MISSISSIPPI STATE UNIVERSITY
PHYSICS AND ASTRONOMY
LECTURE DEMONSTRATIONS
Listed here are items of equipment and lecture demonstrations available. Locations are given in red to the right of each item. Most items are in room H231; their locations are given by shelf number. If the item is in a drawer, the shelf number is followed by a drawer number. For example, (4D-dr3) indicates drawer 3 on shelf 4D. For items not in H231 location is indicated by a room number in the form H#. Some demonstrations use such common and unspecific equipment that no location is specified. For most demonstrations there are links to pictures and/or instructions.
In addition to the equipment listed, there are video projectors, overhead projectors, slide projectors, 16mm movie projectors, several notebook computers, two large TV sets, a laser disc player, a DVD player, two VCR's, and a Flex Cam (for extreme close-ups of small items). Audio-visual equipment and some large pieces of demonstration apparatus are in H240. Oscilloscopes, other electronic equipment, PC's with interfaces, and other items are available in elementary and advanced laboratory storage areas.
Each of the large lecture rooms (H150, H250, and H350) has a ceiling-mounted video projector, an overhead projector, a PA system, a very large ring stand, and a very strong ring in the ceiling.
Send questions or suggestions to Joe Ferguson
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LINKS TO SUBJECT AREAS
(Click on a subject link to move to a more detailed list.)
(In each list click on individual demo links for more details.)
MECHANICS
Miscellaneous , Units, Weights, Measures , Inertia-First Law , Projectile Motion ,Circular Motion, Gravity, Impulse and Momentum , Center of Mass , Third Law, Energy Conservation , Rotational Motion and Angular Momentum , Simple Machines ,
Oscillations and Resonance
ELECTRICITY AND MAGNETISM
Electrostatics , Capacitors , Circuits , Magnetism , Faraday's and Lenz's Law
WAVES AND SOUND
Waves , Sound , Standing Waves-Resonance
LIGHT
Reflection and Refraction , Total Internal Reflection (TIR) , Image Formation ,
Spectrum and Color , Interference and Diffraction , Polarization
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GENERAL USE ITEMS
Laser Pointers (22C)
Room Specific Wireless Microphones (22C)
Blocks of Wood for Supporting Equipment (22E&F)
Ring Stands and Clamps of Various Sizes (5A, 5F, 21E, 22E, H230, H240)
Belt-driven, variable-speed rotators (12E, H240)
Rotator Accessories (14D-dr6)
Fog Hog (11A)
Flex Cam Video Camera (H240)
Pasco Computer Interface (21C)
Vacuum Pump on Cart (cart and 20D-dr 1-3)
Vacuum Accessories (20D-dr 1-3)
Power Supplies and Battery Eliminators (8F, 12F)
Tools and Expendables (20E&F drawers)
Household Chemicals (20B, 21B)
ASTRONOMY
Earth Globe (1)
Scale Model of Solar System (Second Floor Hallway)
Celestial Spheres (1&2)
Hoola Hoop (Collapsible) as Orbit Plane (2)
Several Large Styrofoam Spheres (2)
Two Ball Drop: Planetary Slingshot Analog (6D-dr1, 6C)
90mm Telescope With Tripod and Solar Filter (H330A)
Several 8in Telescopes with Tripods and One Solar Filter (H330A)
Howell Observatory with 14in Celestron Telescope (South Farm)
Sunspot Viewing with Telescope (H330A)
Solar Eclipse Viewing (2, 18E-dr9 , H330A)
35mm Slide Collection (H240)
MECHANICS
Miscellaneous
Various Springs (6D-dr6)
Various Balls (6D-dr1, 6C)
Various Electric Vehicles (4C)
Various Spring Toys (4C)
