Atmospheric Processes — Convection

Modified from Convection: A current event from the Lawrence Hall of Science, Great Explorations in Math and Science. Copyright permission pending.

In this two-part activity, students will observe convective currents in water and observe the air as a fluid.

Background

Heat moves in fluids through several processes, including convection. Convection is the transfer of heat by the actual movement of the heated material.

Any substance that flows is considered a fluid. This includes such things as water, shampoo, sunscreen, and even honey. Although not necessarily obvious, even gases, such as air, can be classified as fluids.

Consider what happens to the water in a pot as it is heated over an open camp stove.

The water at the bottom of the pot heats up first. This causes it to expand. Since the warmed water has a lower density than the water around it, it rises up through the cooler, dense water. At the top of the pot, the water cools, increasing its density, which causes it to sink back down to the bottom. This up and down movement eventually heats all of the water. The continual cycling of the fluid is called a convection current.

Convection currents are found in many places and on many scales, from huge convection currents in the atmosphere, oceans, and even in the earth's interior to smaller convection currents found in a cup of hot cocoa or a fish tank. Meteorologists usually use "convection" to refer to up and down motions of air. Heat gained by the lowest layer of the atmosphere from radiation or conduction is most often transferred by convection.

Convective motions in the atmosphere are responsible for the redistribution of heat from the warm equatorial regions to higher latitudes and from the surface upward.

In this two-part activity, students will observe convective currents in water and observe the air as a fluid.

Learning Goals

Part 1

1. Students will understand that temperature changes can cause density changes in water.
   
   
2. Students will be able to express that the same phenomenon happens in air.
   
   
3. Students will understand that temperature-driven density changes will produce currents in a fluid medium.

Part 2

1. Students will understand that air is a fluid and behaves in ways we expect of other fluids.
   
   

Alignment to National Standards

National Science Education Standards

• Physical Science, Transfer of Energy, Grades 5 to 8, pg. 155, Item #2: "Heat moves in predictable ways, flowing from warmer objects to cooler ones, until both reach the same temperature."
  
  
• Earth and Space Science, Grades 9 to 12, pg. 189, Item #3: "Heating of earth's surface and atmosphere by the sun drives convection within the atmosphere and oceans, producing winds and ocean currents."

Benchmarks for Science Literacy, Project 2061, AAAS

• The Physical Setting, Energy Transformations, Grades 6 to 8, pg. 85, Item #3: "Heat can be transferred through materials by the collisions of atoms or across space by radiation. If the material is fluid, currents will be set up in it that aid the transfer of heat."
  
  
• The Physical Setting, The Earth, Grades 9 to 12, pg. 70, Item #2: "Weather (in the short run) and climate (in the long run) involve the transfer of energy in and out of the atmosphere. Solar radiation heats the landmasses, oceans, and air. Transfer of heat energy at the boundaries between the atmosphere, the landmasses, and the oceans results in layers of different temperatures and densities in both the ocean and atmosphere. The action of gravitational force on regions of different densities causes them to rise or fall - and such circulation, influenced by the rotation of the earth, produces winds and ocean currents."
  
  

Grade Level/Time

• Grade level: 5 to 9
  
  
• Time:
  
  
  • Part 1
    
    
    • Teacher introduction/instructions: 15 minutes
      
      
    • Student activity: 30 minutes
      
      
  • Part 2
    
    
    • Teacher instructions: 10 minutes
      
      
    • Student activity: 20 minutes
      
      

Part 1: Currents in Water

Materials

For the class:

• Pitchers or jugs for water
  
  
• A source of very hot water, such as an electric teakettle, hot plate and regular kettle, or coffee maker (hot tap water is usually not hot enough)

For each team of students:

• Clear plastic plant saucer 8 to 10 inches wide. Caution: DO NOT use saucers with concentric raised rings on the inside bottom; radial ridges are okay
  
  
• Pitchers or jugs for hot and cold water
  
  
• Food coloring
  
  
• Small container for food coloring (a small cup would work fine)
  
  
• Medicine dropper or pipette
  
  
• Four Styrofoam cups
  
  
• Data sheet
  

Procedure

1. Tell the class that they will conduct several experiments to trace the currents when water is not evenly heated.
   
   
• In the first experiment, they will observe the movement of a drop of food coloring in still water.
  
  
• Then they will add a heat source and drop the food coloring in different places in the saucer.
  
  
2. Place three styrofoam cups upside down on a piece of paper.
   
   
3. Place the plastic plant saucer on top of the cups as shown. The cups should be near the outer edges of the saucer and evenly spaced.
   
