4. Convection in a Pan - Lesson Plan
Hot air balloon demo
- Start class by asking for 4 volunteers.
- Have 1 volunteer hold the top of the pre-tested bag with paper clips attached and 2 others to hold the bottom edges open.
- Have the third volunteer turn on the blow dryer to the lowest setting and hold it under the opening. Make sure the blow dryer isn’t too close to the opening that it melts the bag or overheats your volunteers hands.
- As the bag fills, have the other students predict what will happen.
- When the bag is full, have the blow dryer volunteer turn the dryer off.
- On the count of three, have the other 3 volunteers let go and watch the balloon fly.
- When the bag finally comes back down, discuss what happened. Some questions to consider include:
- Why did the bag fly? What powered it?
- How is the hot air balloon the same or different than a helium balloon? Was helium used?
- What would happen if we used a regular fan blowing room temperature air into the bag? Why?
- What would happen if we let an air conditioning vent fill the bag? Why?
- Where is it hottest above a fire – directly above the flames or an equal distance to the sides of the flame? Why?
- What happens to the steam above a pot of boiling water? Where does it go? Why?
- If hot air rises, what do you think will happen to cold air?
- Do you think this happens in a liquid? Do you think hot water will rise among cooler water? How about in a solid?
Convection in a pan exploration
- Tell students that they are going to watch what happens when a candle is placed under a pan of soapy water. Get the students into groups and assign each a work area to assemble around.
- Have one member of each group get a pie pan and the other needed materials.
- For the candle version of the activity, spread out the 4 film canisters on the table. Place the pie pan on top. It should look like a circular table on peg legs.
- Go around to each group and fill each pan a little more than half full with soapy water. Tell the students what is in the water to make it pearly. Ask them not to touch the surface so that the liquid can settle and fluid motion can cease.
- Pass out the handouts. Tell students that they are going to light the candle and put it under the pan. Something will happen to the liquid. Explain that their job is to carefully watch the liquid and draw arrows on the diagrams in the handout to show how the liquid is moving in different places.
- When the water in the pans are still, have students light the candle and slip it underneath the pan, right in the center.
- Have students watch what happens. The food coloring is a tool that can help them figure out what is going on by placing a drop in different places around the pan and watching how the coloring moves away from that spot.
- Let the students experiment, discuss and draw for up to 10 minutes then blow out the candles. After 10 minutes or so, the water in the pan will begin to get too hot and the pearlescent molecules (glycol stearate) will begin to break down.
- Give students a little more time to finish their drawings.
- Optional: See the variations suggested in the going further section.
- Clean up and dispose of the soap solution down the sink (or save it for the next class).
- Discuss the students’ observations of convection currents together as a class. Together, draw 3 different views of the fluid motion on the board (top view, bottom view, and side view.
- Mantle Convection: Figure 32 from "This Dynamic Earth". Image courtesy of the USGS.Relate this activity to what the students saw with the hot air balloon. Hot air rises. So how does that relate to what is going on in the pie pan? My students found it helpful to label the side view drawing of the pie pan convection cells with labels describing what is happening at every place. (For example, label the water right above the candle in the center “Water near candle flame gets hot.” Label an arrow rising from the candle to the surface “Hot water rises.” Etc.)
- Discuss what is going on in the mantle of the Earth. On the board, draw a cross section through the Earth like the diagram shown. Relate this picture to the convection cells observed in the pie pan. Show how the super hot core of the Earth is like the candle heating the mantle above it until the hot mantle rises towards the surface of the Earth.
- Optional: Discuss other examples of convection in the Earth’s atmosphere and oceans.