String (6D-dr3, 21F-dr7)
Pulleys (6D-dr5)
Units, Weights, Measures
Stop Clock (4F)
Stop Watches (4C)
Meter Stick (5F)
Metric Tape (under 2)
Various Masses (4E, 6D-dr7, 20F)
Triple Beam Balance (4E)
Large Dial Spring Scales (4F)
Various Spring Scales (6D-dr9)
Inertia-First Law
Embroidery Hoop Drops Pen into Bottle (4F)
Showing Inertia of Heavy Metal Block (4D)
Toss Styrofoam Block That Students Think Has Great Inertia (4D)
Handle Setting (4D,4F)
Card Flick-Ball Bearing Drop (4D)
Break String below Hanging Block (4D)
Magician's Table Cloth Trick (4D)
Pulling Dollar Bill from between Coke Bottles (4C)
Inertia of Four Eggs (4D)
Cuicchi Crash Dummy Whiplash (4A)
Large Bed of Nails-Cinder Block Smash (H25)
Projectile Motion
Throw Various Balls (6D-dr1)
Walking Super Ball Drop (6D-dr1)
Spring Gizmo That Drops One Ball and Shoots Another Horizontally (4B)
Dart Guns (4C)
Circular Motion
Cut String on Swinging Ball (5B)
Ball through Slot in Pie Pan Cover (5B)
Tumbler of Water on Board Swinging in Vertical Circle (7C)
Bucket of Water Swinging in Vertical Circle (5B)
Distortion of Spinning Circle of Spring Steel (5A)
Conical Pendulum
Gravity
Weightlessness of Leaky Bottle (4C)
Weightlessness During Jump from Chair While Holding Spring Scale Supported Mass (4C)
Show Cavendish Balance from Advanced Lab (H230)
Impulse and Momentum
Air Track Collisions (H240, H325)
Newton's Cradle (6A)
Billiard Ball Newton's Cradle (6A)
Egg Drop (4A,6D-dr2)
Happy and Sad Balls (4C)
Track and Low-Friction Carts with Computer Interface (7C)
Two-Ball Drop: Planetary Slingshot Analog (6D-dr1, 6C)
Center of Mass
Styrofoam Object with LED's (6A)
CM of Mississippi Map Suspended from Several Points (6B)
Pendulum Cart (6D)
Third Law
Electric Vehicle on Sheet of Paper (4C)
Pendulum Cart (6D)
Propeller-Driven Cart (4D,7C)
Air Track Collisions (H240,H325)
Newton's Cradle (6A,6F)
Energy Conservation
Various Spring Toys (4C)
Chain Reaction of "Dominoes" of Increasing Size (7D)
Hoop and Cylinder Race Down Incline (6C and under 3)
Conversion of Gravitational Energy to Kinetic Energy for Descending Spools (6C)
Two-Ball Drop: Planetary Slingshot Analog (6D-dr1, 6C)
Rotational Motion and Angular Momentum
Large Gyroscope (7E)
Bicycle Wheel Gyroscope (7E)
Speed Up of Spinning Student When Moment of Inertia Decreases (6E,7E)
Angular Momentum of Bicycle Wheel and Spinning Student (6E,7E)
Hoop and Cylinder Race Down Incline (6C and under 3)
Conversion of Gravitational Energy to Kinetic Energy for Descending Spools (6C)
Direction of Travel of Pulled Spool (6E)
Center of Percussion of Baseball Bat (7A)
Simple Machines
Box Including: Pulley System, Gear Windlass, Wedge, Meter Stick Balance (7B)
Oscillations and Resonance
A Box with Various Pendula (7B)
Mass hanging by Spring (6C)
Inertia Balance Oscillations (14D)
Resonance of Mass on Spring (6C)
Inverted Coupled Pendulum Resonance Device (13A)
Tuning Fork Resonance (13B)
Wilberforce Pendulum Resonance (7B)
Connection between SHM and Circular Motion (7E, 13A, 12E, 14D-dr6, 6C)
Standing Transverse Waves on a Long Spring (13C)
Video Tape of Tacoma Narrows Bridge Collapse
FLUIDS
Bell Jar (cart)
Ping-Pong Ball Cannon (left of 19)
Water Manometer (19E)
Mercury Manometer (19E)
Three Hole Bernoulli Column (19E)
Box of Bernoulli's Principle Demos (19E)
Magdeburg Hemispheres (19E)
Blood Pressure Gauge (19F)
Pascal's Principle (19F)
Cartesian Diver (19E)
Archimedes Principle (19F)
