Convection Currents

Vanderbilt Student Volunteers for Science Fall 2011

Pre-Lab

• Divide Students into 8 groups

• Fill the Styrofoam cups 1/3rd of the way with dry ice.

• Assemble the chimney apparatuses – DO NOT PASS OUT UNTIL: Colliding Warm

Air and Cold Air Masses

• Have a group member write “Convection” on the board

I. Introduction

• Ask, What do you know about convection?– Convection is the transfer of energy by a moving

liquid or gas– Heat moves by convection currents in liquids

and gases– Ex. Convection Oven, Hot Air Balloon

I. Introduction (Cont.)

• Ask, Who knows how a hot air balloon stays up? – From www.eballoon.org: Hot air is propelled

towards the balloon. As it rises, it stays trapped in the balloon and causes the hot air balloon to rise. The pilot must keep firing the burner at regular intervals to ensure that the balloon continues to be stable. The hot air will not escape from the hole at the very bottom of the envelope because the hot air rises.

I. Introduction (Cont.)

• But, How does a hot air balloon land?– The ‘Parachute Valve’ at the very top of the

balloon is what is used to bring the balloon down towards the ground. If the pilot wants to bring balloon down he simply pulls on the chord which will open the valve, letting hot air escape, decreasing the inner air temperature. This cooling off of air causes the balloon to slow its ascent.

II. Liquid Crystal Thermometers

• Before handing out strip thermometers:• Ask, Have you ever seen anything that

changes color with temperature?– Answers may include: mood rings, strip

thermometers– Students used strip thermometers in 5th

grade lesson last year• Show the students the strip

thermometer– The Dark Blue temperature is room

temperature– Write on the board the room temperature

• Measurements will be compared to this number

II. Liquid Crystal Thermometers (Cont.)

• Hand out 1 strip thermometer per group– Tell Students to hold the strip gently at the top

edge of the strip– DO NOT clasp in their hand!

• Tell students to record the temperature of their strip thermometers on their observation sheets– As temperatures change, so do the colors of

the temperatures– Temperatures may not be the same, but are

close and accurate enough to measure changes in temperatures

III. Convection Currents in Gases

• Draw a diagram of the chimney apparatus on the board– One board member should write

temperature changes and air flow patterns on the diagram as the experiments are conducted

• Hand out to each group1 Chimney Apparatus1 Thermometer Strip1 Styrofoam cup containing Dry Ice

*Tell students that dry ice is extremely cold

III. Convection Currents in Gases (Cont.)

1. Give each group a cup of dry ice. Tell them not to touch the ice – it’s cold! Have the students tilt the cup sideways so that they can see “fog” flowing out of the cup.

2. Explain that “Fog” = cold mixture of CO2 gas, water vapor, and air. The white color is from the condensed water vapor.

3. Fog flows down toward the ground because it is colder and denser than the warm air surrounding it. Set dry ice aside while steps 4-11are completed.

III. Convection Currents in Gases (Cont.) 2

4. Distribute chimneys to the groups.5. Place the metal box on a surface away from

combustibles6. Insert chimneys into metal box7. Tell students to take the temperature 2 inches above

each chimney and to record this8. VSVS members should light candle and place under

left hand chimney9. Slide glass window into place10. Tell students to record the new temperatures 2

inches above each chimney. Discuss any changes in temperature.

IV. Colliding Warm Air and Cold Air Masses

1. Tell students to hold the dry ice cup 6 inches above the left (hot) chimney. What is the direction of the fog?

2. Hold the cup 2 inches above the chimney. What happens now?

3. Hold the dry ice cup over the right hand chimney and “pour” cold air out of it.

4. What path does the fog take? What happens to the candle?

5. Tell the students that warm air rises and cold air sinks.

V. Observing Convection Currents in Liquids

1. Hand out to each group 1 jar of rheoscopic fluid, 1 heat pack, 1 plate, and 1 ice cube.

2. Tell the students that fluids follow same pattern as gases: hot fluids rise and cold fluids sink.

3. Gently shake the jar, then open it, insert the ice cube, and then close it tightly.

V. Observing Convection Currents in Liquids

4. Observe the motion of the liquid

5. Activate the heat pack and place the jar on top of the heat pack. Keep the jar as still as possible!

6. Observe the motion of the liquid.

7. Draw the motion on the board and have the students draw the motion on their observation sheets.

VI. Conclusion

1. Review: Do convection currents in liquids follow the same pattern as convection currents in gases?

2. Pack up – careful to pack chimneys so that glass doesn’t break