Explore the World of Soft…e: Engineering & Science/Explore_the_World_of_Software_Engineering_and_Science_HRS_Software_1998.iso/programs/physics/wwdem.exe/WWDEM.TXT

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      PHYSICS DEMOS FROM THE WOODROW WILSON PHYSICS INSTITUTE

                                   compiled by Pat Cannan
                       submitted by Preston "The Boom" Boomer

Physics Institute
Woodrow Wilson National Fellowship Foundation
Box 642
Princeton, NJ 08542

Banana Drop:
     When introducing acceleration of gravity, discuss it in terms of a
falling banana (or rutabaga, or whatever).  Demonstrate the fall and
then compare to a heavy banana (filled with lead shot and rubber
latex or aquarium sealant).  Drop both bananas at once by quickly
pulling a book out from under them.
Conclusion:  All bananas accelerate at the same rate.  This can then
be quoted for the rest of the year to remind students of the
demonstration.

And/or another variation:
     Galileo's home country-- Italy.  National fruit of Italy-- Grapes.
So all grapes fall at the same rate whether dropped individually or
in a bunch.  Show it.  Bunching them makes no difference!  Each atom
accelerates at g regardless of its companions.

Ellipses:
     Kepler's dad was a plumber.  So take two plumber's helpers
(preferably with short handles) and stick them onto the chalkboard
for the foci of the ellipse.  Then using a looped string and chalk,
draw the ellipse around the two helpers.

Lead Banana:
     How can an astronaut distinguish between a lead banana and one
that is just a hollowed out (and reinforced) peel?
By sensing their resistance to motion by shaking them.  Pass bananas
around so students can FEEL the mass.

Gravity discovered-- the real story.
     One day, young Isaac Newton, then in his mid-twenties, was
sitting under the banana tree in his back yard...

Centripetal hang-ups:
     Bend a coat hanger and its hook so that a penny will balance on
the upturned hook.  Hold the hanger by your index finger and swing it
in a circle.  The penny will (with practice) remain in place.

Swingin' big scare:
     Suspend a small (25cm diam) board from three strings so it can
be vertically swung around.  Place objects on the board and scare
everyone!  Practice this before trying beakers of water etc.

Car Parts:
Cars come equipped with a positive accelerator, a negative
accelerator, and a change sign lever.  When the lever is in the +
position, the positive and negative accelerators work as designed.
When the lever is in the - position, the positive accelerator produces
a negative acceleration, the negative accelerator a positive
acceleration. (for those doubting students, what would have
happened in the second case if the - accelerator had indeed
accelerated the car negatively?)

TP Rip-off:
    Single-ply toilet paper takes a force of about 10 newtons to
separate.  A rapid linear acceleration of the paper takes advantage
of the rotational inertia of the roll to help stretch and tear the
paper.  The build-up to the breaking point must occur quickly so that
angular velocity of the roll is kept small and paper is not dumped
onto the floor. As the roll is used up, the moment of inertia
decreases making it increasingly difficult to get paper off with one
hand.
  Place a new roll of TP and an almost empty roll on a bar held by
two students.  Give the new roll a yank, and the paper should tear
nicely. Give the small roll a yank, and it should unravel onto the
floor.
   Discuss the moment of inertia.  The new roll approximates a disk,
the old roll a hoop.


Sound Thinking:
   With a small transistor radio blaring away, enclose it in a cage of
wire mesh.  The Faraday cage will shield the radio from any electric
fields and hence will shield it from radio waves.  (The electric
waves of light enter and leave the cage because their wavelengths
are much smaller than the mesh size.)

When Chocolate Chip Speaks, Students Listen:
   Take about 50 turns of fine, insulated wire and tape to the back of
an ice cream carton (or whatever), leaving the two leads of the wire
to attached to the output of an amplifier.  Bring a large magnet up to
the back of the voice coil when the amplifier is signaling
appropriate music.
  You may construct the speaker in class, discussing it in abstract
terms so students are taken by surprise.  If you do not have a carton,
the daily bulletin or and administrative pronouncement will do.

Mixed Nuts:
   Tie large steel hex nuts (of varying mass) on a string at spaces of
S=1/2gt2 where t=.1,  .2, .3, etc. Release the string onto a noisy flat
plate and listen for a constant rat-a-tat-tat as th nut hit.  First
drop a string with evenly spaced nuts.

Instant Parabola:
   Put tape on a meter stick at intervals similar to the Mixed Nuts.
Mark off a parabola on the board by moving equal horizontal
distances as you mark off vertical positions from the tape.
   Challenge students to toss an object that matches the parabola, or
drag a student in a wagon while she tosses an object straight up.

The Beat Goes On:
   Drafting supply stores have stick-on tape with parallel bars.  By
Xeroxing this and shrinking it various amounts, waves of different
length are obtained.  Make transparencies and place them over each
other to project beats or group velocity onto the screen.
   By choosing l = fl (l= lambda) where f is a simple fraction, beat
frequencies are harmonic with the generating frequencies and give a
pleasing sound.  (Thank you, Pythagoras).

Running Interference:
   Concentric ring patterns may be purchases from drafting suppliers
of about $3 per sheet. Make different wave lengths by enlarging or
reducing the pattern.  With these made into overheads, you may
demonstrate (1)  2-slit interference, (2) the effect of changing slit
spacing or wavelength (3) n-slit interference (4) diffraction grating
(5) effect of telescope aperture and incident wavelength on
resolving power.

Golden Rule:
   Measuring the wavelength of light with a diffraction grating
demands and act of faith-- are there really all  those lines on the
grating?  You can diminish those concerns by using a metal ruler
with scored divisions of less than a millimeter or so ($2.50 at a
hardware store).  Allow laser light to reflect off it at a grazing
angle and project the pattern onto a wall.

Kitchen Scale Equilibrium:
   Take a two-meter or so 2X4, mark it at 30 cm intervals and show
your class its weight on the scale.  Now support one end on the
scale, one on a block, and ask the class to predict weight on the
balance before you actually release the board.  Then reverse block
and scale and ask again. Try various locations of the block and scale
and even add extra weights to the beam.  This demonstrates
moments under a variety of conditions.

Back to Normalcy:
   Clamp a weight to the scale and tilt it to show normal force
variance with angle.  In general, you will find a kitchen scale to be a
frequently used piece of apparatus for all sorts of phenomena. They
are available with metric readouts.

Reflections on the Wave Nature of Light:
   Reflect a laser beam off a flat mirror, make incident and reflected
beams visible with clouds of chalk dust. Then reflect light off a good
quality diffraction grating.  Ask students what is going on.

Polarizing Influence:
   Kids do not outgrow the desire to take something home with them.
diffraction gratings and polarizing materials are so cheap that they
should be given to every student. Tape 1cm X 1cm pieces of each
behind punched holes on a card.  Challenge students to write down
observations when they look at clouds, reflected light, neon or
sodium lamps, stars, etc.

Trapped in the corner:
   A corner reflector may be made by cementing three mirrors
together at right angles to each other. Use aquarium sealant for
adhesive and drafting triangles to insure accurate right angles.
Reflect laser beam off the interior of the cube.  Students will be
able to see how three reflections are required for it to work, and
they will actually be able to follow the light path.  Rock the cube
around so it is clear that the return path is not dependent upon
orientation.

Bubble Dome:
   Make a soap solution as follows: 70ml of Joy, 200ml glycerin, 230
ml water.  Roll a cone from a piece of paper and blow a large bubble
onto a glass plate on an overhead projector.  Ignore projection on
screen and look at beautiful, iridescent interference on the bubble
itself.  With the right mixture of bubble soap the bubble should get
thin enough to become totally transparent to reflected light, just
before it breaks.

Canned music:
   Reflect a strong light off a soap film across the end of a can.  With
a lens of appropriate focal length, focus an image of the bubble on a
screen. A series of spectra characteristic of a thin film is visible.
Now bring a speaker to the back of the can, and interesting
distortions of the image will occur. If you hook up a signal
generator, very the frequencies to get resonance on the soap film.

The Swing Era:
   Hang several pendula of different lengths from a semi-rigid
support.  Challenge students to get a particular pendulum swinging
to the exclusion of the others by pulling on a rubber band attached to
the support.

Spring String:
   The classic demonstration of a mass suspended between two
strings, protecting the upper string from breaking by its inertia does
not communicate the importance of a stretchable string.  If the
string were absolutely unyielding, the upper string would break
every time.
   By replacing the upper string with a spring, a slow motion of the
mass downward stretches the spring and visibly puts tension on it.
A rapid jerk on the string breaks it without significant stretch of
the spring.

The Big Attraction:
   With a charged lucite rod, rubber rod, or golf tube, attract an
empty pop can. Balance objects on the dome of a watch glass and
observe the effects-- everything up to a 2-meter 2X4 will work.
   Small charged objects may be discharged with an anti-static gun
available at a record store.  The gun has a piezoelectric crystal
connected to a sharp pin.  The potential developed on the point
creates ions that stream off the point and discharge whatever.

Funneling Momentum:
   Suspend a large funnel from a support so that it can spin freely.
Fill it with sand and release it, giving it a small initial angular
velocity.

Soda Straw Symphony:
   Clip the flattened end of a drinking straw to a point, forming a
double reed.  Pinch the reed end slightly with your lips as you blow
HARD to get something like a high-pitched duck call. (No self-
respecting beaver would respond to such a noise).  While blowing,
clip the other end off to change resonant frequencies.  Remind the
class that shorter tube lengths produce higher resonant frequencies.
in the straw just as it did on the soap bubble.
   Connect multiple straws and flex straws for good bass notes,
insert smaller straw to make a slide trombone, cut holes for
advanced work.  Cut appropriate lengths so class can play school
fight song, Christmas carols, etc.