Hydrometers (20E-dr8)
Balloons (21D-dr1)
Pressure and Bed of Nails (7F)
Air Resistance on Coffee Filters (19E)
Aneroid Barometer (20E-dr7)
Smoke Ring Cannon (12A)
HEAT AND THERMODYNAMICS
Crooke's Radiometer 19B)
Hero's Engine (19B)
Stirling Engine (19B)
PVT Surfaces (19A)
Box of Thermal Expansion Demos (19C)
Thermal Expansion of Long Wire (19C)
Thermocouple Powered Magnets (11B)
Adiabatic Expansion-Cloud Formation (19C)
Thermal Conduction (19C)
Plastic Hypodermic Syringe Pressure Chambers (19E)
Various Dewars (18F)
Boiling Water at Room Temperature in Bell Jar (cart)
Freezing by Boiling (19B)
Electrical Conduction by Glass at High Temperature (10A)
Temperature-Sensitive Liquid Crystal Sheets (20E and 20E-dr4)
Sling Psychrometer (20E-dr5)
ELECTRICITY AND MAGNETISM Electrostatics
Classic Pith Ball Experiment (8C,8D)
Charged Balloons and Glass Flask (8D,21F-dr1)
Uncharged Objects Attracted by Charged Objects (8D)
Various Electroscopes (8B,8C)
Testing for Conduction (8B,8C,8D)
Conducting Spheres and Other Shapes on Insulating Stands (8C)
Charging by Induction (8C,8D)
Electrophorus (8B)
Whimshurst Machine (9E)
Charge Confined to Surface of Conductor (8C)
Electrostatic Bells (9F)
Capacitors
Effect of Whimshurst Capacitors (9E)
Dissectable Flat Plate Capacitor (8E)
Box of Various Types of Capacitors (9C)
Large High Voltage Capacitors (8F, H240)
RC Circuit (9C)
Circuits
Clip Leads (8E-dr1)
Assorted Hook-up Wires (under 2)
Switches (8E-dr2)
Batteries and Bulbs (8E)
Lead-Acid and Lantern Batteries (8E)
DC Power Supply (8F,12F)
Large Cenco Lecture Table Galvanometer (10F)
Various Projection Galvanometers (10E)
Various Bulbs with Sockets (10C)
Resistor Collection (9C)
Testing for Conduction (8B,8C,8D)
Electric Pickle (10C)
Parallel and Series Bulbs (10C)
Swinging LED Demonstration o f AC Current (8B)
Chassis Ground: Lighting a Bulb with Cart as Return Lead (8E)
Household Ground: Lighting a Bulb with Water Pipe (10B)
Pasco Interface Display of Voltages in RLC Circuit (9B, 21C)
Conduction by Glass at High Temperature (10A)
High Temperature Superconductor (9A)
Magnetism
Variety of Magnets (11F)
Magnetic Deflection of Electrons-Discharge Tube (11A,11C)
Magnetic Deflection of Electrons-Oscilloscope (9F)
Magnetic Force on a Current (11E)
Magnetic Force on a Wire Heated by an AC Current (19C)
Audio Speaker (14D)
Point out Mechanism of the Large Cenco Galvanometer (10F)
Magnetic Field of a Current (12D, 11C, 8E-dr1)
Iron Filing Demo (12D)
Collection of Coil Shapes for Overhead Projector (12D)
Magnetic Force Between Two Currents (10B and under 10A,11A, 12A)
Thermocouple-powered Electromagnets (11B)
Strong Electromagnet (11C)
Box of Solenoids (10D)
Box of Toroidal Coils (10D)
Suspended Button Magnet Stack Compass (12B)
Box of Compasses for Overhead Projector (12B)
Dip Needle (12B)
Various Electric Motors (12C)
Cenco Motor Generator (12C)
Faraday's and Lenz's Law
Step-down Transformer (10B)
Box of Earth Inductors (10D)
Jacob's Ladder: Step-Up Transformer (H240)
Variac (8F)
Small Tesla Coil and Bulbs (10B)
Large Tesla Coil (H240)
Several Induction Coil High Voltage Sources (11C)
Flashbulb Faraday's Law Demo (11C and left of 19)
LED's on Pipe Lighted by Falling Magnet (11C and left of 19)
Eddy Currents in Pipe Due to falling Magnet (11C and left of 19)
Eddy Currents in Swinging Copper Plate (11E)
Eddy Currents in Spinning Disk (11F)
Point out Eddy Current Damping of Triple Beam Balance (4E)
