Air: Demonstrating Its Presence and Effects

Porter Johnson                 Illinois Institute of Technology
                               Biological Chemical Physical Sciences Dept
                               Chicago IL 60616-3793
                               (312) 567-5745

Objective:

To examine the effect of air pressure in a series of experiments that highlight 
the consequences of the presence of our atmosphere, aimed at grades 6-12. 

Materials Needed:

1.  A supply of sturdy 6" or 9" balloons [available at "party stores"]
2.  Heat Source [hot plate or Bunsen burner], tongs
3.  A supply of aluminum soft drink cans, a water bucket
4.  Heavy flat smooth rubber mat material [available at American Science Center]
5.  String, heavy scissors, metal washers, metersticks, stopwatches
6.  A supply of coffee filters, tea bags, matches
7.  Stick and propeller blade [available at American Science Center] 

Strategy:

We live at the bottom of a 10 km ocean of air.  The density of air is about 
1/1000 that of water, so that air pressure corresponds to the pressure of a 
water column of 10 meters [40 feet].  In these experiments we examine the 
effects of air pressure. 

Ethnic Rocket Launch:  Remove the cord and staple from a dry tea bag [Lipton 
or other unflavored] and empty its contents.  Form the bag into a hollow 
cylindrical "silo" and stand it erect.  Light the top of the bag.  The bag will 
burn quickly with little residue.  As fire reaches the bottom, the bag will 
rise.  A column of warm air aids the launch, which can be quite spectacular.  

Dropping Coffee Filters:  Modern physics began when, some 500 years ago, Galileo 
Galilei dropped metal balls of different sizes from the "leaning tower" of Pisa, 
and observed [or at least claimed to see] that they hit the ground at the same 
time, in contrast to expectations that the heavier ball would fall more quickly.  
Galileo had in mind neglecting air resistance. 

By contrast, when an empty coffee filter [Mr Coffee or clone] is dropped [nose 
pointing down], air resistance is not negligible.  Drop a coffee filter from 
heights of one meter and two meters, and measure how long it takes to hit the 
ground.  [We observed that it took circa 1.15 seconds to hit the ground from one 
meter, and about 2.30 seconds from two meters.]  Note that, as the distance is 
doubled, the time required also doubles.  This is an indication that, for most 
of its travel, the filter is moving with constant speed, the force of gravity 
[downward] being balanced by air resistance [upward]. 

Next, put several filters together, so as to increase the mass of the system, 
while keeping its profile fixed.  Measure the time for several coffee filters to 
fall.  We observed the following times from a height of 2 meters:

                 Number of Filters      Time
                        1             2.30 sec
                        2             1.60 sec
                        3             1.30 sec
                        4             1.15 sec    

We saw that one coffee filter falls through one meter in the same time as the 
four coffee filters took to fall through two meters.  Drop them simultaneously 
to determine whether they hit at the same time.  [Note:  the force of air 
resistance appears to be a quadratic function of the velocity of the filter.] 
        
Crushing the Can:  Put about 50 cubic centimeters [2 ounces] of water in the 
bottom of an aluminum can.  Heat the can until the water inside begins to boil.  
Then, take the tongs, turn the can upside down, and push it directly into the 
water in the bucket.  Observe the resulting collapse of the can.  The air inside 
the can has been displaced by water vapor, which condenses when the can enters 
the cool water.  Air pressure on the outside pushes the can inward.  This 
crushing force of air pressure is always present in our environment. 

Rubber Mats:  Get a supply of heavy flat smooth rubber mat material [about 3-5 
mm thickness].  Cut the material into rings of diameter about 30 cm with heavy 
scissors.  Punch a hole through the center of the disc, and push a fairly heavy 
string through the hole.  Tie a metal washer to the string, so that the string 
will not pull back through the hole.  Place the disc on a smooth solid surface, 
and press the air out from under the disc.  Pull up on the cord. If the seal is 
properly made, you will not be able to pick up the disc with the cord, because 
you must overcome air pressure [approximately 15 pounds per square inch, or 
10000 kilograms per cubic meter]. 

Measuring Lung Volumes with Balloons:  Have every member of the class take a 
standard 9" balloon, making it limber by blowing it up a few times.  After some 
practice, each class member should fill his/her lungs, expel one full breath 
into the balloon, and measure the diameter of the balloon.  [Note: Not everybody 
knows how to blow up a balloon!]  Calculate the volume V of the balloon [assume 
a spherical shape]  from its diameter D using the formula: 
              
      V   =   PI D3 / 6

where PI = 3.1416, and D3 is "D-cubed".  Then, record the age and height of 
each participant.  Here is a data table for the current SMILE class. 

                            LUNG VOLUME TABLE

Participant        Age        Height      Diameter    Volume    | Fit Volume
  Number         (years)       (in)         (cm)      (liters)  |  (liters)
                    A            H           D           V      |     V-fit

  #1                54           71          17         2.57    |      2.58
  #2                37           64          18         3.05    |      2.85
  #3                60           66          17         2.57    |      1.91
  #4                28           70          20         4.19    |      3.75
  #5                28           56          14         1.44    |      2.66
  #6                42           62          15         1.77    |      2.46
  #7                 7           40          18         3.05    |      2.43


The last column in the table is calculated from a "least squares" fit to data 
with the formula

     V    =   -.048 A   +   .078 H   -  .353   .

The standard deviation of this fit was +/-  0.66 liters.  A variation of this 
method of measuring lung volume and comparing with standard formulas is used by 
medical clinicians to detect lung damage. 

Helicopter Blades:  

Attach a light plastic propeller to a stick.  With the propeller held up, launch 
the stick by giving it a spin and throwing it up in the air.  Depending upon the 
direction of the spin, the rocket will either accelerate upward, or else plummet 
to a quick crash.  Try launching the rocket with the propeller pointed down, and 
note the direction of the spin in relation to its motion. 

Performance Assessment:

In medieval times in the city of Magdeburg [in Saxony Province; formerly East 
Germany], two metal hemispheres of diameter circa 30 cm were connected through 
an airtight seal, and the air was pumped out of the interior.  Teams of horses 
were then connected to each of the hemispheres.  The teams of horses could not 
pull the hemispheres apart.  Using the concept of air pressure, explain this 
result. 

Conclusions:

The effects of air pressure are sometimes subtle and sometimes dramatic.  There 
is a wide variety of classroom demonstrations of the existence of air and its 
effect in our environment.

References:

    Robert E Ehrlich Why Toast Lands Jelly-Side Down
    Zen and the Art of Physics Demonstrations
    [Princeton 1997] Paperback:  ISBN 0-691-02887-7