No Strings Attached:
   Poke a hole 2/3 of the way into a Nerf ball and imbed a 30g sinker
attached to a string in the middle of the foam. Swing the ball in a
circle over your head and ask students at what moment in its path
you should release it to hit a target.  The demonstration is more
forceful if some of the student predictions result in the ball flying
into the class.

Shifting to Doppler:
   Get a code oscillator circuit (e.g. Radio Shack #20-115), a 5cm
speaker, a small switch and a 9volt battery clip.  With a sharp knife
slice into a Nerf ball and imbed all parts inside. Turn on the switch
and throw the ball to students in the class. Pitch will change
noticeably depending on whether the ball is approaching or receding.
(Total cost is about $9).

Electric Washtub:
   Mount a length of piano wire under tension (from a spring or
weights).  Place a large magnet over it and hook the ends of the wire
to the input of an amplifier.  Plucking the wire will induce currents
that will amplify as musical sounds.

Splitting Hairs:
    A human hair held in the laser beam will produce a single-slit
interference pattern.  (The hair forms a single thin barrier.) The
width of the hair can be determined by measuring the spacing of the
secondary maxima and using the single-slit equation.

Learning the Ropes:
   A convincing session in vectors:
       Have two burley guys pull a rope between them as tight as they
can.  Then have your smallest kid pull sideways in the center of the
rope.  He will have no trouble pulling the burleys toward each other.

Bubble Battle:
   If two soap bubbles (or balloons) are connected at opposite ends of
a pipe, the smaller bubble (or balloon) will force air into the larger
one.  The pressure inside a bubble varies inversely as the radius of
the bubble. It's neat to have a valve in the pipe and set up the
bubbles or balloons and ask the kids to hypothesize on what will
happen when the valve is opened and why.

Soap Film Trampoline:
   Use a coat hanger or a large wire frame. Dip it into a bubble
solution. With practice you can cause the large soap film to undergo
many interesting modes of vibration.

Football Spin:
   Try spinning the following objects on a bare floor or on a smooth
table top:  Small toy football, hollow egg-shaped plastic container
(l'eggs), full-size football.  though the football begins its rotations
about its short axis, it reorients itself to a lower energy state by
standing up and rotating about its longer axis.  (see The Physics
Teacher, Vol. 15. p 188, 1977).

The Levitating Screwdriver:
   When various objects are individually placed in a narrow stream of
fast moving air, they seem to float.  Objects which have been used
include: golf balls, small footballs, styrofoam balls, rubber balls,
steel balls, hollow egg-shaped plastic containers, and smooth
handled screwdrivers.

Pat the Pipe:
   Pat the end of the pipe with the flat palm of your hand.  Use two
distinctly different motions:  1. Leave the hand against the end of
the pipe after striking it.  2. Quickly remove the hand away from the
end of the pipe immediately after striking it.  If you listen carefully,
you should hear two different octaves because an open pipe has
antinodes at both ends while a closed pipe has a node at one end.

Singing Rod:
   Hold on to the midpoint of a solid aluminum rod (18mm diameter is
good) with the thumb and forefinger of one hand.  Stroke the rod with
the thumb and forefinger of the other hand.  A LOUD tone emerges
from the longitudinal oscillations set up in the rod.  By holding the
rod at other locations, higher harmonics are heard.  It is helpful
(essential to get resin, stick-um, or some kind of frictional material
on the rod.  Consult your local sports store.

Baffle the Speaker:
   Purchase an ear phone attachment for a cassette player. Cut off
the ear piece and in its place solder a small (5cm) speaker (Radio
Shack). Plug  this speaker into the cassette player and listen to the
musical sounds before and after the speaker is placed near the
opening of each of the following objects: plastic pipe, bottomless
styrofoam cup, a sheet of 60cm square cardboard with a 5cm hole
cut in the center.

Light My Balloon:
   Fill one balloon with water.  Fill another balloon with air.  Place a
lighted match beneath each of the balloons.  Explain.

Follow the Vector:
   Use a bicycle wheel or a circular disk (such as a disk stroboscope).
Attach a styrofoam ball and arrows radially inward and tangential to
the circle.  Use shadow projection and watch the length of the arrow
s  shadows simulate the acceleration and velocity vectors of a body
in simple harmonic motion. A 1cm hole in an aluminum slide in the
projector helps narrow the beam of light.

An Uplifting Experience:
   Use duct tape to attach a plastic garbage bag to the outlet hose of
an air blower (vacuum cleaner). Have someone sit on the bag as you
turn on the air.  Watch the person being lifted as the bag fills with
air.  An alternative is to attach several hoses and have classmates
blow up the bag with lung pressure.  This method is sometimes used
to lift automobiles in accident situations.  Big total force from a
small pressure over a large area.

Smoke Cannon:
   How to construct a smoke cannon:  Remove one end of a cardboard
box. Attach long rubber bands to each of the remaining 4 corners.
Use duct tape to attach a sheet of flexible plastic across the open
end of the box.  Attach each of the four free ends of the rubber bands
to a knob near the middle of the plastic sheet.  Cut a circular hole  in
the side of the box opposite the plastic sheet.  Saturate the interior
of the box with smoke and blow smoke rings across the room.

Vortex Generator:
  A simpler version of the Smoke Cannon.  Use a plastic milk bottle.
Whap it on the side. Nice vortex rings shoot out.  Fill the bottle with
methane from your gas jets.  Shoot the vortex rings at a burning
burner.  Watch the rings ignite with nice sound action!

Better still super great Vortex Blammer:
   Take a plastic bucket (10 liter) and cut a 25cm hole in the bottom.
Cover the top with rubber (wet suit) material (surfers can usually
supply you with old wet suits). Tie or clamp the rubber on (with hose
clamp stock).  It should be tight as a drum.  Bam the rubber and WOW!
Great vortex rings.  Shoot the kids across the room.  Fill it with
methane, shoot the Bunsen Burner and oooohhhhhh.... NEAT!  Big
roaring rings of fire!

Diet Lite:
   Drop two full cans of pop into a tank of water.  One a can of diet
pop, the other regular.  The diet drink will float. The sugar content
of the regular drink will increase its density sufficiently to sink it.

Instant Recycling:
   Take an empty pop can, put in a few ml of water.   Heat over a
burner until steam emerges vigorously from the opening.  Quickly
invert the can into a pan of cold water.  Condensing of the steam is
so rapid that the low pressure created allows the atmosphere to
crush the can instantly. (Unless you are a real man, you'll want to
hold the can in tongs).  The pressure of the atmosphere is 1kg/cm2
(9.8n)/cm2.

Sweet Success:
   The sugars in Karo syrup rotate the plane of polarized light and
rotate different colors different degrees.  Place a bottle of it on a
polarizing filter on the overhead.  Students looking at the bottle
through their own polarizers see spectacular colors.
   The trick may also be done by stretching scotch tape and layering
it randomly on a glass plate.  The stretching is essential to
straighten out long coiled organic molecules.

The Silver Lining (or a cloud in a 4-liter jug):
   Take a 4-liter jug (you may first have to consume the contents the
previous weekend), swirl a few ml of water around in it and pour out
the excess to raise the humidity. Introduce some smoke from a
freshly blown out match for hydroscopic particles. With your mouth
over the mouth of the jug (mouth-to mouth) blow hard into the jug
and then release the pressure.  Ah, fog in a 4-liter jug!
   Or connect a rubber tube to the jug with a one-hole stopper and
glass tube and blow or suck on the tube.  Nice adiabatic warming and
cooling.
   Discuss capacity of air, absolute humidity, relative humidity, and
the variance of the capacity with temperature. And do not forget the
dew point.

Sticky Situation:
   Mix corn starch (from the grocer) with water to make a fluid
paste.  Pour it into a beaker and challenge students to QUICKLY punch
their finger into it and withdraw.
   Under rapidly applied force the mixture becomes an elastic solid.
Scoop out some of the fluid and squeeze it into a ball.  As soon as
you quit squeezing it will become a liquid in your hands.
   This is completely washable, so you can really get into it.

Vanishing Charge:
   Take a commercial disectible Layden jar and charge it by holding
the base in your hand and reaching toward a Van de Graaf generator
with the central plate.  Be sure your body is grounded or it won t
charge.
   With a wire between the inside and outside plate, show it is
charged.  Great Spark!  Set it on a table and with faked care lift the
center plate out with an insulated rod and hand it to a reluctant
student.  Remove the glass and with the insulated rod hand the outer
cup to another student.  Ask them to touch hands.  Nothing happens.
   Now touch the cups to show there is no charge.  Nothing happens.
Reconstruct the jar and VOILA there is a spark.
   You can make your own jar with the bottom half of a soda can in a
peanut butter jar with a steel can on the outside.
   The charge resides on the glass, not the metal.

Change in the Wind:
   Place a dime about 3 cm in from the edge of a table.  Set a can at
an angle near the coin.  Challenge students to get the coin into the
can without touching either.
   Solution is to blow SHARPLY over the top of the coin. The Bernoulli
effect will lift it and pop it into the can.  A real expert can lift a
quarter.