Jumping Ring-Heated Ring (11E)
Turn-by-Turn Transformer Demo (11E)
Hand-crank Generators (12C)
Cenco Motor/Generator (12C)
Tri-color LED Lenz Law Demo (11C)
Super Sensitive LED Lenz Law Demo (11C)
Magnetic Microphone (11B)
WAVES AND SOUND Waves
Transverse and Longitudinal Waves on Long Spring (13C)
Waves on Slinky (13C)
Waves on Rope (13C)
Antique Hand Crank Transverse Wave Simulator (13F)
Torsion Wave Device (under 2)
Sound
Various Small Tuning Forks (14D-dr3)
Tuning Forks with Sound Boxes (13B)
Nail Violin-Nail Guitar (14B)
Coconut Mandolin (14B)
Double Sonometer (H240)
Eight Pipe Octave Organ (240)
Synthesizer (H240)
Audio Speaker (14D)
Matched Vibrating Bars (13B)
Barphone (H240)
Decibel Meter (14)
Cassette Tape Player (14)
Ear Model (14)
Transmission of Sound Through Long Spring (14A)
Martin's Spinning Speed of Sound Measurement Device (14F)
Sound Spectrum with Audio Oscillator and PA System
Doppler Football (14C)
Swinging Doppler Buzzer (14D-dr5)
Speaker Baffle Interference (14D)
Two Speaker Interference (14E)
Standing Waves-Resonance
Antique Mechanical Standing Wave Projection Device (14D-dr2)
120 Hz Standing Waves on a String (13F)
Computer Controlled Standing Waves on a String (14D, 21C)
Standing Waves on a Long Spring (13C)
Double Sonometer (H240)
Singing Rod (left of 19, 14D-dr1)
Effect of Holding a String Against a Fret of a Guitar (H240)
Air Column Resonance (14E, 13E)
Sound Box on Big Tuning Fork (13B)
Assorted Organ Pipes (14C, H240)
Eight Pipe Octave Organ (240)
Twirl-A-Tune (14E)
Heat-Driven Singing Pipes (14E)
Breaking the Beaker
LIGHT Reflection and Refraction
Various Lasers and Tripods (15C)
Various Water Tanks (15B)
Blackboard Optics (16D)
Bending Light Beam (15B, 15C)
Total Internal Reflection (TIR)
TIR in Blackboard Optics Semicircular Block (16D)
TIR in Blackboard Optics Prism (16D)
Rigid Plastic Light Pipes (15D)
Flexible Fiber Optic Light Pipes (15D, 18D-dr6)
Water Stream Light Pipe (15D)
Various Reflecting Prisms (18D-dr3&6)
Frustrated TIR (15C, 15D, 18D-dr6)
Magic Drawing Board or Glo-Doodler (17C)
Image Formation
Plane Mirror Flying Trick (15F)
Two Plane Mirrors at an Angle (16F)
Illusion Box (17C)
Large Concave and Convex Mirrors (15F)
Real Image of Light Bulb (16A, 17A, 18A)
Assorted Lenses (18D-dr5&8)
Half Lens (18D-dr5)
Box of Plastic Magnifiers (17B)
Binoculars (18B)
Slide Projector Lens Replacement (16A, 18D-dr5&8)
Blackboard Optics Ray Tracing Through Lenses (16D)
High Resolution Vision Only at Center of Retina
Spectrum and Color
Color Vision Only at Center of Retina
Spectrum Formed by Prism (15D)
Spectrum of Overhead Projector Light Formed by Grating (17B)
Hand Out Gratings (18D-dr7)
Colored Shadows (16B)
Color Mixing Box (16A)
Tinker Toy Color Mixing Device (16B)
Colored Filters on Overhead Projector (17B)
Spectrum of Light Transmitted by Color Filters (17B)
Colored Object in Spectrum (17B)
Strobe Light on Spinning Colored Fan Blades (17E)
Sodium Lamp (17E)
Spectral Lamps (17E)
Projected Spectrum of Mercury (17B)
Electromagnetic Spectrum-Ultraviolet (18C)
Electromagnetic Spectrum-Infrared and Microwaves (17C, 17F)
Interference and Diffraction
Near Point Source-Concentrated Arc Lamp (18B)
Double Slit Interference (H330A)
Effect of Slit Width on Width of Diffraction Pattern (H330A)
Various Diffraction Gratings (18D-dr7)
CD As Grating (18D-dr7)
Soap Bubble Gun (16C)
Soap Film Interference Projection (16C)
Gasolene Film on Water Projection (16C)
