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ExposeExciteIgniteMay2012

1. Balloons with

1. Balloons with Messages [] A message is attached to the H2 balloon that will faithfully float and fall with it as the balloon pops. If someone finds it and takes the trouble to call, then you will know how the winds have blown on the launch day. You may want to place a map and pins in the classroom to record the locations of the balloons found. What you will need • Balloons filled with hydrogen. You could fill them with lab grade H2 or He (but that would be so boring!), or prepare your own as indicated in the previous chapter • Masking tape • Red or yellow cardboard • Pen, scissors Here’s how 1. A red or yellow note (for maximum visibility) (Fig. 4.1) is attached to the balloon’s string with masking tape (Fig. 4.2). Scientific Experiment If found, please phone John Smith and advise on the location Tel: 98989898 Figure 4.1 2. If the balloon does not float with the attached note, then increase either its gas volume, remove some excess rubber from the balloon's lip or cut the note smaller. 3. Check the forecast wind direction and strength with the local weather bureau. Launch the balloon and make observations as it soars (Figure 4.2). 4. From phone calls and visual observations students can draft a table with data: • How many seconds pass until the balloon is out of sight? • Does this period change during the day? (cool morning / hot afternoon) • What is the direction of upper air currents? 35

• The distance a balloon is found from the launch site. Teaching Extension Students might be surprised to learn that all soaring balloons return . . . very soon . . . popped! No exception. Why? As elevation from sea level increases there are exponentially fewer and fewer air molecules. The air gets thinner (air pressure falls) the higher the balloon goes. Most people know that additional oxygen is required in the ‘thin’ air of Mount Everest at an altitude of 8,848 meters (29,030 feet). Figure 4.2 There are some remarkable facts about air pressure: The air pressure at an altitude of 10 km (32,810 ft) is only 26.2% of that at sea level. No wonder commercial airliners fly at this altitude to reduce air friction. But in exchange they have to supply air under pressure to the cabin. At an altitude of 30 km (98,430 ft) the air pressure drops to only 1.1% of the pressure at sea level. This confirms that 98.9% of the earth’s atmosphere is contained 30 km from sea level. This is assumed by many scientists as the “edge” of our atmosphere. So thin, so fragile! Altitude Pressure % of 1 atm Balloon Volume dm 3 or L Table 4.1 Since P ! 1 /V when T is constant (Boyle’s Gas Law), the balloon’s volume will increase dramatically as the pressure drops (Table 4.1) and since the rubber’s stretch is limited, it pops! In a simple simulation, you can place a small inflated balloon in a vacuum flask or desiccator and suck the air from the flask with a vacuum pump. The photos in Figure 4.3 tell the story: 36 0 m 100 % 14.14 3,000 m 69.2 % 20.43 5,000 m 53.4 % 26.50 10,000 m 26.2 % 54.00 30,000 m 1.1 % 1,285.00