Milk, Motion, and Molecules:
   Dilute homogenized milk with about 10 parts water to 1 part milk.
Place a drop on a microscope slide, cover and view under high power.
The tiny fat particles will be seen to dance continually from the
collisions of the water molecules.  The effect is called Brownian
Motion and was crucial evidence for the existence of molecules.
Einstein did the math on it.

Absorbing Interest:
   Dissolve about 10g of erbium chloride or any rare earth chloride in
a large test tube.  Hold the tube in front of a showcase bulb while
students look through diffraction gratings.  They will see a beautiful
absorption spectrum.

The Incredible Inverting Pin:
   Take a plastic film container and poke a small pinhole in the
bottom center. Then push the pin through the side near the front so
the head is centered in the opening.  Hold it up to you eye and look at
the pin silhouetted in the little circle of light.  It will appear to be
inverted.

Bloogle:
  Take a short piece of flexible plastic hose (60 cm long) and swing
it briskly in a circle.  Three or four resonant frequencies can be
heard.  With a little practice you can play Taps on it.

Acoustical hang-ups:
   Tie about 1.5m of thread or very fine wire to a coat hanger.  Loop
it over your head as you lean forward, and with your index fingers
stick the tread into you ears.  Have a friend strike the hanging
hanger with a pencil, and revel in the sound produced.  Ah, Great Tom
of Westminster!

Physics Transferred:
   Take two PSSC air core solenoids. Attach lamp cord directly to the
terminals of one. to the other attach the terminals from a low-watt
household light bulb.  Through the first solenoid place a bundle of
straightened coat hanger wires. Plug the assembly into the 120 volt
line.
   The coil will jump a little  onto the iron core.

Pressure Situation:
   To find the pressure on a balloon, simply press the inflated balloon
down onto a balance until it flattens to a circle of known radius
(cm).  Read the balance for force and calculate pressure. P = fA.

Eggstra Eggsitement:
   Tow students hold a sheet by its four corners. Someone throws a
raw egg into the sheet as hard as possible.  It does not break.  Be
careful not to miss the sheet.

Tubular Resonance:
   The is a good follow-up to the closed-tube resonance one gets
from reflecting tuning-fork sound into a graduated cylinder with
water in it.
   Just take two cardboard tubes that fit one inside the other.
Adjust the total length by sliding them in and out until resonance
with the tuning fork is found.  Your open tube will resonate at twice
the length of the closed tube.

Look Look!
   Print  DICK  JANE  on a card.  DICK in blue, JANE in red.  When
viewed through a water-filled test tube, JANE appears inverted,
while DICK appears normal.  Let the kids hypothesise this one.  The
fact is that they are both inverted, but DICK looks the same either
way.

Lake Level:
   Famous Archimedian problem.  Ask class... A row boat has an
anchor in it as it floats on a lake.  When the anchor is thrown
overboard, will the lake level rise, fall, or stay the same?
   Collect student answers and justifications. Then try this
experiment.
   Float a 400 or 500 ml beaker with mass in it (say 200g) in a larger
beaker of water (say 2000 ml beaker). Mark the water level.  Then
remove the mass and place it in the bottom of the larger beaker (the
lake).  The lake lowers because in the boat, the anchor displaces its
weight of water but in the lake it displaces its volume of water.

Pin-Point Discharge:

   Get a Pom-Pon from a cheerleader.  Place it on the Van de Graaf
machine and charge it up.  Approach the charged pom pon with a
sharp pointed object. It will discharge fast.  Ben Franklin found that
charge accumulates on sharp points... The Lightning Rod.

Galloping Gourmet:
 Cook a hot dog by inserting nail electrodes at each end and
connecting to the 120 v line.  Watch out for shock hazard!
it takes only a few monents to cook.

Electric Jump Rope:
   Connect a long wire to the terminals on a galvanometer.  Swing it
like a jump rope.  In the earth s magnetic field it will generate
current.  Swing only one loop of the wire so that the opposite loop
won t cancel the current.

PHYSICS INSTITUTE 1987          GREAT DEMOS              DEPARTMENT



                           THE DISCOVERY OF ATMOSPHERIC PRESSURE

                                                      or

                                        The Earth Sucks Not

(Theses demos takes a full period)


     Demonstrate a piston pump (tire pump or glass demo pump), and
tell how the ancients knew why it worked--  Aristotle had told
them...

                                  NATURE ABHORS A VACUUM .

     The story of the Duke of Tuscany s pump (17 th cent).  The Duke
had his pump moved from the water source up the hill into his castle
(so foul foe couldn t mess with it).  Well, it wouldn t cause water to
rise higher than 10 meters above the source.  Nature had pooped out
at 10 meters of water.  He called in Torricelli (a student of Galileo)
to solve the problem. Torri experimented with inverted tubes, closed
at the top, filled with water and mercury.  The water always ran
down to 10 meters, and the mercury to about 760 mm.

     Set up this device by filling a glass tube, with one end sealed
whose length is at least  78 cm, with mercury (use a dropper).  Place
the  finger of Torricelli  over the open end and invert the tube into a
beaker containing mercury (be careful not to allow any air to enter).
When the finger and tube end are beneath the mercury in the beaker,
remove the former.  Clamp the inverted tube of mercury to a
ring stand, and place a meter stick next to it to measure the
millimeters from the mercury in the beaker to the top of the column
of mercury.

     On closer examination, Torri found the mercury level varied from
day to day, dropping with rainy weather and rising with fair
weather.  Ah, Yaz, it must be a function of the atmosphere. Hence he
deduced that the atmosphere must exert pressure, and therefore
must have weight!  (Aristotle said,  Air is Levity , so Torricelli has
shot down two of the ancient wrongs, and invented the Barometer).

     On the same ring stand, attach an 80 cm glass tube (with a 90 deg
bend at the top is nice) with one end in the same mercury reservoir
with the Tube of Torricelli (Ooohh, the Barometer), and connect the
top of this tube to an aspirator (or other vacuum pump) with a piece
of heavy walled rubber tubing. Place a safety beaker under the
aspirator to catch any mercury that accidentally comes through the
system.

     Start the aspirator and watch the mercury zip up the tube. Gasp,
it stops in the nick of time (we hope), and guess where? Ah,
interesting, about the same level as the barometer.

     Well, if Torri is right, air must have weight (not levity), so let s
weigh air.  Use the aspirator to remove air from a hollow metal ball
(obtainable from the catalogs) or a separatory funnel, close the cock
thereon, and weigh it  empty . Then, let air in (nice hiss), and
re weigh it.  There is quite an increase in weight.  You can even
calculate the density of air by dividing the mass of air in the ball by
the volume of the ball.


                          MORE DEMOS ON ATMOSPHERIC PRESSURE:

     Give these equivalents of atmospheric pressure:

    One Atmosphere = 760 mm of mercury (sea level average)
                              =  10 m  of water   (sea level average)
                              =   1 kg of mass per square cm of area
                              = 101 kpa   (of that kind of pressure)
                              = 1013 mb   (for weather)

     Point out that a vacuum is nothing, and nothing cannot do
anything, which includes  suction .  Vacuums do not suck, the
atmosphere pushes!  One does not  pull a vacuum   or  put a vacuum
in  something. One creates a lower pressure outside, say by
expanding the volume in a pump, and molecular motion (kinetic
theory) causes the air to move into the lower pressure region.

     Evacuate the Magdeburg Hemispheres (obtainable from catalogs)
and have two burly guys try to pull them apart.  Tell the story of
Otto von Guerrike, Mayor of Magdeburg in the 17 th century, setting
eight horses to the task.  His hemispheres were much larger, and the
horses failed to do it.

     Now, take back the hemispheres from the burlys, and, while your
back is turned, let the air sneak in. Face the class and tell them that
only the strongest can succeed.  Tear them apart with considerable
moaning, etc.

     Calculate the number of kilograms needed to really separate them
by measuring the radius in cm, using pi r squared for the area in
centimeters squared, and that the atmosphere exerts 1 kg per
centimeter squared.  It take about 75 kg, which may be about the
mass of the instructor.  Hence, Spider Man is in trouble with these
things as suction cups.

     To illustrate this, moisten one hemisphere and place it on the
smooth surface of the demo table, remove the air, and get on the
table, and pull (watch your back), and you can do it (or a burly can).

     Point out, again, that there is no mysterious  force of vacuum ,
but merely the force of the atmospheric pressure, which is not all
that strong.  Also, that it is the surface area and the pressure that
determine how many horses are needed.  But it can be done!

     Crush a ditto juice can (or a plastic jug) by removing the air from
it with rubber stopper, glass bend, and pump. (Great fun).

     Then, wrap a 4-liter glass wine jug in a safety towel, place it in
the sink (as a safety precaution) and remove the air from it as with
the above can.  It doesn t crush!  This shows, again, that there is no
mysterious irresistible force of  vacuum .  The strong construction
of the jug enables it to withstand the great total force of a large
number of centimeters of surface area on which there is about 1 kg
of atmospheric action on each. WOW!

     Now for the mysterious Siphon.  Take two large beakers and a
transparent plastic tube, and fill one beaker with water. Immerse
most of the tube into the beaker of water allowing it to fill to the
water level. Then block off the upper end of the tube
with a finger.  Pull the blocked end down until the water level in it
is below the water level in the first beaker, and holding it into the
empty beaker, open the blocked end.  Ah, the water flows.  This is
the proper way to start a siphon, not with the mouth (unless it s
wine being siphoned). Perhaps the action is caused by the heavier
 water piston  in the longer tube lowering the pressure so that the
atmosphere will push the water into the shorter end which has the
lighter  water piston .


