Galileo Thermometer

Learning Objectives:

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.



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.  

Prior Knowledge:


(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?"

Interpretation:  "How can you explain what you saw?"

Here is the "official" answer:   

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.

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.

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?"

Tough Questions:

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 (kastens@ldeo.columbia.edu).
May be freely used for educational purposes provided appropriate credit is given.