   
   
   
4. Fill the plastic saucers three-quarters full with cool water. To make certain the water is still, let it sit before the experiment. Be careful not to bump the desk or table at any time during the experiment.
   
   
5. Using a dropper, slowly release a small amount of food coloring at the bottom of the saucer of water. Slowly remove the dropper, taking care not to stir the water.
   
   
6. Observe and record on the data sheet what the drop does as it sits in the tray. Ask students to draw what happens.

Variations

Repeat the experiment with the following variations. Ask students to record their observations after each variation. Make certain they start each trial with a clean saucer of water. Having several dump buckets placed around the classroom will be helpful.

For the following three trials, place a cup of hot water under the center of the saucer as shown. Fill the cup almost to the top.

Trial A: Place a drop of food coloring on the bottom of the saucer in the center, over the cup of hot water. Take care not to stir the water.

Trial B: Place a drop of food coloring on the bottom of the saucer about halfway between the center and the side. Take care not to stir the water.

Trial C: Place two drops of food coloring on the bottom of the saucer, one halfway between the center and side of the saucer, the other in the center. Take care not to stir the water.

Observations and Questions

1. For each trial, ask students to draw their observations from an overhead view, including an explanatory caption for each drawing. The drawings should show the movement of the colored water (currents) and its relationship to the hot water or heat source for convection.
   
   
2. Have students repeat the exercise, showing their observations from a side view.
   
   
3. What effect does the hot water in the center under the saucepan have upon the currents?
   
   
4. What type of heat transfer is taking place? How do you know? (Convection is occurring because we can observe the movement of the colored water within the liquid. Convection transmits heat through the movement of molecules.)
   

Preliminary Exercise

If you have access to a large aquarium, do the following preliminary exercise to demonstrate the concept of density and temperature in water.

1. Fill the aquarium with cool tap water and allow it to settle.
   
   
2. At each corner, attach a foam cup to the inside of the aquarium so that the bottom of the cup is slightly above the surface of the water.
   
   
   • Fill one cup with ice water (use food coloring to dye it purple).
     
     
• Fill the second cup with very hot water (dye it red).
  
  
• Fill the third cup with warm water (dye it yellow).
  
  
• Fill the fourth cup with cool water (dye it blue).
  
  
3. Ask students to predict (orally or in writing) what will happen if you let water from all four cups drip into the aquarium.
   
   
4. Then use the sharpened tip of a pencil to punch a SMALL hole in the bottom of each cup and allow the liquid to drip into the aquarium water.
   
   
5. In a few moments, an impressive layering of the water will be observed.
   
   

Part 2: Air is a Fluid

Materials

• Baking soda
  
  
• Vinegar
  
  
• 500 ml beaker or glass jar of similar size
  
  
• Candle (a small votive candle is ideal)
  
  
• Matches
  
  
• Strip of poster board or cardboard about 12" by 3" (old file folders work well)
  

Procedure

1. Discuss the physical properties of a fluid with students. Be sure to include the idea that fluids can be poured. Ask students if they think air is a fluid. Ask how it could be demonstrated.
   
   
2. Fold the poster board or cardboard lengthwise.
   
   
   
   
3. Place the candle on a plate and light the candle.
   
   
4. Put about a tablespoon of baking soda in the glass jar or beaker.
   
   
5. Pour about 1/4 cup of vinegar in the jar or beaker. (The vinegar and baking soda will react immediately filling the jar with carbon dioxide gas.)
   
   
6. When the fizzing subsides, hold the poster board "funnel" at an angle so that one end is near the candle flame and the other end is slightly higher.
   
   
   
   
7. "Pour" the gas in the beaker or jar down the funnel. The flame will go out in a second or two.

Observations and Questions

1. What happens when the vinegar and baking soda are mixed? (The mixture froths and bubbles, producing carbon dioxide.)
   
   
2. Explain how the flame was extinguished. (There was no more oxygen available for the flame, so it went out. Pure carbon dioxide is denser than air, so it flows like a liquid from the jar or beaker along the funnel. Carbon dioxide is used in fire extinguishers because it is effective at smothering flames.)
   
   

Assessment Ideas

• Ask students to imagine a situation similar to Part 1, except this time, a cooling unit is substituted for the heating unit.
  
  
• How would the circulation look? (Cold water should flow along the bottom of the saucer until it nears the warmer sides. Then it will rise up. As it reaches the top, the water will cool and sink, drawing water from the sides in toward the center.)
  
  

Modifications for Alternative Learners

• Limited language students should be able to rely on drawings and diagrams for explanations.