****  Great new video--
                  (1991)
Shows the ten-meter water action with glass pipes.  The water
                 boils, too!
Crushes a steel oil barrel... Neat.


                 Atmospheric Pressure
                                    from
             Films for Humanities & Sciences
             Box 2053 Princeton, NJ  08543-2053
                  around $49.

---------------------------------------------------------


                                  ATMOSPHERIC PRESSURE & HUMIDITY

PRESSURE--

     The density of air is 1 gram per liter (at room temp and press).

     The pressure due to the weight of 300 km of atmosphere above is
equal to that of:

                 1  atmosphere
                 1  kg per square centimeter
                10  meters of water
               100  kpa
               760  mm of mercury
              1000  millibars (where a bar is 1 atmosphere)

     The total force on the human body due to the pressure of 1g/cm2
is about 22 megagrams (metric tons).

     It is the force of atmospheric pressure that causes the
phenomenon commonly called  suction .  The atmosphere pushes air
into regions of lower pressure.

HUMIDITY--

     Capacity of air is the number of grams of water vapor that can be
dissolved in 1 cubic meter of air at a given temperature.  The
capacity increases with temperature.

     Absolute Humidity is the actual number of grams of water vapor
dissolved in a cubic meter of air at any moment.

     Relative Humidity is the ratio of Absolute Humidity to Capacity
expressed in percent.
          100% Relative Humidity means that the air is saturated as in
a cloud or in fog.

          Example determination of Relative Humidity:

               At 20oC,  the Capacity is 18 g/m3.  If the Absolute
Humidity is 9 g/m3,

               The Relative Humidity is 9 g/m3/18 g/m3 = 50%

               Here is a table of Capacities at various Temperatures:

                    Temp in oC   Capacity in g/m3

                                   0             5
                                   5             8
                                  10          10
                                  15          14
                                  20          18
                                  25          22
                                  30          30
                                  35          40


ADIABATIC TEMPERATURE CHANGES--

     Adiabatic refers to changing the temperature of air by either
compressing it or expanding it. Compression warms air, expansion
cools air. (An example of adiabatic heating is in the ignition of a
diesel engine.  The air is heated so hot by compression that it
ignites the fuel on contact. Adiabatic cooling is noticed when air is
allowed to escape from a tire).

          Upslope fog is formed by adiabatic cooling of the air as it is
blown uphill. As the air is cooled, its Capacity is lowered until it
equals the Absolute Humidity.  Then we have 100% Relative Humidity
and further cooling will cause fog to form.  Fog, like clouds, is
composed of LIQUID droplets (NOT water vapor).



-------------------------------------------------------------



                                       BAROMETER STORY

METHODS OF MEASURING THE HEIGHT OF A BUILDING WITH A
      BAROMETER:


Atmospheric Pressure diminishes 1 cm per 100 meters of ascent.

Lower the barometer on a rope and measure the rope.

Drop the barometer and time its fall.   S=1/2gt2  to calculate the
height.

On a sunny day, measure the shadow of the barometer on the ground
and the shadow of the building.  Use proportions of right triangles
and the height of the barometer to calculate the height of the
building.

Going up the stairs, use the barometer as a  meter stick  to mark off
on the walls the height of the building.

Tie a string to the barometer, use it as a pendulum to determine g at
the street level and at the top of the building.  The difference in g
can be used.

Take the barometer to the superintendent of the building and say,
 Sir, I will give you this fine barometer if you will tell me the
height of this building.


------------------------------------------------------------


                                                 PULLEY STORY


Respected Sir,

    When I got to the building, I found that the hurricane had knocked
some bricks off the top.  So I rigged up a beam with a pulley at the
top of the building and hoisted up a couple of barrels full of bricks.

     When I had fixed the building, there were many bricks left over.

     I hoisted the barrel back up again and secured the line at the
bottom, and then went up and filled the barrel with extra bricks.
Then I went to the bottom and cast off the line.

     Unfortunately, the barrel of bricks was heavier than I was and
before I knew what was happening, the barrel started down, jerking
me off the ground.  I decided to hang on and halfway up I met the
barrel coming down and received a severe blow on the shoulder.

     I then continued to the top, banging my head against the beam and
getting my finger jammed in the pulley.  When the barrel hit the
ground it burst its bottom, allowing all the bricks to spill out.

     I was now heavier than the barrel and so started down again at
high speed.  Halfway down, I met the barrel coming up and received
severe injuries to my shins.  When I hit the ground I landed on the
bricks, getting several painful cuts from the sharp edges.

     At this point I must have lost my presence of mind, because I let
go the line.  The barrel then come down giving me another heavy blow
on the head and putting me in the hospital.

     I respectfully request sick leave.



-------------------------------------------------------


PHYSICS CAMP 1987                                                     GREAT DEMOS
DEPARTMENT


                                     BACK ELECTROMOTIVE FORCE
                                                 by the Boom

     When an electric motor is running, its armature windings are
cutting through the magnetic field of the stator.  Thus the motor is
acting also as a generator.  According to Lenz s Law, the induced
voltage in the armature will oppose the applied voltage in the stator.
This induced voltage is called BACK EMF. The strength of the Back
EMF depends upon the number of magnetic lines cut per second.  So
the faster the motor turns, the greater the Back EMF.

     Back EMF is very important in electric motors.  When the motor is
first turned on, there is no Back EMF inasmuch as the armature is not
yet turning.  This means that the motor will have a high starting
torque since there is no opposition to the applied voltage.  Then
when the motor is running at speed, the Back EMF will oppose much
of the applied voltage and the net result is a relatively small amount
of power consumption.  A big load will slow the motor, reducing the
Back EMF, so more power is applied to maintain the torque. (The
inverse feedback principle). This is the reason why a motor will burn
out if it is not allowed to run.  The windings are designed to operate
at the net voltage determined by taking the difference between the
Applied Voltage and the Back EMF. The starting motor takes several
times as much current as the windings can tolerate for more than a
few seconds.

     Set up your 120 volt power supply, switch, and light sockets and
bulbs (see the apparatus in ELECTRIC CIRCUITS).

     Use an ordinary 120 volt AC motor (such as an electric fan or
power drill).  Connect it in SERIES with a large clear glass light bulb
and your switch.  With the bulb in series with the motor, you should
be able to prevent the motor from starting with your hand (better
use a cloth on the shaft just in case it s stronger than normal).

     Start the power, and note the intensity of the bulb.  Then allow
the motor to run at full speed.  The bulb should be brightest when the
motor is stopped because there is no Back EMF to oppose current
flow.  At full speed, the Back EMF will cut down on the power
consumption, and the bulb will dim. You may need to experiment with
different wattage bulbs to find the one that works best for your
motor.

     Add load to the motor with friction, and note the bulb gets
brighter as Back EMF is reduced.

     You may wish to use AC meters to measure quantitatively the
above results.

---------------------------------------------------------


PHYSICS INSTITUTE 1987                                              GREAT DEMOS
DEPARTMENT


                              VAPOR PRESSURE & BOILING POINT

                                      a super demo by the Boom


     This is a fun demo that takes a full period and covers the
following concepts:  Vapor pressure, Boiling Point, Cooling by
Evaporation, Liquid to Gas Phase Change, and the Equilibrium
between liquid and gas in a closed system at constant temperature.


APPARATUS:

     Ring stand, 2 utility clamps, a 500 ml thick-walled round-bottom
flask, two glass bends, two pieces of thick walled rubber tubing
(about a meter in length), rubber stoppers for the flask [a solid, a
one-hole, and a three-hole (I had to drill my third hole)], a tall glass
cylinder, a straight glass tube at least 80 cm long (a 90 deg bend at
one end is nice), a meter stick, a 100 ml beaker containing about 30
ml of mercury, an aspirator or other vacuum pump, a thermometer (0
- 110 deg), a pinch clamp, and a burner.


PROCEDURE:

     Set up a 500 ml thick-walled round-bottom flask on a ring stand
over a burner.  Insert a 3-hole rubber stopper therein. One hole gets a
thermometer, the other two are for glass bends for attaching rubber
tubing.

     Using the same ring stand, set up a glass tube at least 80 cm long
with one end resting in a 100 ml beaker containing about 30 ml of
mercury.  Mount a meter stick along side the tube.  This will be a
manometer for measuring the vapor pressure.

     Fill the 500 ml flask about half-full with water and preheat the
water to about 50 deg C.

     Attach an aspirator or other vacuum pump via rubber tubing to
the top of the manometer tube. Start the pump and note how many
millimeters the mercury in the glass tube is pushed up above the
mercury in the beaker by the atmospheric pressure.  Call this reading
 atmospheric pressure .
     Explain that when there is very little air left in the tube, the
atmosphere will push the mercury up until the pressure of the
mercury equals the pressure of the atmosphere.

     Allow air to enter the manometer by removing the rubber tubing
from the pump.  (Warning: do not lift the glass tube out of the
mercury or the mercury in the tube will flow to the pump. If you are
using an aspirator, place a safety beaker under the water flow to
catch any mercury that comes through).

     Now, attach the manometer to the flask with a rubber tube, and
attach the pump to the flask with the other rubber tube.

Explain the following two definitions:

     Vapor pressure is the force exerted by evaporating molecules
escaping through the surface tension of the liquid.

     Boiling point is the temperature at which the vapor pressure
equals the atmospheric pressure (or the gas pressure within a closed
flask).


