Purpose:
- The purpose of this lab is to explore some of the basic thermal properties of different materials and how they are used to create temperature sensitive devices like thermostats and thermometers. We will also consider what this all has to do with the temperature control knob in the refrigerator.
Coefficients of Linear Expansion:
- Procedure:
- There will be two or three stations set up along the counter (possibly two versions of the same thing). One is a linear expansion apparatus and the other is a form of a thermometer that would have kept Tom Hanks a little more sane on his island.
- Station 1: Linear Expansion
- This a classic physics lab apparatus. In our lab this is NOT about getting a tremendously accurate result but rather the process of figuring out how to use the tool to determine the coefficient of linear expansion of the metal rod. Your task is to design a process that makes sense as well as figuring out what all the scales measure. After that it would be a shame to forget to actually calculate the coefficient of linear expansion. When you compare your number to the accepted value you will know right away whether you have a valid process. You will be asked what temperature change would be needed to yield the accepted value of the coefficient. We will also explore the use of electrical characteristics to identify contact.
- Station 2: Volumetric Expansion
- This not a classic physics lab apparatus. Once again our lab this is NOT about getting a tremendously accurate result but rather the process of figuring out how to use the tool to determine the coefficient of volumetric expansion of the water in the flask. Your task is to design a process that makes sense as well as figuring out what all the scales measure. After that it would be a shame to forget to actually calculate the coefficient of volumetric expansion for water. You will of course recognize that the container has changed size as well which messes up your calculation -- please unmess your calculation by including the expansion or contraction of the pyrex flask. When you compare your number to the accepted value you will know right away whether you have a reasonably valid process. You will be asked what temperature change would be needed to yield the accepted value of the coefficient.
- There will also be a standard thermometer available at this station which also depends on volumetric expansion. Determine the coefficient of volumetric expansion for the fluid inside by actual measurement. Some tiny volumes to calculate to be sure but it's just math....Identify the fluid inside the thermometer from your calculated coefficient.
- Station 3: The Refrigerator
- Now it's time to apply your understanding of thermal expansion to an actual engineered tool. The temperature sensor in a fridge is an important tool in assuring that it works well. Begin by contemplating the design requirements for this temperature sensor. We will discuss this collectively. Then look at the various bits of sensor that are with the dissected fridge. Observe their features both large and small. Based on the observed features and the design requirements determine the most likely operation of the sensor. Feel free to search the web for additional information about these temperature sensors but it is your understanding of the evidence supporting your conclusion that is important not the actual answer. Part of the point of this part of the lab is for you to think about the engineering design requirements of a refrigerator thermostat and compare that to the physical evidence in deciding which form of expansion is being used. I agree, modern fridges use electrical temperature sensors which I will make available to you as well for comparison.
- LAB DELIVERABLES:
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I) Your clear description of how you determined the coefficient of linear expansion of the Al rod using the equipment provided. This description will make it clear the order of the operations you completed to gather the data as well as the data that resulted. Include a determination of the temperature change needed to make your data yield the accepted value for the coefficient of linear expansion for Al. Discuss and comment on the reasonableness of this result and the likely sources of error. Please note that if you mention human error I will return your lab to you unevaluated:)
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II) Your clear description of how you determined the coefficient of volumetric expansion of the water using the equipment provided. This description will make it clear the order of the operations you completed to gather the data as well as the data that resulted. Include in your calculations the effect of the change in volume of the flask assuming it is the same temperature as the water inside. Include a determination of the temperature change needed to make your data yield the accepted value for the coefficient of volumetric expansion for water. Discuss and comment on the reasonableness of this result and the likely sources of error. Please note that if you mention human error I will return your lab to you unevaluated:)
III) Your clear description of how you determined the coefficient of volumetric expansion of the fluid in the thermometer. This description will make it clear the order of the operations you completed to gather the data as well as the data that resulted. Discuss and comment on the likely fluid in the thermometer based on your data. Please note that if you mention human error I will return your lab to you unevaluated:)
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IV) Discuss the engineering design requirements for the temperature sensor. Then describe clearly and completely the physical features and characteristics of the temperature sensor. Present an analysis of the observed features you documented and how they lead to a behavior which could turn a switch off and off. Be conscious, in a reverse engineering sense, that any feature that looks more complicated to fabricate must be essential to the functioning of the temperature sensor to make it worthwhile. Based on your analysis articulate whether the underlying principle of the sensor is linear expansion, volumetric expansion, or some other process.