Thin Air Film Between Pieces of Plate Glass (16E)
Newton's Rings (18D-dr8)
Lenses With Anti-Reflective Coating (17C)
Ferguson's Multi-Concept Two Prism Demonstration (15C)
Polarization
Linear Polarizer for Microwaves (17F)
Polarization by Scattering (15B)
Large Polarizers for Overhead Projector (18D)
Polaroid Sunglasses (18D-dr9)
Polarizer 3-D Glasses (18D-dr9)
Random Polarization of Internal Mirror He-Ne Laser (15C)
Polarization of Laser Pointer (15C)
Calcite Crystal (18D-dr9)
Stress Birefringence of Squeezed Lucite (18D-dr6&9)
Stress Birefringence of Cellophane and Clear Plastic (18D-dr9)
Interference of Polarized Light-Double Slit
Interference of Polarized Light with Calcite Crystal
MODERN PHYSICS
Fluorescence (18C)
Spectra Seen Through Hand-Out Grating (18D-dr7)
Projected Spectrum of Mercury (17B)
Electric Pickle (10C)
X-Ray Tube (18)
Laser Tube (18D)
Cloud Chamber (18F)
Falling Domino Analog to Nuclear Chain Reaction (7D)
Crystal Models (20A, 21A, 22A)
High Temperature Superconductor (9A)
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BRIEF INSTRUCTIONS AND ILLUSTRATIONS
FOR SELECTED DEMONSTRATIONS GENERAL USE ITEMS Room Specific Wireless Microphones
The microphones can easily transmit to a room's PA system from other parts of the building. Therefore, make sure you have the one for your classroom or you may broadcast to someone else's class; this has happened.
Also be careful that it is not on if you wear a microphone outside the classroom, for example, in the rest room.
Belt-driven, variable-speed rotators
Shown at right is a rotator (12E,H240) and several
accessories (14D-dr6). The rotator shaft can be
adjusted so that it is horizontal, vertical, or in
between. The speed of rotation is adjustable by
means of a leather disk that moves against a drive
plate. If the disk slips, try cleaning its rim or using
belt dressing on it.
Caution: there is no belt guard.
Keep your fingers and tie away from the belt.
Fog Hog
This device generates a fog that is useful for showing up laser beams and for filling the smoke
ring cannon. Unfortunately, the fog sets off the supersensitive smoke detectors in our building
resulting in a clearing of the building and a visit from the cops and fire department. Therefore,
the fog machine is useful only for demonstrations done in other buildings.
Flex Cam Video Camera
Vacuum Pump on Cart
This camera is easy to connect to the video projector either by means of
usual RCA video connectors or by means of the S-Video connector. The "goose neck" allows the camera to be positioned near objects of interest and the
focusing ring on the lens allows anything from extreme closeups to
distant views.
The vacuum pump has attached to it a large vacuum hose that
has a valve for use in repressurizing evacuated vessels. There
are fittings (20D-dr 1-3) that allow smaller tubing to be
attached to the large hose. A mechanical gauge gives the
approximate pressure. The large plexiglass screen is meant to
protect the class in case there is an implosion of a bell jar.
Caution: protect the pump by never leaving it under vacuum
when it is off and by not leaving it on when it is not
connected to a closed vacuum chamber.
ASTRONOMY
Scale Model of Solar System
The scale of the model is based on the sun being represented by the yellow light bulb in the
ceiling outside H250. Planet orbits are represented by colored pvc pipe in the hallway ceiling.