     Explain now that we shall measure the vapor pressure in the
flask by pumping the air out, pinching off the aspirator, and waiting
for the system to reach equilibrium, reading the manometer, and
obtaining the vapor pressure by subtracting the manometer reading
from the atmospheric pressure reading.

     Note: with  no air  in the manometer, the mercury goes so many
mm high.  Then, when vapor molecules from the flask enter the
manometer, they will push the mercury down so far.  The difference
between the two manometer readings is the vapor pressure in mm of
Hg.

     Start the pump... Oh Oh, What s this, the water is BOILING! WOW! A
STATE OF SHOCK!

     Now explain what boiling is and how it is possible to boil a liquid
by TWO methods: by either raising the vapor pressure to the
atmospheric pressure or by lowering the atmospheric pressure to
the vapor pressure (say by a pump).

     Back to the vapor pressure measurements. Pump the system until
boiling occurs, then pinch off the rubber tube to the pump, stop the
pump and allow the system to reach equilibrium (boiling stops at
equilibrium).  Read the manometer and calculate the vapor pressure
by subtracting the manometer reading form the atmospheric
pressure reading.

     Heat the system and note how raising the temperature increases
the vapor pressure and pushes the mercury further down in the
manometer.

     Take several readings at different temperatures and note the
vapor pressures. Then note the boiling temperature at one
atmosphere of pressure when the mercury is pushed all the way
down the manometer.

     Here s a good place to discuss why it takes so long to boil an egg
on a high mountain (At 5000 meters elevation, it takes a half hour or
so), and the principle of the pressure cooker.


ENDOTHERMIC REACTION:

     Now let air back into the system, and heat the water to boiling
with the burner.  Remove the burner, and note for a minute that the
water cools very slowly. Then start the pump, and read off how fast
the temperature DROPS as the water is boiling!

     Here is a good place to discuss that evaporation is endothermic,
and boiling is a cooling process. Give examples of cooling by
evaporation, the wind-chill factor, and refrigeration. (Inasmuch as
the typical teenager will attack the fridge frequently, he can be told
to listen for the sound of boiling liquid around the ice trays when
the fridge is running).

     Water + 538 cal/g <----> Vapor


GREAT PHASE CHANGE:

     To demonstrate the tremendous volume change when water goes
from liquid to gas, remove the stopper from the flask and boil the
water with the burner. Remove the burner, and place a solid rubber
stopper in the flask before the steam stops coming out.

     Carefully cool the flask with a damp cloth and note how COOLING
makes the water BOIL FASTER!  GASP! What s happening?? When the
flask is cool enough to handle, remove it from the ring stand and
hold
it under the cold tap.  It really boils now! The steam within is
condensing and lowering the pressure inside to boil the water by
reduced atmospheric pressure.


MORE ACTION:

     Now replace the solid rubber stopper with a one-hole stopper and
attach a piece of rubber tubing.  Boil the water with the burner, and
show that LIVE STEAM is an INVISIBLE GAS by noting that the visible
CONDENSATE, which is composed of tiny droplets of LIQUID water,
shooting out of the tubing is preceded by a few millimeters of
invisible region.  Also the vapor inside the flask is invisible.

     Place the end of tubing with the steam shooting out into a tall
cylinder of cold water. ROAR! BIG NOISE!  The bubbles of steam are
condensing in the cold water with much rattling.  Here, again, you
may discuss the big volume change when a gas becomes a liquid, and
the water is collapsing onto itself.

     For the next mystification, and another illustration of the big
volume change, keep the steam tube at the bottom of the cylinder of
cold water and remove the burner from the flask. Soon the water
will be pushed with much vigor by the atmospheric pressure into the
flask.

     This demonstration is a blast to do and the kids love it. Good luck!

----------------------------------------------------------


PHYSICS INSTITUTE 1987          GREAT DEMOS           DEPARTMENT


                                       ADIABATIC ACTION

                                                      or

                             THE FOUR LITER CLOUD CHAMBER

                                             by the Boom

     We shall use two variations of adiabatic temperature change to
produce a cloud in the jug. Adiabatic expansion by pump, and
adiabatic compression using Boyle s Law with a water piston.

     Take a clear glass 4-liter wine jug (you may have to dispose of
its contents on a previous weekend), fit it with a one-hole rubber
stopper with a glass bend therein.  Attach it with a rubber tubing to
an aspirator or vacuum pump.
     Give it a safety test run in the sink covered with a towel to be
sure there are no flaws in the glass.

     Discuss adiabatic temperature changes, capacity of air, absolute
humidity, relative humidity, saturation of air, condensation nuclei,
and in general why a cloud forms.  The capacity of air at 20 deg C is
about 5 grams of water vapor per cubic meter, and increases with
temperature.

     Introduce a few ml of water into the jug, insert stopper, and
shake it for a few seconds.  Point out that the air will be saturated
with vapor, that is at 100% relative humidity.

     Now attach aspirator and lower the pressure somewhat (if you
are worried about an implosion, don t pump it all the way.  I ve done
this demo five times a year for the past 30 years with no mishaps).

     Not much fog forms.  Oooh, we need some condensation nuclei.
Remove the stopper and allow some match smoke into the jug.  Pump
again and, nice, there s the fog. Perhaps most noticeable when it
disappears when air is re-admitted.  Repeat this a couple of times.

     Point out that when air is blown up a hill, the lower atmospheric
pressure allows the air to expand, cool adiabatically, and upslope
fog may form.

     Also explain that fog and clouds are not water vapor, which is
invisible, but tiny droplets of liquid water.  They can be seen in a
beam of light.

     Now try this for even better fog in the jug.  Add some more
smoke, attach the rubber tubing to the water jet.  Holding the
stopper on the jug to prevent its blowing out, force in enough water
to fill the jug one-third full. (You would have to fill it half-full to
acquire the same pressure differential that you had when it was
evacuated).

     By Boyle s Law you have increased the pressure in the jug, this
will adiabatically warm the air therein, increase its capacity, and
cause it to dissolve more vapor and reach a new saturation
equilibrium.

     After a few seconds, pop the top. The sudden drop of pressure
inside will give a quick adiabatic cooling, and, WOW, great fog.
Some will be pouring out of the neck of the jug. Four liters of London
Fog!

-----------------------------------------------------------

PHYSICS INSTITUTE 1987                                               NEAT LABS
                                                 DEPARTMENT



                                    LAWS OF HEAT EXCHANGE

                                  THE COFFEE-CREAM PROBLEM


This lab takes about half an hour.

NEWTON S LAW OF COOLING:
     The rate of heat conduction is proportional to the temperature
difference between an object and its surroundings.

 THE STEFAN-BOLTZMANN LAW OF RADIATION:
     The rate of heat lost by radiation is proportional to the fourth
power of the absolute temperature.

THE HISTORIC PROBLEM:

     Ah, you see, there is this business man who likes a large amount
of cream in his coffee, and he wants the resultant mixture as hot as
possible. (Alas, there is no microwave oven available).

     He has just prepared his boiling coffee when he is called by the
boss for a quickie conference of ten minutes duration.  The boss
tolerates no coffee in his presence.

     What to do?  To keep the coffee as hot as possible should he add
the cream now or wait until after the conference?

     Have the students try to answer this problem for part of their
homework assignment the day before the lab. Then they can test
their theories in lab.

     Use two 250-ml beakers for coffee cups and two 100-ml beakers
for creamers.  Put 200 ml of water (coffee) in the cups
and 40 ml of water (cream) in the creamers.  Heat the coffee to
boiling (try not to evaporate much).

     To one cup add the cream immediately, set both cups on the same
table (for the same conductivity and radiation conditions), and
record the temperatures in deg C for each cup every 30 seconds for
ten minutes.  Then add the cream to the creamless cup, and continue
to record temperatures for another two minutes.

     Graph the temperature-time curves on the same set of axes, and
evaluate the results.

-----------------------------------------------------------

PHYSICS INSTITUTE 1987                                            GREAT DEMOS
                                                DEPARTMENT


                                MAGNETISM, THE DOMAIN THEORY

                                               by the Boom


Here we shall do a series of neat evidences supporting the domain
theory of magnetism. It needs most of a period.

THE DOMAIN THEORY states that groups of atoms join to make tiny
magnets in the metal.  When these domains are lined up they combine
their feeble magnetic forces to make the total magnet.

Equipment:  A beaker of iron filings, a demonstration compass,
various pieces of iron (such as nails, screwdriver, knife, etc.), a
good
permanent magnet, a burner, a hammer or two, a couple of good 6-
volt batteries, a hookup source of 120-volts AC (a plug-in cord with
roach clips on one end or use the switch board described in
ELECTRICAL CIRCUITS).

You can make your own coil (solenoid) by acquiring enamel covered
wire (#22 or #24 is good) from a motor shop and winding it about 8
cm deep on a cardboard tube about 5 cm in diameter and about 20 cm
long.  You will need ends on the tube to keep the wire on. Blocks of
wood drilled and glued onto the tube are ideal.  This solenoid will be
useful for many electrical experiments. A fat coil like this can
operate directly on the 120 volt line without overheating. It will be
quite powerful.  You may need to test it as you wind it.  Too many
turns will cause it to weaken as the resistance and reactance
increase.  Too few and it will overheat. You ll probably want to bolt
it into a slow running drill press to wind it. (A lot of work but it
will last you for your career. I m still using the one I made as an 8th
grade shop project in 1945).


