The question in my mind when I started this lab was to try and understand how the refrigerator we are studying controls the temperature. I knew from experience that something tells the refrigerator to turn off and on at various times as needed. I hoped by understanding a variety of temperture sensors and switches thatI would recognize the specific device used in the refrigerator. Boy, was I ever wrong -- but you'll probably be much more thoughtful than I was the first time. Good Luck!

As you have gathered from your textbook or class thermal expansion is an effect you are relatively familiar with. We know it in our daily lives as the concept that hot things get bigger (in general). Figuring out how to use this effect to create useful objects for our lives is a different story. One might imagine making a rod out of metal that reaches out (expands!) and turns off a switch when it reaches some temperature. A reasonable idea that turns out to have practical difficulties. The practical difficulty is that the coefficient of linear expansion is so small for most materials that the rod doesn't get all that much longer. Linear expansion effects only become noticable in our everyday world when we are concerned with tremendous accuracy or when we are dealing with very long pieces of material.

An example of the first scenario comes from some work I did a Lincoln Labs in Massachusetts. We needed to zap an integrated circuit (like those in your calculator) with a laser and it had to be in the right spot. The laser had to hit within 2 microns (micrometers) of the correct spot. To do this we use a very nice microscope to watch the process and position the laser. We found that it would work for a few minutes but then our accuracy would drift. After days of puzzling over this someone happened to be watching through the microscope when another worker opened the door and let some warm air into the laser room. What we noticed was that the position of the microscope immediately drifted 10 microns! We found an even more dramatic effect could be created by breathing on the microscope mount. As it turned out, the microscope body was made of aluminum which has a (relatively) large coefficient of thermal expansion. Small changes in temperature due to the microscope illuminator were causing all of our problems.

We see examples of the second scenario every time we cross a large bridge. At either end, and sometimes in the middle, are some toothy metal plates. There is nearly always a gap of several centimeters between the bridge side and the land side. For the longest time as I was growing up I had no idea what they were there for. Later, in college, I learned about thermal expansion and expansion joints. Concrete expands by 1 part in 100,000 for a temperature increase of 1 degree Centigrade. For a 500 m bridge this means it get longer by 5mm! (check my math.) Over the course of a summer day this means the bridge might lengthen or shorten by 10 -15 cm. Concrete is strong but brittle and if it flexes too much it will break. The expansion joints allow the concrete to get longer without flexing and creating undo strain.

Some of the stations in the lab will illustrate this basic thermal effect. Another use of the same idea in a more sophisticated way is the bimetal switch. In this case two thin strips of different materials are rigidly joined together. At some temperature the two individual strips are of exactly the same length and the switch is flat. As the temperature changes one of the two strips will become longer than the other due to its different coefficient of thermal expansion. Because the two strips can't separate from each other they must find some way to accomodate the fact that one is longer than the other. The switch does this by bending so that one strip is on the "outside' of the curve and the other is on the "inside". Is the longer or shorter strip on the "outside"? Some of the lab stations illustrate this effect.

In order to get the most out of this lab you will need to be curious and very observant about the details of the behavior of the device you are examining.

Reading:

Find the section(s) on temperature and thermal expansion. These will cover:

Temperature scales (F,C, and K)

Linear and Volumetric Expansion

Bimetal switches

Start by developing a conceptual understanding of each topic. Then find the formulas which mathematically describe linear and volumetric expansion and be sure that you understand what each term means. Be sure you have some understanding of the history of temperature and what it means to say two objects are at the same temperature. What is absolute zero? How do bimetal switches work on a conceptual level? What are typical values for the linear coefficient of thermal expansion?