Old-Fashioned Cooling (~2 hrs.)
Before the days of modern thermos technology, folks sometimes used cloth-covered canteens. They soaked them in water to keep their drinking water cool. How did these old-fashioned canteens work?
To investigate, consider how the temperature might change in a model canteen: a plastic cup and lid represent the canteen and a paper towel represents the cloth cover. How much of a difference do you think it makes to have a cover that's wet?
If you wrap a thoroughly soaked, room-temperature, paper towel around a glass beaker of room-temperature water, how do you think the temperature will change over the next 10 minutes in the water in the cup? What would happen in the following 10 minutes if a breeze came up? (In the model system, you could use a fan to create a breeze.)
For comparison, what will happen if you wrap a dry paper towel around an identical beaker of room-temperature water?
Draw prediction graphs for the temperature in the 2 glasses of water, showing the beginning temperature, the final temperature, and the shape of the temperature vs. time curves. Draw a diagram to help you think about where and how energy is transferred, and record your rationale for your predictions, telling the story of the energy in each water/cup/towel/air system.
B. Collect data
Use your probes to collect data from the model wet canteen and the comparison cup. (A warm, dry spot works best --- a sunny spot in the desert in summer would be great.) You can tuck an aluminum backed thermometer into the paper towel in each case to get an idea of the temperature there.
Measure the temperature for 10 minutes. Then place your fan (it could be a hand-made one from folded paper) a foot away from the cups, turn it on, and continue to monitor the temperature for another 20 minutes.
Record details of your test set-up (air temperature, humidity, fan, lighting, drafts, etc.), your observations, and questions.
Tip for Logger Lite
Click and drag the y-axis to stretch it in order to see small variations of temperature change.
C. Take a molecular perspective
Imagine you could zoom in on the system you're investigating until it is magnified a billion times and you can observe evaporation on a particle scale.
Here is a quote from Conceptual Physics that describes evaporation and energy transfer:
Water in an open container will eventually evaporate, or dry up. The liquid that disappears becomes water vapor in the air. Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid.
The temperature of any substance is related to the average kinetic energy of its particles.....Molecules at the surface that gain kinetic energy by being bumped from below may have enough energy to break free of the liquid. They can leave the surface and fly into the space above the liquid. In this way they become molecules of vapor....Thus, the average kinetic energy of the molecules remaining in the liquid is lowered --- evaporation is a cooling process.
In your journal: Keeping Hewitt's description of evaporation and energy transfer in mind, consider the cup/water/towel/air system. Use a molecular perspective to explain any temperature change in your model canteen in terms of molecules and evaporation.
D. Interpret, make sense, and explain
Reflect on the data you have collected. In your journal, take notes about the following:
- Compare the two temperature over time curves. What evidence can you find for heat transfer due to evaporation in your temperature data?
- In view of your observations, the temperature data, and your work from a molecular perspective, describe where and how you think that energy moves among the components of each system. (What part do conduction, convection, radiation, and/or evaporation play?)
- Go back to your prediction graph and compare it to your data. How well does the evidence support your predictions? What have you learned that would shape your prediction another time?
- Would the water in the canteen cool more quickly if the paper towel were soaked with warm water? Why or why not?
Turning Up the Heat (~1 hr.)
Compare the cooling canteen to another case where phase change occurs: consider a pot of ice that's placed on a burner and heated until first the ice melts, and then the water in the pot boils. In the canteen investigation, phase change occurred when the water evaporated. As water heats up and boils, water also evaporates.
In Session 4, you investigated how energy was transferred by convection and conduction in a beaker of water on a burner. In this case, there is a much larger range of temperature change, and the contents of the pot change phase twice - from ice to water, and from water to vapor. In view of your work with radiation in Session 8, and your work with evaporative cooling in this session, it appears that there's more than conduction and convection going on here.
Watching temperature change with time as water heats up will give you insight about how energy is transferred both to and from water every day when you make your morning coffee. The challenge is setting up a safe system to record temperature change.
If you have a heavy thermometer (e.g., a candy thermometer) or you have room on your stove to safely set up the thermometer stand and glass thermometer, fill a small pot half full of crushed ice, heat it, and watch how the temperature (and the contents of the pot) change as time changes. In your journal, record your observations every 30 seconds.
Temperature vs. Heat?
Note that in this temperature change graph, the horizontal axis shows heat, measured in calories, and not time. A calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C.
However, In this scenario, heat is continuously transferred from the burner to the water, and so time increases concurrently with heat.
The graph below shows temperature vs heat data for water. This is an idealized graph showing temperature change for just 1 gram of water. What can we learn about energy transfer from the temperature change data?
Tell the story of the graph.
Tell the story of the temperature vs. heat graph above. As you work, be aware of energy transferred to the water as well as energy transferred from the water.
- What common patterns of temperature change do you see in the graph?
- In this scenario, additional energy is transferred continuously to the contents of the pot. How does the change in temperature reflect energy transfer among the components of the system?
- Select 4 points on the curve and explain what is happening at those 4 points in time.
- Explain why you think the temperature is constant when it's constant.
Read and Report on Your Investigation (~1 hr.)
Refer to Conceptual Physics for more information about change of phase (chapter 17 in the tenth edition). In your journal, note down insights from the reading that help you to explain your investigations.
For morel information about phase change, read Are we just going through a phase? by Judah Schwartz.
B. Report on your investigation
Use your notes to write a report on your cooling canteen investigation. In your report, address the following:
- Explain the temperature change and heat transfer occurring using both macroscopic observations and a particle model. How does the "canteen" use evaporation to transfer heat? What happens to the energy in the system when phase change occurs?
- Compare your findings to your prediction. Explain any changes in your thinking about energy transfer.
- How is evaporative cooling in the canteen the same as, or different from, the pot of boiling water?