NOW FOR THE ACTION:

First, demagnetize your nails, knife, and screwdriver by activating
your coil with 120 v AC and pulling the metals through the tube
while the coil is ON.  Test them by dipping them in iron filings.


                          EVIDENCES SUPPORTING THE DOMAIN THEORY:


MAGNETIZING BY RUBBING ON ONE DIRECTION ONLY--

     Use the permanent magnet to magnetize the knife or screwdriver
by stroking in one direction.  Check it with compass and iron filings.
The domains have been lined up in the same direction.
     Reverse the direction by stroking the metals in the opposite
directions.


BREAKING A MAGNET GIVES TWO MORE MAGNETS--

     Breakable magnets can be had from the catalogs. (I fake mine by
having a previously broken one assembled in the demo drawer, and
pretending to break a good one above the drawer, and it s slight-of-
hand time).
     The domains maintain the two pole magnetism even when the bar
is broken many times.


HEATING DESTROYS A MAGNET--

     Magnetize a large nail.  (It won t be very strong as it is soft iron,
but you don t want to heat your knife or screwdriver and destroy
their temper).
     Test it in iron filings, then heat it red hot.  Test again with
filings and it s no magnet.
     The thermal agitation of the domains has scrambled them. (The
temperature  at which this happens is called the Curie Point).

MECHANICAL SHOCK DESTROYS A MAGNET--

     Magnetize a large nail, test it, and pound the heck out of it with a
hammer. (Placing it on concrete or another hammer for an anvil
works)  Well, what do you know, that magnet s dead. The domains
have been persuaded to scramble.


SATURATION OF A MAGNET--

     When all of the domains are lined up, the magnet gets no stronger.
Weakly magnetize the screwdriver by passing the permanent magnet
nearby, test it and then be more serious about it.

DEMAGNETIZING WITH AN ALTERNATING FIELD--

     When the magnet is pulled through the coil charged with AC, the
domains are scrambled as the bar moves through the alternating
magnetic field.  It s good and dead!

MAGNETIZING BY THE DC COIL--

     Place the screwdriver inside the coil, and energize it with two 6
volt batteries in series.  This will do it if the current is strong
enough and the bar is not too thick.  Now the domains
are aligned in one direction by the constant electromagnetic field.


MAGNETIZING BY SUDDEN AC JOLT--

     Place the screwdriver inside the coil and give the AC circuit a
quick zap.  Because the AC is sinusoidal, we know not wherein the
cycle we zap it.  It might be high on the positive side or anywhere
else.  We find out by testing the magnet produced to see if it is
strongly magnetized and which pole is which. Repeat this several
times to see the variance.


SIZE OF MAGNET DETERMINES ITS STRENGTH--

     Make different sized magnets using different sized screwdrivers

and compare.  The more metal, the more domains are adding their
magnetisms.


HARD STEEL VS SOFT IRON--

     Compare the strengths of nail magnets Vs screwdriver ones. The
alloy elements in the hard steel tend to hold the domains in place.
Hence, hard steel is not only more permanent but harder to
magnetize.

ELECTRON MICROSCOPES HAVE PHOTOGRAPHED THE DOMAINS.


------------------------------------------------------- ---

                                 GREAT DEMOS DEPARTMENT

                                     ELECTRICAL CIRCUITS

                                            by the Boom


CIRCUITS IN SERIES, PARALLEL, AND COMBINATIONS.

A nice way to allow students to see the results of combining
resistors in series, parallel, and compound circuits, is to use
standard clear glass 120-volt light bulbs as well as electrical
meters.  This way, they can see the changes in voltage drops by the
intensity of the filament glow.

Screw a sturdy double-pole knife switch to one end of a board about
one meter long.  Attach to the switch the leads of a cord that has a
plug on one end.  This will enable you to activate your circuits by the
switch rather than by pulling the plug.

To the rest of the board, attach five or six surface sockets in a row.

Radio Shack supplies bags of inexpensive connectors (with Roach
clips at each end) that are ideal for making your circuits.

Acquire five or six clear glass light bulbs of the same wattage and
several bulbs of different wattage.

If you want to do quantitative measurements, you ll need AC meters
for at least 120 volts, and 10 amps.

Now you re ready for electric action, and do not include yourself in
any of the circuits.

Set up first several series circuits with the same wattage bulbs,
then include some different sized ones, note the intensities of the
bulbs and measure the voltages drops and amperages in the circuits.
See if the sum of the voltage drops equals the applied voltage.

Set up parallel combinations and likewise measure the voltages and
amperages. And see if the total amperage is the sum of the
individual ones.

Set up compound circuits and do likewise.

Calculate wattages by multiplying voltage drops by amperages.

Another interesting measurement is the difference between the
resistance of a light bulb when the filament is cold and when it is
white hot.
To do this, determine the resistance of a single bulb by measuring
its voltage and amperage and using Ohm s Law to find its resistance.
For the cold bulb, use a battery as your voltage source, and for white
hot, use the line voltage on the bulb.  The resistance is very much
higher at higher temperatures.

After comparing the two resistances for the same bulb, ask why an
electric appliance such as a stove, a heater, or a toaster, will warm
up to red hot but not keep heating further and melt down. It is
because the resistance increases with temperature until it finally
opposes further heating (the inverse feedback system).

To demonstrate the action of a fuse, cut a very thin (couple of
millimeters wide) piece of aluminum foil about three or four
centimeters long, fold over the ends a bit to increase their strength,
and clip the ends to two connectors.  Being sure your switch is open,
and connect the roach clips to the switch.  Stand by to execute the
fuse...  Throw the switch and watch it blow! Neat action!  (Be sure
you know where your circuit breaker is located as you may have to
re-set it).

---------------------------------------------------------


PHYSICS INSTITUTE 1987                                       GREAT DEMOS
                                             DEPARTMENT


                                    PROBLEM OF THE BALANCE

                                          By Preston Boomer
                                       Solved by Jearl Walker

Ah, we are about to learn something that very few people know!

BACKGROUND:

     For many years, The Boom wondered why an object, say a
meter stick, will balance horizontally.  A force is needed to rotate
the stick a few degrees, and when the stick is released, it returns to
the horizontal.  Why?

     When the balanced meter stick is turned somewhat, one end is
closer to the earth by a few centimeters and should therefore be
ever so slightly heavier, while the other end, being a few
centimeters farther from the earth, should be somewhat lighter.
This torque will tend to keep the stick turning toward the vertical,
and not return it to horizontal balance.

     Also, when the meter stick is rotated, its center of gravity is
shifted to one side of the fulcrum, and this torque should encourage
the stick to continue rotating to the vertical.

     Over the years, The Boom has asked knowledgeable people about
this and received the same answer,  Gee, I don t know, I never
thought about it before.

     After reading an anthology of Isaac Asimov articles, The Boom
wrote to him and received a nice reply on 9/7/76, in which he
stated,  I am but an amateur physicist but it seems to me that
torque depends on weight and distance from fulcrum,  If the bar
tilts, horizontal distance from fulcrum decreases and torque moves
back to maximum--- but I may be all wrong.

     In May, 1987, The Boom re-read his copy of FLYING CIRCUS OF
PHYSICS by Jearl Walker (who also is editor of THE AMATEUR
SCIENTIST in SCIENTIFIC AMERICAN), and noticed that in it he invites
questions.  The Boom sent him the problem, and immediately
received an enthusiastic answer.  He said that he didn t known
either, but he proposed the following explanation:

      It must be that the fulcrum is not microscopically sharp.
Instead, it must have a slightly flat top, although it may look sharp
to the unaided eye.  When you lower the right side, the stick actually
pivots around the right side of the flat top, with the center of mass
always slightly to the left of that pivot. When you then release the
stick, gravity pulls on the center of mass, causing a torque that
returns the stick to the horizontal. This action by gravity
overwhelms the slight difference in the weight of the two halves of
the stick when the stick is released.


PUT IT TO THE TEST:

     So we clamped a razor blade in a vice and attempted to balance a
meter stick on it.  After 15 frustrating minutes, we gave it up.  We
could not balance the stick on a super sharp fulcrum (where the shift
of the fulcrum to a new fulcrum would by negligible).

     We then we tried it with kilogram masses hanging at each end of
the meter stick.  At first it was easy to balance,  but then we
noticed
quite a bend in the stick due to the heavy weights.  So we turned the
stick onto its narrow edge to reduce bending.  We were unable to
balance it.  We would get very close, but never succeeded in having it
balance for more than a few seconds.


Here are the diagrams provided by Jearl:

-----------------------------------------------------


PHYSICS INSTITUTE 1978                                          GREAT DEMOS
                                                 DEPARTMENT



                           EVIDENCES FOR THE KINETIC THEORY

                                              by the Boom


     The KINETIC THEORY states that molecules are in motion, have
elastic collisions, and the warmer they are the faster they move.

                 (These demonstrations a full period to perform).


1.   GAS PRESSURE.  Inflate a balloon and ask why it expands as air is
added.  If it is just because the balloon must make room for the air,
then why is the air so very compressible? A sand-filled balloon
would not be compressible.  Ah, there must be much space between
the molecules, but then why doesn t the balloon squeeze  em tightly
together?  HMO, the molecules must be moving and going BAM
BAM BIFF BIFF and pushing each other apart and the balloon too.

