Conceptual Questions in Thermal Physics

Below are a series of questions that I have asked in both written and oral form. The most enlightening results came from oral interviews. Students give remarkably similar answers to these questions. The procedure used is as follows. I ask for five volunteers from an introductory physics course and pay them $10 each for having a 30 minute interview. Our students have been more than willing to participate. In addition, I have used the same questions with physics majors at the beginning of a junior/senior level thermal physics course. The answers from these more advanced students were only slightly different from those of introductory level students. I have not yet done follow-up interviews with students after they have taken the thermal physics course, but will do so in the future.

During the interview I start with the following questions below, but I always ask the students to explain what they mean. At some point in almost all interviews I eventually ask students to explain what they mean by pressure, temperature, potential energy, heat, and many other terms. Thus, the questions are only starting points. I have been amazed at the answers. I think you will be too. Also, you will have a greater appreciation for the need to help students develop microscopic models of the phenomena discussed in thermal physics courses.

If you do try out these questions, please let us know, and share your results. Send your results, comments, or questions to Jan Tobochnik at jant@kzoo.edu.

Questions

Please answer the following questions as best you can. Our goal is to use your answers to help improve the quality of teaching in thermal physics. Thank you for your assistance.

  1. A thermos bottle with a piston instead of a lid contains a fixed amount of gas. Because it is a thermos bottle, no heat can enter or leave the bottle. The piston is then pushed in, compressing the gas.
    1. Does the pressure of the gas increase, decrease, or stay the same?
    2. Does the temperature of the gas increase, decrease, or stay the same?
    3. Describe the behavior of the molecules of gas during the compression and support your answers to (a) and (b) with an explanation in terms of their behavior.
    4. Are there any other properties of the gas that change?

  2. Consider the same system except now the bottle allows heat to go in or out of the bottle. The piston is moved slowly, and the temperature of the gas is maintained constant.
    1. In terms of what is happening to the molecules, how is this situation different from the situation in the first question?
    2. Does the pressure of the gas increase, decrease, or stay the same?
    3. Support your answer to (b) with an explanation in terms of what is happening to the molecules of gas.
    4. Are there any other properties of the gas that change?

  3. A closed bottle contains water at the bottom and air above it in equilibrium at the same temperature.
    1. Do the water molecules have more energy per molecule than the air molecules, less, or the same? Explain your answer.
    2. Is the potential energy per molecule of the water molecules more negative than that of the air molecules, less, or the same? Explain your answer.
    3. Do the water molecules have more kinetic energy per molecule than the air molecules, less, or the same? Explain your answer.
    4. Explain how the water and air can be at the same temperature.
    5. Explain what happens when the water and air are heated slowly so that they both continue to have the same temperature. Does the temperature change? What is happening to the molecules?
    6. Now consider a glass of water filled with ice sitting on a table at room temperature. Describe what happens. Is the temperature of the ice the same as that of the water? What happens as the ice melts? Why does it melt?

Updated 15 May 2000.