Transcript Convection - S3 amazonaws com

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 group
1 Chimney Apparatus
1 Thermometer Strip
1 Styrofoam cup containing Dry Ice
*Tell students that dry ice is extremely
cold
III. Convection Currents in Gases
(Cont.)
1.
2.
3.
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.
Explain that “Fog” = cold mixture of CO2 gas, water
vapor, and air. The white color is from the condensed
water vapor.
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.
5.
Distribute chimneys to the groups.
Place the metal box on a surface away from
combustibles
6. Insert chimneys into metal box
7. Tell students to take the temperature 2 inches above
each chimney and to record this
8. VSVS members should light candle and place under
left hand chimney
9. Slide glass window into place
10. 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.
2.
3.
4.
5.
Tell students to hold the dry
ice cup 6 inches above the
left (hot) chimney. What is
the direction of the fog?
Hold the cup 2 inches
above the chimney. What
happens now?
Hold the dry ice cup over
the right hand chimney and
“pour” cold air out of it.
What path does the fog
take? What happens to the
candle?
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