Heat Transfer - Convection Part 1

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Objective

SWBAT use a variety of models to explain transfer by means of convection currents.

Big Idea

This series of mini labs helps to introduce students to the complexity of convection.

Getting Started

In this series of investigation developed by Ann Bykerk-Kauffman at the University of Chicago, students explore convection through different models.

She identifies the following goals:

By the end of this lab lab, your students should be able to…

  1. define density, buoyancy and convection.
  2. describe how density affects buoyancy.
  3. describe how and why temperature affects density.
  4. explain how, why and under what conditions convection happens.
  5. relate how convection serves as an effective mechanism for transporting heat energy.

What I like most about these investigations are that they keep pushing student thinking back to the same phenomena by means of different models. I added the split demo tank activity at the beginning to grab kids' attention. They really love watching the hot and cold water mix. If you are unable to acquire the split demo tank, feel free to improvise with an empty aquarium or other water tank. You can also use fresh (tap) water and salt water as an alternative (here's a good online resource for this alternative). 

Most of the materials are easy to come by. 

You wil need:

  • Split Demo Tank (mine is from Spangler Science)
  • 1 clear plastic bottle containing corn syrup (light colored) and SAE 50 Motor Oil (dark), turned upside down. 
  • Food coloring (blue, yellow, green, red)
  • small clear glass bottle filled with green-colored water, capped with a rubber stopper that has a glass eye dropper inserted into the hole
  • overhead transparency pen (water-soluble)
  • 2 large (1000 ml) pyrex beakers
  • hot plate
  • crushed ice
  • 2 clear cylindrical “pill bottles,“ each with two holes in the cap
  • stirring stick
  • red, yellow, green, and blue colored pencils

Engage

15 minutes

SPLIT DEMO TANK

Begin with an empty tank in front of the class along with two smaller tubs (about a gallon each) of water, one cold blue and the other warm yellow water. If you have not used these tanks before they have a removable center divider that keeps the liquids separated until you are ready to mix them together. You can use hot/cold water or salted/unsalted water. 

Fill one side of the tank with warm water and the other with cold water. There is no need to heat the water, just use the hottest water from the tap. In order to keep one color from leaking into the opposite chamber it is important to pour the two colored liquids in their separate chambers at the same time. The pressure from the two liquids will push against the center divider and prevent the colors from leaking until you pull out the divider. 

Take a moment to ask the students to (silently) predict what they think will happen when the liquids are free to mix. Will both liquids mix to make green? Will the liquids form two layers? Which color will be on top?

Have them open their journals and draw their before and after predictions. You could have them think about what will happen a few seconds after mixing and then 30 minutes after mixing. This requires them to dig more deeply into their own understanding, rather than making a quick, less thoughtful, prediction.

OK, when you are ready, remove the divider and watch the show!  

Explore

20 minutes

In the next part of this activity students observe two fluids of different density that will not mix (motor oil and corn syrup) to help them visualize and explore the relationship of buoyancy and density.

Students are given their bottle (pre made) and are instructed (by the Convection Investigation Lab Sheet) to turn it over and observe what happens.

Before they begin, it is critical to demonstrate how to look at these bottles. I model (without actually doing it with a real mixture) removing the bottle from the rack, holding it up and observing it carefully (just as it is when it is removed) and then slowly turning it upside down, watching carefully throughout the process. The bottle is returned to the rack in this new orientation because students need to make a second observation after some time has elapsed. You want them to notice the movement of the fluids around each other. This don't just switch places. There is movement to their displacement. 

They record this exchange by drawing on the lab handout. 

Explain

20 minutes

Be sure to preview the follow up questions and be ready to talk to your students about buoyant forces. 

  1. Motor oil and corn syrup have different physical properties such as color, clarity, odor, density, mass, volume. Which of these properties determines the buoyancy of the fluid?  Explain
  2. Combining your answers to questions 2 and 3, explain which of the two fluids is more buoyant and why.
  3. If we took this bottle of corn syrup and motor oil up in space where there is essentially no gravity, how would the results be different? Why?

In the video below I demonstrate how I explain buoyancy to my students for the first time.