How is heat transferred by conduction?
There are two parts to your investigation today: The first involves heating water. The second involves melting ice. Heat is often transferred simultaneously by multiple modes. This week you consider heat transfer by conduction as well as convection. Your challenge is to find where heat is transferred and how heat is transferred in each phenomenon you investigate.
Explore: Where is there heat transfer when you heat blue water? (~1 hr.)
In a glass of water, heat is transferred by means of currents in the fluid itself. But that's not the whole story.
Take a look at another phenomenon involving heat transfer. Watch the video of Heating Blue Water to see what happens when a pot of cool tap water is placed on a warm burner. (To be able to see the movement of the water as it heats up, we pipetted blue water into the bottom of a beaker of clear water.)
Spend a few minutes thinking about heat transfer in the whole system of the hot plate, the beaker, the water, and the air around them. Where is heat transfer occurring? How is it occurring?
- In your journal, create a sketch of the beaker/burner/water/air system where you can explain what you think is going on and note any questions you have. Begin by using arrows and notes to show heat transfer by convection, based on your experiences with convection in Session 3.
But is there more? This week you focus on a second mode of heat transfer: conduction. Where is heat transferred by conduction in Heating Blue Water? To help you think about this, read definitions of conduction and convection from Conceptual Physics:
"Conduction: Energy transfer from particle to particle within certain materials, or from one material to another when the two are in direct contact."
"Convection: Means of heat transfer by movement of the heated substance itself, such as by currents in a fluid."
(Hewitt, Conceptual Physics)
Return to the video of Heating Blue Water and your annotated sketch. Keeping the definition of conduction from Conceptual Physics in mind, consider heat conduction in and between each of the materials in the beaker/water/hotplate/air system.
- On your sketch, use arrows and notes to show where you think heat is transferred by conduction among the components of the system.
Investigate: Which ice cube melts fastest? (~3 hrs.)
Here is a situation to investigate that will help you to think further about the ability of materials to transfer energy by conduction: If you place 3 identical ice cubes on a metal surface, a wood surface, and a stone surface, will one ice cube finish melting first or will they all melt at about the same rate?
To get started with your prediction, sketch the 3 ice cube systems (ice cube, surface beneath it, air around it). Note the starting temperature of the ice (freezer temperature) and of the three surfaces (room temperature), and use arrows to show where you think energy is transferred.
Take a particle point of view: Imagine you could zoom in on the system you're investigating until it is magnified a billion times and you "see" the particles that make up the ice cubes, the surfaces beneath them, and the space around them. Make sketches that represent your ideas about how heat is transferred by conduction. What do you think is moving? Explain your rationale.
Use your ideas about heat transfer, and your experience with these materials, to predict: Which ice cube do you think will finish melting first, second, and last? (Or will they all melt at about the same rate?) Record your prediction and rationale.
Metal: a cake pan or a frying pan
Wood: a bread board or a 2x4
Stone: a marble counter, an unglazed tile, or a flat stone
*If you don't have one of these materials at home, select another material that interests you.
Design an experiment to test the effect of changing just one variable: the material of the surface. What would you keep the same to create a fair comparison (e.g. ice cube mass and shape, contact between ice cube and surface, etc.)?
When you've set up your three surfaces, take out your ice cubes and test your ideas.
C. A Closer Look at Conduction
Heat transfer can happen quickly, and invisibly. Scientists use models to isolate salient features of a phenomenon and to generate possible explanations for them.
Use the following resources to verify and expand your understanding of what's going on.
A Molecular Workbench Model of Conduction
- What do the models show you about how heat is transferred in the case of conduction? What is moving?
- Thermal conductivity refers to the ability of a material to transfer heat by conduction. What can you conclude about which material is the most and least conductive?
D. Interpret, Make Sense, and Explain
In your journal, take notes explaining your ice melting investigation.
- In view of your test results and your work with Molecular Workbench, describe where and how you think heat is transferred by conduction at the molecular level in the metal, wood, and stone surfaces, and in the air. How does that correspond to your observations of the melting ice cubes?
- Conduction and convection often occur simultaneously in the same system. Describe where you think there is convection in the ice cube systems.
Review the section in Conceptual Physics on heat transfer by conduction and convection (pages 306-310 in Chapter 16 of the tenth edition). Add any new insights to your explanation of what happened in the ice/surface/water/air systems.
To bring together your understanding about conduction, tackle this week's Challenges.
Return to the investigation question: If you place 3 identical ice cubes on a metal surface, a wood surface, and a stone surface, will one ice cube finish melting first or will they all melt at about the same rate? Review your sketches and your notes and report on your investigation.
- Describe your test set-up and your observations.
- Use evidence from your investigations and your work with models to explain where and how you think that heat is transferred among the components of the systems.
- Look back at your predictions. In what ways, if any, has your thinking changed?