More About Air (~1.5 hrs.)
At various times in this course, you have considered conduction in air, radiation through air, and more often, heat transfer by convection in air. In general, however, you considered heat transfer through the air to various solids and liquids. Think about what happens in the air itself.
What happens when you put a soda bottle containing air, with the cap screwed on, in the freezer?
What happens when you plunge it into a hot (~80°C) water bath?
In your journal: Spend a few minutes reflecting on the system of the bottle, the air inside it, and the air around it. In each case, how is heat transfer involved?
- Think microscopically: How will the energy of motion of the particles in the bottle change?
- Think macroscopically: Describe any changes you might expect in the volume, the mass, and the temperature of the air.
Screw the lid tightly onto a soda bottle, and put it into the freezer for 10 minutes. When you take the bottle out, feel it, squeeze it, open the lid and listen to it.
In your journal, record your observations of any possible changes in:
- The mass
- The volume
- The temperature
Then open the bottle, squeeze it to reduce the volume a bit, put the lid back on, and plunge the bottle in a hot water bath, or place it under hot running water, for a couple of minutes. When you take it out, record your observations of changes again.
C. Take a molecular perspective
Imagine you have a pair of magic eyeglasses that allow you to observe these changes on a microscopic scale. What would you see? First, note your thoughts in your journal.
Then, use the PhET Gas Properties Model to explore relations between temperature, and volume. This model is provided courtesy of the Physics Education Technology (PhET) Project at the University of Colorado.
Pump the handle once to add air molecules to the chamber and wait until the system equilibrates. (The temperature of the gas will be 300°K.) Select "Pressure" as the Constant Parameter and then the "Heat Control" switch to transfer heat to or from the chamber. Note any changes in the mass, the volume, the temperature, and the pressure.
- What is the effect when heat is transferred from the chamber?
- What is the effect when heat is transferred to the chamber?
Use your molecular tests to help you explain the results of your soda bottle investigation.
D. More to Explore - How do pressure, temperature, and volume relate?
NOTE: Pressure = force/area - For more information about pressure in a gas, you can refer to Chapter 14, pp. 267-276 in the 9th edition of Conceptual Physics.
In a gas, heat transfer and temperature change often go hand in hand with other striking changes - changes in volume, and in something else: pressure. You can use the PhET model to explore these changes.
Take the case of your car tires. When you pump air into a tire, the air applies a pushing force over the surface area of the tire, applying a pressure to it. At the same time that the air pressure increases, the mass of the air in the tire increases. When those changes are rapid, the temperature of the tire temporarily changes.
Click on the graphic below to view an animation of pressure increasing in a tire as it is inflated.
Explore cause and effect relations between pressure, temperature, mass, and volume using the PhET Ideal Gas Molecular Model.
Consider the following scenarios:
- What happens to the air in a tire when you pump it up? When you deflate it? Set the volume constant, and add air. Then open the lid to let air out.
- What happens to the air in a jar with a screw-on lid when it's placed in ice water and then warm water? How would you model that? What parameter would you set constant?
- What happens to air when a piston is pulled back? When it is pushed in? You can pull and push the handle on the chamber to change the volume.
Change just one variable at a time, and record its effect on the others. Use the Cause and Effect Relationships Among Pressure, Volume, Temperature, and Mass Chart to organize your thoughts and your findings.
To think more about pressure and volume relations from a molecular perspective, read pages 273-274 in Conceptual Physics. As you read, note any insights you gain about your investigation.
Post any questions you have about your explorations with gas in the science forum.
Heat and temperature in your life (~2 hrs.)
This assignment provides an opportunity to revisit this semester's science experiences and bring together evidence of how your understanding has evolved over the past few months.
How have your experiences in this course changed the way you understand some aspect of heat and temperature in your life?
You've investigated only a handful of phenomena involving heat and temperature in this course, but we hope that the tools and concepts you have worked with have enabled you to inquire more deeply about heat and temperature phenomena that are all around you every day.
Throughout this course you've kept a running log of your encounters with heat transfer in the Heat Transfer in Your Life section of your journal. Set aside a quiet hour to spend with your journal, reviewing your encounters. As you read, choose 3 examples that interest you. Record any insights and questions about these examples that you may have in view of your subsequent course work.
Then, select just one example to report on.
- How has your understanding of this phenomenon changed?
- What questions about it would you like to continue to explore?