Ice Water (~ 1 hr.)
To begin this week's investigation, consider a familiar, everyday glass of water. Does an ice cube melt faster when it's floating at the top of the water, or when it's forced to the bottom?
Watch the videos of Melting Blue Ice. Compare what happens to the ice and the water in the two glasses. In each case, how do you think that energy moves?
This is a case of convection, that is a "means of heat transfer by movement of the heated substance itself, such as by currents in a fluid" (Conceptual Physics). Keeping the definition of convection from Conceptual Physics in mind, describe where and how you think that energy moves among the components of the two ice water systems.
Convection: Means of heat transfer by movement of the heated substance itself, such as by currents in a fluid.
Hewitt, Conceptual Physics
Below are some questions that may help you formulate a description. In each case:
- What exactly is the system (e.g., the boundaries and components)?
- Do you think the temperature difference between the water and the ice is important?
- What do you think are all the factors that affect how the energy moves?
Speeding Up Heat Transfer (~3 hrs.)
The movement in a glass of ice and water is one example of heat transferred by the "movement of the heated substance itself." This is a case of density driven convection. Heat can also be transferred by the forced movement of a fluid (liquid or gas), for example when coffee is stirred with a spoon or when your furnace blows hot air through your house. Heat can also be transferred from an object by the movement of the fluid surrounding it, for example when a fan blows hot air away from an engine.
With convection in mind, consider this situation in the kitchen: The baby’s crying with hunger and her soup is too hot. You’ve got some ideas about how to cool the soup – but what is the fastest method? How much can you cool the soup in 10 minutes?
This week you resolve the hot soup question by investigating a cooling strategy using convection, and report your findings to your group. While you could use any number of things to stand in for the soup, the easiest material to use for multiple explorations will be cups of hot water.
A. Plan an Investigation
Begin this week's investigation by brainstorming and exploring ideas for ways to change the temperature of hot water using convection.
Take 20 minutes to explore your ideas about ways to optimize cooling of a cup of hot water in 10 minutes that rely primarily on convection. List four different strategies. Then, select 2 strategies that interest you, and design a fair test to compare them.
- Does convection in a water environment (e.g., a bowl of water) cool the hot water faster than convection in an air environment (e.g., the air around the counter)?
- Is heat transferred more quickly when there's stirring or no stirring?
We've put a few constraints on this investigation:
1. Keep the hot water in identical cups (e.g., don’t pour it into a baking sheet).
2. Don’t dilute or dissolve anything (e.g., no adding ice or cold water).
3. Only use other items that are at room temperature (e.g., don’t put the cup in the fridge).
4. Allow the test to run at least 10 minutes, or until thermal equilibrium is reached if you have time.
In your notebook, a.) write your investigation question, b.) describe the systems (e.g., the boundaries and components) that you will investigate, and c.) describe your experimental design.
Sketch prediction curves showing change of temperature with time for each of the two cases. Be sure to predict the starting temperature, the final temperature, and the general shape of each curve.
Use words and sketches to describe how you think that heat is transferred in each case.
You've got your probes. Test your predictions about temperature change for the two strategies.
More to Explore?
As you investigate, you may be surprised by unexpected results. This is where the fun in science lies! You might try following up your surprises, as this investigator did:
“The cup without stirring transferred heat much more quickly than the cup with stirring. What’s going on??? Is heat transferred more quickly when there's no stirring? Perhaps, BUT the two cups were in different locations, and perhaps the change in temperature was caused by a draft near the cup that wasn't stirred. The movement of the air in a draft wasn’t one of my 'cooling by convection' ideas, but I need to distinguish between these two theories. To see if it’s true that a draft matters, I need to test and compare still air and air with a draft.”
D. Interpret, make sense, and explain
Now analyze your data, looking for patterns.
Analyze each temperature over time curve separately, and then compare them qualitatively over different time spans - the first few seconds, the first minute, the entire time span of data collection.
- Compare the patterns of temperature change.
- Compare the temperature.
- Compare the rate of temperature change (the change of temperature per minute).
We don’t have a molecular model that displays convection to show you, but imagine you could zoom in on the systems you're investigating until they are magnified a billion times and you "see" the particles that make up the materials. What would you see?
- How is heat transferred at a molecular level? How does that correspond to changes in the temperature graphs?
- When thermal equilibrium is reached, what’s not changing with time?
Finally, read the section in Conceptual Physics on heat transfer by conduction and convection (pages 306-310 in Chapter 16 of the tenth edition). As you read, jot down thoughts and questions in your journal. How do these ideas relate to your investigation? What new questions does the reading raise?
To bring together your understanding about heat transfer by convection, tackle this week's Challenge.
Report on your investigation. Don't forget to tell your colleagues:
- What was your investigation question and the system you investigated, and how did you set up a fair comparison?
- What does the data tell you about heat transfer in the two cups of water? Describe your temperature data, and describe your ideas about heat transfer.
- Compare your prediction to your data. What have you learned that would shape your prediction another time?
- Go back to the soup. Based on your results, what's the best way to use convection to cool a cup of soup? Explain your reasoning.