2.   EXPANSION UPON HEATING.  Use the standard ball and ring device
to show expansion of solids.  A flask fitted with a one-hole stopper
and a piece of glass tube, and filled completely with colored water,
will show liquid expansion (a Galileo thermometer).  A balloon
immersed in hot water will show gaseous expansion.
     Note: Be sure to point out that the molecules themselves do not
expand, but it s the impact of their collisions which push them
further apart to cause expansion.
     Then there is the famous discussion on whether a hole in a metal
plate gets larger or smaller when the plate is heated. It gets larger
along with the rest of the plate since molecules are pushing each
other farther apart in all directions.

3.   DIFFUSION.  At the beginning of the period, fill a large beaker (i.e.
3000ml) with water and, when the water is still, add a couple of
drops of food coloring.  Later in the period the color will have
diffused throughout the water.  Ah the intermingling of moving
molecules.
     For gaseous diffusion, pour a few ml of 15 Molar ammonium
hydroxide onto the floor in the middle of the room.  The students
will soon notice that the ammonia has diffused through the air.

4.   OSMOSIS. Diffusion through a porous membrane can be shown
either with the usual biological methods, or with the unglazed
porous cup made for this purpose obtainable from scientific
catalogs.  Insert a one-hole rubber stopper in the cup with a glass
tube about 50cm long.  Hold the cup up with the tube sticking into a
beaker of colored water (the
diffusion beaker above is ideal).  Then invert a 1000ml beaker over
the cup which you are holding with your hand. Then with the other
hand, put a piece of rubber tubing onto the gas jet, start the gas, and
hold the end up inside the beaker containing the cup and your first
hand.  After a few seconds, you may turn off the gas jet. Inasmuch as
the low density methane will diffuse into the cup faster than the
higher density air will diffuse out, an osmotic pressure is built up
shown by the bubbles coming out in the water below.
     Then, when the bubbles stop, remove the upper beaker.  Now the
gas inside the cup will be diffusing out faster than the air outside
will diffuse in, the osmotic pressure inside is less than
atmospheric, and the water in the beaker below will dramatically be
pushed up the glass tubing.  Neat!


5.   MORE DIFFUSION.  Wearing your goggles, pour a couple of ml of 12
M hydrochloric acid into a collecting bottle. Likewise pour a couple
of ml of 15 M ammonium hydroxide into another collecting bottle.
Cover both bottles with glass plates.  Carefully shake the bottles to
encourage some evaporation of the liquids.  Point out that the HCl
fumes from the acid are twice as dense as the ammonia fumes from
the ammonium hydroxide.  Next remove the glass plates, invert the
ammonia bottle, and place its mouth on top of the mouth of the HCl
bottle.  The white fumes (ammonium chloride) formed show the
meeting of the gases.  Even though gravity would have the gases
remain apart, due their densities, there is diffusion in both
directions. Those molecules are moving!

5.   HEAT CONDUCTION.  Place drops of candle wax on a bar of metal
(the standard heat conduction demo bars are nice). Heat one end and
see the progress of heat conduction by the melting of the wax drops.

6.   VAPOR PRESSURE.  Toy steam engines are great for this (there is
usually a student that can bring one in). Or make a steam generator
with a thick walled flask, a one-hole stopper with a glass bend.  Boil
the water and shoot the steam on a pinwheel.  If you have a Hiero s
Steam Ball, use it. Steam power is neat!

7.   BROWNIAN MOTION.  Try the Biology department for the apparatus
for this. Otherwise, describe it (cop out time). It is usually shown in
your films anyway.  But it does need a microscope or microprojector.


----------------------------------------------------------

PHYSICS INSTITUTE 1987                  NEAT  DEMOS  DEPARTMENT


                         AQUEOUS RHEOSTAT AND HEATING

                                          by the Boom


THE SALT WATER RHEOSTAT--

     Set up a circuit with switch, large clear glass light bulb (i.e. 300
watt), or several smaller bulbs in parallel (the board in ELECTRICAL
CIRCUITS is ideal), the 120 volt source and the following rheostat:

     The aqueous rheostat is a 500 ml beaker containing dilute salt
water, and a pair of electrodes that you can raise and lower into the
solution.  The electrodes can be pieces of metal (screwdrivers are
nice because of their insulated handles), or carbons from flashlight
cells.

     Connect the electrodes in series with your light bulb(s). (Be sure
your bulbs are in parallel for maximum load on the circuit).

     Raising and lowering the electrodes into the solution will cause
the lights to vary their intensity, and illustrate the principle of
ionic solution conduction and surface area of contact to vary
resistance.

     This principle was used in 1946 to control the lights in the
Boomer Gambling Palace, a somewhat dubious operation in the
grandfather s basement, whose objective was for the 9th graders to
relieve the loose change from the underclassmen.

     The above usage led to another discovery. That soon the solution
became heated by the current flow of many lights.  The intensity of
the lights increased as the temperature rose.  Then the lights began
to flicker as a result of the rheostat s commencing to boil.


So here is another neat demo with this apparatus:


              ZEE QUEEK BOIL


     Increase the concentration of the salt solution, connect the
electrodes directly to the AC switch, immerse the electrodes all the
way, be sure that they do not touch each other, and let  er ROAR.  We
should have quick boil in a matter of seconds.  If your circuit breaker
kicks out, dilute the salt solution (always know where your breaker
is located).

     This principle was used some years back in high speed egg
cookers and baby bottle warmers.

----------------------------------------------------------


                                          CHAOS

                                   James Gleick


THE BUTTERFLY EFFECT--

     Sensitive Dependence upon Initial Conditions

          The movement of a butterfly s wings in China may affect the
weather in Paris.


     For want of a nail, the shoe was lost;
     For want of a shoe, the horse was lost;
     For want of a horse, the rider was lost;
     For want of a rider, the battle was lost;
     For want of a battle, the kingdom was lost!
-------------------------------------------------------------
----

TRIBOELECTRIC SEQUENCE

Left hand substances tend to lose electrons, Right hand ones tend to
gain electrons.


bunnyfur/lucite/glass/quartz/wool/kitty/silk/cotton/wood/amber/
re sins/metals/teflon  -

-------------------------------------------------------------

                                     THE LOCOMOTIVE PROBLEM

                                                       or

Why does the Train move forward when the piston pulls backward?


     The Steam Pressure pushes the piston in BOTH directions,
forward and reverse.

     On the Forward Push, the piston rod is connected to the driver
wheel above the axle forming a lever of the second class where the
fulcrum is at the track and the forward thrust (the load) is at the
axle.  This gives the forward thrust of the piston a mechanical
ADVANTAGE.

     On the Backward Push of the piston, we now have a lever of the
third class where the piston rod is connected to the driver wheel
between the axle and the track, that is the force is between the load
and the fulcrum.  This gives the backward pull of the piston (which
is trying to make the train move in reverse) a mechanical
DISADVANTAGE.

     Take an example where the connecting rod joins the driver wheel
half way between the axle and the rim:

     During the forward movement of the piston.

          Inside the cylinder there is a balance of steam pressure
against the piston and the cylinder head.  However the force exerted
against the wheel has a lever arm whose length is THREE compared
to the load arm which is TWO.
               Hence the MA is 3/2 or 1.5.

               Subtracting the backward force against the cylinder head
of say one unit from the forward force caused by the lever action of
1.5 units gives a net forward force of 0.5 units.

     During the backward movement of the piston.

          Now with the load arm equal to the distance from the track to
the axle, and the force arm equal to half the distance from track to
the axle, we have a third class lever whose MA is 1/2 or 0.5. A
disadvantage!

          Again the forward force of steam pressure is equal against
piston and cylinder head.  However it is now against the FORWARD
cylinder head.

          Taking the difference between the wheel s MA of 1/2 trying to
push the train in reverse, and the full force of 1 unit pushing
forward against the cylinder head (which is firmly attached to the
locomotive) we find a net forward force of 0.5 units.  The same as in
the first case above.


     Hence, both halves of the cycle contribute equally to the forward
propulsion of the locomotive.

------------------------------------------------------------

   FUNDAMENTAL INTERACTIONS


   FORCE        RANGE (m)  STRENGTH           ROLE


Strong Nuclear    10-15      1041  Binds Protons &  Neutrons


Weak Nuclear      10-15      1028  Radioactive Decay


Electromagnetic   Infinite   1039  Binds Atoms to form
                                     molecules, propagates
                                     light, electromagnetic
                                     waves.

Gravitation       Infinite     1   Holds planets and stars
                                     together.


                                 BUILDING BLOCKS OF MATTER

PARTICLE          DESCRIPTION                 EXAMPLE


Lepton      Dimensionless , <10-35m.      Electron
           Do not participate in the      Muon
           strong force.                  Neutrino

Quarks     Small, <10-18m. Do             Hadrons (three
           Participate in strong force.     quarks)
                                          Mesons (two quarks)
------------------------------------------------


                                    ATOMIC STRUCTURE
                                                  and
                  The Discovery of Sub-Atomic Particles

ATOMIC THEORIES:
     Democritus
     Dalton  (Modern)

LAW OF CONSERVATION OF MASS  (Exper 8)
    The Total Mass Entering a chemical reaction = the         total
mass leaving the reaction.

LAW OF DEFINITE PROPORTIONS (Dalton s Law)
    Every compound has a definite ratio of elements by
    mass. (Ah... Formulae!)

LAW OF MULTIPLE PROPORTIONS
    Some compounds exhibit whole number ratios of elements,

        H2O, H2O2, NO, N2O, N2O3, N3O4, FeO, Fe2O3, Fe3O4

THE LAW OF GAY LUSSAC
    The combining ratios in chemical reactions are in the
        ratio of small whole numbers.

