WILL THEY FLY?
Plastic Hot Air Balloon Lab
How Gases Respond to Heat
In the Split-Level Candle Lab, why did the CO2 rise?
Density = Wt / Vol
• Heat
• Expansion
molecular motion
Volume =
•Lower density CO2 will rise.
expansion of gas.
Density
How Gases Respond to Heat
Air inside a heated can………
• Expansion occurs, pushing molecules of air out of can.
•Volume of air inside can is same.
•But with fewer molecules….
•Weight of the air inside can goes down.
•Weight drops, so Density drops.
Hot Air Balloon is like the can….
• The volume of a filled balloon is pretty much
fixed.
• So, for a balloon to rise, what has to occur?
• The weight of the heated air has to be low
enough to lift the balloon up.
What is necessary for a hot air balloon to rise?
• Hot air
– Hot air has to at a lower weight (lower density)
• Must be made of Lightweight materials
– How lightweight?
– As a math equation:
• Balloon weight + HotAir weight < OutsideAir weight
Your Mission
1. Build a hot air plastic balloon.
2. Weigh the materials of you balloon.
3. Determine the weight the heated air inside
the balloon compared to the weight of the
outside air
4. Calculate the lift capacity of your balloon to
determine if it will fly.
Finding the Weight of Air at Different
Temperatures
• Charles Law
– With a change in temperature there is a
proportional change in the weight of air.
- Once we know the proportion or ratio, we can find
the weight or air at any temperature.
Finding the Weight of Air at Different Temperatures
175o F provides a 1/1 proportion
FACT: 1 cubic foot of air at 175o F weighs 1 oz.
-This fact helps set our proportions
The Rankine Scale provides the correct
number scale
Rankine Scale Explained
Absolute Zero :
• 0o Kelvin (-273.15oC) is where all molecular
motion stops. Coldest possible temperature.
• 0o Rankine (-460oF) = 0o Kelvin
• Kelvin is based on Celsius scale
• Rankine is based on Fahrenheit scale
• To convert from Fahrenheit to Rankine – add 460
175o F provides the 1/1 proportion.
The Rankine Scale provides the number scale.
Establishing “635” Factor
• Because the change in weight of air is proportional to
change in temp…
• Because 1 cubic foot of air at 175o F weighs 1 oz…
• Because the Rankine scale is the correct number scale…
• Hence 635 becomes our “base” factor…
• “635 Factor”
• Dividing the Rankine temperature of the measured
air into “635” will give you the weight of 1 cubic ft of
that air (in ounces).
EXAMPLE : Weight of Air at 72OF
• Convert Fahrenheit temp to Rankine temp
– 72 + 460 = 532
• Divide converted temp into 635 (“635 factor”)
– 635 / 532 = 1.19 ( SO, 1 ft3 of air weighs 1.19 oz)
635 Factor to Find the Weight of Air
To find the weight of a cubic foot of air at a
particular air temperature:
• Convert that temperature to the Rankine
Temperature Scale
• Divide that number into 635 Factor
– (635 / Rankine Temp)
• The result is the weight of 1 cubic foot of air at
that temperature (in ounces).