A Case of Insulation
Ms. C’s class had been studying heat transfer and temperature for several weeks. Imagine that you have dropped in on her class on the day they are investigating thermal insulation. By the end of the class, Ms. C hopes her students will understand that a good thermal insultator is a good insulator no matter which direction heat flows. An insulating wrap will slow down heat transfer from a hot liquid inside a cup to the cooler surrounding air and to a cold drink in a cup from the warmer surrounding air.
As you enter the classroom you notice this question on the board.
What insulating wrap should we choose to keep a hot drink hot and a cold drink cold?
The class is assembled; their desks have been pushed together so that groups of 4 children are seated around a “table.” Each child has a journal and pen or pencil. There are some handouts on each table. Bins of materials, one for each table, are lined up at the front of the room.
Ms. C begins.
Ms. C: On your tables you’ll find copies of today’s challenge. Take a look. (On their paper is a sketch of three children gathered near a snowman. The children are debating the idea of whether an overcoat could keep the snowman from melting.)
What do you think of each child’s point of view? In your journal, explain which child’s idea you think makes the most sense and seems the most scientific and why you think so. Do you think there are problems with some of the children’s ideas? Explain this too. Take 4 or 5 minutes working individually. When everyone in your group has finished writing, talk over your ideas and see what you agree on and where there are differences.
The room is quiet as the children study the sketch and write in their journals. Ms. C doesn’t say anything as she moves from table to table. Soon there is a hum of conversation as the groups compare their opinions of the strengths and weaknesses of the various arguments presented in the scenario. Ms. C moves from group to group, pausing to listen to the conversation and posing questions from time to time. After about 5 minutes she asks someone from each table to summarize the group’s consensus about the strengths and limitations of each child’s argument. She asks if there seems to be agreement. One student says there is some agreement but that some groups thought of things that others didn’t. For example, she says, some groups talked about whether the air temperature would make a difference. Our group forgot to consider this.
Ms. C then has her students reflect on the understandings they’ve been developing about heat transfer over the last several weeks. She uses an ice cube held in the palm of her hand to focus their discussion. The ideas they discuss are:
- What happens when objects or systems that are different temperatures come in contact;
- That heat always flows from warmer objects or substances to cooler ones; and
- We use temperatures to tell when thermal equilibrium is reached.
Then she introduces the day’s investigation.
Ms. C: Today’s investigation question is: What’s the best wrapping (insulation) to use if we want to keep a hot drink hot and a cold drink cold? The materials you can use to wrap the cups are aluminum foil, newspaper, and cotton fabric.
Ms. C: Before we begin, let’s make some predictions using the chart on the board. You’ll find stickers of different colors on your tables. Each person should take 2 red, 2 yellow, and 2 green stickers. Put your initials on each sticker. You’ll use your stickers to predict which material you think will make the best, second best, and third best wrap to keep a hot drink hot. Then, you’ll predict which you think will be best, second best and third best to keep a hot drink cold.
When I call your table group, come up and record your predictions by putting your stickers on our class prediction chart. Use a red sticker for best, yellow for next best, and green for worst.
Ms. C then asks for a few volunteers to explain their rationale for their predictions.
Micah: I know aluminum foil is good at keeping things cold because my mother always wraps things in foil when they go in the freezer.
Eva: I think foil will be best for keeping hot things hot because people cover hot dishes with aluminum foil when they take them out of the oven. I think fabric will be best at keeping cold things cold because cold will have a hard time escaping from the cup through the fabric.
Ms. C: After you make your predictions, look over the investigation sheet on your tables.
Testing the prediction
Ms. C: In the supply bin for each table you’ll find thermometers, plastic cups of various sizes, foil, newspaper, and fabric, tape, and scissors. Hot water is in the coffee urn and you can scoop cold water out of the bucket of ice water (be sure to leave the ice cubes behind).
Before starting, decide on a plan.
As a group, decide how you will make a fair test. What will be the same about each of the cups? What will be the one variable – what will vary from cup to cup? Don’t forget to include a control that you can use as a comparison. What will the control cup look like?
OK, go ahead with your planning.
In their groups, students sketch their set-ups. They discuss how to keep everything but the insulation the same so the test is “fair.”
Ms. C moves from group to group.
Ms. C: How are things going?
S: We’ve decided that the material around the cup needs to be about the same thickness to make it a fair test. The fabric seems a little thicker than the newspaper or foil but we’ve decided to wrap the cups in 4 layers of each material and leave the control cup unwrapped.
Ms. C: What do you plan to measure?
S: The temperature of the liquid inside each cup.
Ms. C: When will you start to measure and how often will you measure?
S: Well, if it’s a fair test, the water should be the same temperature in every cup when we start. Then we’ll measure every 5 minutes and see how the temperatures change.
Ms. C: How will you decide which material is the best insulator?
S: The best insulator will keep the hot water hottest. That means the temperature won’t drop very much. So the cup that changes temperature the smallest amount is the one with the best insulator.
30 minutes later, Ms. C brings the class together.
Ms. C: Everybody has a nice set of data. Some of you have been wondering how your results compare with others’. Let’s pool our data and see if any patterns emerge.
Each table reports and Ms. C records the data on a class table.
Explaining the Evidence
Ms. C: It looks like there’s a pattern emerging. In your journal, write a sentence or two that describes the pattern you see.
Ms. C: What are you finding?
Based on your results, are some materials better insulators than others? What’s happening in terms of heat transfer in each case? Look back at your predictions and compare them with your results. Were there any surprises?
S: We were surprised that newspaper insulation was best at keeping the hot drink hot and was best at keeping the cold drink cold.
Ms. C: I heard some other groups say they, too, were surprised by these results. Why is that so? Let’s wrap up with a few minutes of writing in your journal. Use your understanding of heat transfer to explain why you think the same insulation could be good at both keeping hot drinks hot and cold drinks cold.
15 minutes later . . . It’s time for lunch, and so we need to wind things up. Here is a question that I’d like you to work on between now and tomorrow: What about thermal insulation at home? How many examples of thermal insultation can you think of? (Think about windows, appliances, lunch boxes.) For homework, I'll ask you to choose one or two examples and describe the insulating “wrap.” Is the purpose to keep heat from being transferred in or out or both? What materials is it made of? How thick is it? What do you think makes it “work?” You'll have a copy of the assignment to take home with you.
Off to lunch now!