The Above Laws indicate that there are basic particles (atoms) that
make up matter.

FARADAY S DISCOVERY--
    Electricity decomposes compounds into elements.  On
    re-forming, electricity is re-generated.

    This shows that the binding force between atoms in a
    compound is electrical.  (Not the mechanical  hooks and eyes
of Dalton s atoms.)


THE DISCOVERY OF PARTS OF THE ATOM:
    (Late 19th and early 20th century action)

    The high voltage Induction Coil,  Sparky , gave the
    energy needed to probe the atom.

     Discharge Tubes-- lowering air pressure in a tube
    allowed the  Cathode Rays  to travel great
    distances.

*** Diagrams ***

THE DISCOVERY OF THE ELECTRON--

    Crookes Tube (the tube of Sir William)
        Showed that the Rays

Travel from Cathode (negative) to Anode (positive).  Hence they are
NEGATIVE.

Are easily stopped by thin metal obstacles (the Iron Cross).

Travel in Rectilinear Propagation (Straight lines) because the
shadow of the cross is sharp.

Are attracted to positive electric charges and repelled by negative
charges placed beside the tube. Hence, again, the rays are Negative.

Are bent at right angles by magnet fields. Cause fluorescence.

 Paddle Wheel Tube (the tube of Jean Perrin) showed that the
Cathode Rays have mass and velocity (momentum) because they push
the paddles. Great fluorescence colors are demonstrated by paint on
the paddles.

The rays therefore must be particles and were named  Electrons .


          Sir JJ Thompson s candy cane-shaped tube bent the cathode
rays in a magnetic field and showed     their path on a fluorescent
screen. This enabled the calculation of the Charge to Mass Ratio.
Higher charge would bend the beam more, higher mass would bend
the beam less.

                   e/m = ratio = 1.8  X 10 8   coulomb/gram

 Millikan Oil Drop Experiment to measure the actual charge on an
electron.  X-Rays would add or subtract electrons to the microscopic
droplets. By varying the electric charge needed to balance the weight
of the droplet, three biggies were discovered:
    All electrons are identical.
    The charge of the electron.
    The electron is a basic particle of electricity.

                        e = 1.6  X  10  -19   coulomb of charge

          Mass of the electron determined from e/m ratio and the
charge:

                                   m = 9.11  X  10 -29   gram/electron

THE DISCOVERY OF IONS AND THE PROTON--

     The Canal Ray tube was built to see if there are rays from the
anode.  There were positive rays, but not from the anode. They were
produced by the impact of electrons with atoms of gas in the tube.

     The collisions knocked electrons off the atoms     creating positive
IONS. The e/m ratios of the ions (in a JJ tube) depended on which gas
was used. When using the simplest gas, Hydrogen, we found the
simplest ion, the Hydrogen nucleus called the  PROTON .


THE DISCOVERY OF ISOTOPES--

     The Mass Spectroscopy is a refined Sir JJ tube built for accurate
measurements. When ions were sent through it, they separated into
several beams according to different masses (their charges were the
same). These different masses for the same element were called
 ISOTOPES  (same place on the periodic table).

THE DISCOVERY OF THE NEUTRON--

   To explain Isotopes, we needed a NEUTRAL particle whose mass
was equal to that of the Proton. This way we could account for a
change in mass without changing which element was present.

   The Neutron was discovered in 1932 by Chadwick using a piece of
jam jar paraffin to react with radiation.

THE DISCOVERY OF X-RAYS--

     Roentgen was experimenting with discharge tubes found
fluorescent substances were glowing around in his lab including
places behind barriers. Powerful!
   X-rays are electromagnetic waves above the Ultra-Violet on the
spectrum.

  X-rays are used to determine:

   The Atomic Number (the number of protons in the nucleus. The
wave length of X-rays depends on number of protons).
   The Structure of crystals  (X-ray diffraction studies).

 THE DISCOVERY OF RADIOACTIVITY AND ITS THREE RAYS--

   Becquerel, experimenting with fluorescent minerals, found that
Uranium ore on his desktop exposed film in the drawers below with
the shadow of a key thereon.

   Marie Curie analyzed Uranium ore and discovered new radioactive
elements including Radium.

  Lord Rutherford, using Marie s Radium, found three rays coming
therefrom:
           Alpha rays--
           Positive Helium ions
           Beta rays--  Electrons
           Gamma rays-- Electromagnetic waves of very high energy.
WOW!

             Properties of Radioactivity:
                 Cause ionization
            Discharge electroscope
            Cause fluorescence
                 Expose photographic film
                 Destroy cells
                 Promote nuclear reactions

THE SIZE OF THE ATOM--

    For Copper, weigh out 1 mole-- 63.5 g/mol.
        Make it into a cube.  Ah, it s 2 cm per side.
        This cube contains 6 X 10 23 atoms (1 mol).
        To find the number of atoms per side of the cube, take
                     the cube root of 6 X 10 23.  It s about 10 8 atoms per
                     side.
        Now divide the 2 cm per side by the 10 8 atoms per side,
                  and you get about 10-8 cm per atom.
                     This is called the Angstrom unit (the size of an atom).

THE SIZE OF THE NUCLEUS--

          Lord Rutherford shoots Alpha Particles (Helium nuclei)
              through a thin piece of gold foil.
               SHOCK! most Alphas go right on through. Hence the atom
must be mostly empty space!
     A very few Alphas are deflected however.  From the statistics of
the deflection patterns, the size of the nucleus is calculated. About
10 - 13 cm diameter! Compared to the size of the atom (10 -8 cm),
the nucleus is  The flea in Yankee Stadium).

DISCOVERY OF THE ORBITALS OF THE ELECTRONS:

 Spectroscopy--

     When electrical discharge is sent through gases in a tube, and the
light emitted passed through a prism, we get a spectrum of bright
lines. These lines are the spectral  finger prints of the atom . From
the line spectra we learn:

    Which elements are present (like in the stars).
         The arrangement of electrons around the nucleus, called the
            Electronic Configuration (the electronic structure chart).



------------------------------------------------------------


NUCLEAR REACTIONS--

    Top numbers are the mass numbers (protons + neutrons).
    Bottom numbers are the charge numbers (atomic numbers).
    The sum of the numbers on the right must equal the sum of the
                   numbers on the left.

     4/2 He  +  9/4 Be   --->  12/6 C  +  1/0 n

     2/1 H   +  16/0  O  --->  14/7 N  +  ?

     4/2 He  +  27/13 Al --->  30/14 Si  +  ?

     2/1  H  +  10/5  B  --->  11/6 C  +  ?

     210/84 Po --->     ?      +  4/2 He

         ?     ---> 212/83 Bi  +  0/-1 e


TRANSMUTATION--

     1/0 n  + 238/92 U  --->  239/93 Np  +  0/-1 e

then:

NUCLEAR FISSION  & THE CHAIN REACTION--

  235/92 U  +  1/0 n --->  141/56 Ba  +  92/36 Kr  +  3 1/0 n  +
ENERGY

NUCLEAR REACTIONS--


     4/2 He  +  9/4 Be   --->  12/6 C  +  1/0 n

     2/1 H   +  16/0  O  --->  14/7 N  +  ?

     4/2 He  +  27/13 Al --->  30/14 Si  +  ?

     2/1  H  +  10/5  B  --->  11/6 C  +  ?

     210/84 Po --->     ?      +  4/2 He

         ?     ---> 212/83 Bi  +  0/-1 e


TRANSMUTATION--

     1/0 n  + 238/92 U  --->  239/93 Np  +  0/-1 e

then:

NUCLEAR FISSION  & THE CHAIN REACTION--

  235/92 U  +  1/0 n --->  141/56 Ba  +  92/36 Kr  +  3 1/0 n  +
                                                                          BIG ENERGY


THE NUCLEAR REACTOR--
         Fuel
         Moderator
         Control Rods
         Coolant/power source
         Shielding

NUCLEAR BOMB--  The Critical Mass and Separation of Isotopes


NUCLEAR FUSION & MASS DEFECT--

     2/1 H  +  3/1 H ---> 4/2 He  +  1/0 n

            Reactants:   2 H    2.01471    g/mol
                         3 H    3.01707
                                _______
                                5.03178

            Products:    4 He   4.00390
                         1 n    1.00893
                                _______
                                5.01283

                     Reactants:         5.03178
                     Products:          5.01283
                                _______
                     Mass Defect        0.01895

                E = mc2

            COMPARISON OF ENERGIES




PHYSICAL--


       Rearrangement of molecules, no new substance formed.

            Involves electrons, van der waals forces,
            and hydrogen bonding.

                 Crystals
                 Melting
                 Evaporation


CHEMICAL--

            Rearrangement of atoms, new chemical substances formed.

                 Involves electron transfer and sharing.

                      Chemical reactions of various types.



NUCLEAR--

            Rearrangement of the atomic nuclei, new elements,
            isotopes and sub-atomic particles formed.

                 Energy is from the Mass Defect in  E = mc 2

                      Transmutation
                      Fission
                      Fusion
                      Particle interactions.



COMPARISON OF ENERGIES--


                 Physical : Chemical : Nuclear

                    1     :   100    : 1,000,000


                               WOW!


------------------------------------------------------------
-------



Any comments, corrections, or additions may be sent to

Preston  The Boom  Boomer
60 Verde Drive
Santa Cruz, CA 95060
408 426-2617