Posters explain the scale and give additional information. Jupiter's orbit is at the far end of the
hall. There is a poster showing where, on campus, the orbits of the more distant planets would
be.
Sun, Moon, Earth Model
The tennis balls can be passed out the students to help them figure out what the moon will look like for a given position with respect to the Earth and Sun. Each tennis ball is painted black on one side, to represent the unlighted side of the moon, and unpainted on the other side, to represent the part of the moon lighted by the sun.
Sunspot Viewing with Telescope
This model comes apart with each piece a different color.
Set up one of the telescopes, with a solar filter, outdoors on the paved area in front of Hilbun Hall. Run a long extension cord through the window of H125 for the clock drive. Take students outside to the telescope or arrange to have
them stop by on their way to class.
Solar Eclipse Viewing
Besides some of the methods publicized in the media and besides using a
telescope and solar filter (H330A) as with sunspots, there are several ways
that have proven effective in making solar eclipse viewing available to a
large group. We have an eclipse viewing stand (2) that can be used with
one side of a pair of binoculars to form a real image of the sun on a white
paper screen.
The method that gives the largest sun image uses a very small plane mirror (18E-dr9) on a stand about 30m away to form an image of the sun on a screen inside the Hilbun breezeway. This involves the mirror equivalent of the pinhole camera. A mirror that works well is one designed to be used as part of an optical lever.
Both of these methods
require frequent adjustments to compensate for the Earth's rotation.
MECHANICS
Inertia-First Law
Embroidery Hoop Drops Pen into Bottle
Showing Inertia of Heavy Metal Block
Balance the embroidery hoop on top of the glass pop bottle and then
stand a (flat-ended) ballpoint pen (or a nail head down) on the top of the hoop directly above the mouth
of the bottle. Grabbing the hoop from the inside, snap it quickly from
under the pen. The pen should fall into the bottle. Motion of the hand
must be rapid and horizontal and must begin before contact with the
hoop. Hand must grab hoop from the inside. For the case shown in the
diagram the hoop will be snatched away to the right.
Pounding the block: place hand palm down on table top; bridge hand a little so that insides of knuckles are not against table. With the block standing on edge on the table near the thumb side of your hand, tilt the heavy metal block over on top of back of hand. Tell students that the block's inertia will protect you when you pound the block with a hammer. Pound on the block and let them see that your hand is not injured. To ham it up a bit, for your first blow slow the hammer and just barely hit the block to let them think you are not confident. After they groan a bit, really whack the block several times.
Shaking the block: indicate to the students that we sense inertia by how hard it is to accelerate objects, not by how hard it is to lift them. Show them how hard it is for you to shake the block in front of you. Explain that this would also be true under weightless conditions even though it would then be very easy to lift the block slowly.
The photo below shows the steel block(with the hammer) and a styrofoam block of the same dimensions and color.
Toss Styrofoam Block That Students Think Has Great Inertia
Have styrofoam block the same size as and painted the same as the massive metal block. While going though the discussion of how the massive metal block would be just as hard to shake under weightless conditions, slyly switch to the styrofoam block and pretend that it is hard to lift and hard to shake. While discussing that this would mean that the block would be very hard to stop if it were moving fast, pretend to trip and send the block into the audience.
Caution: It is possible that students who are fooled can be injured in their attempt
to escape the approaching block.
Handle Setting
Explain how an experienced woodcutter or carpenter tightens the handle of
his tool by pounding on the end of the handle rather than by pounding on
the head, thus utilizing the inertia of the head. Demonstrate this by
tightening the loose handle of a hammer (4D). Alternatively, as illustrated
at right, use the "inertia block" (5F) that consists of a massive block of
wood with an aluminum rod passing through it. Pound on the end of the
rod and show how the block "climbs" the rod.
Card Flick-Ball Bearing Drop (4D)
Break String below Hanging Block (4D)
This demo uses a piece of commercial apparatus consisting
of a base and a pedestal with a small well at the top. Place a
playing card or an index card on the pedestal and place a
steel ball on the card above the pedestal. Press down slightly to make a small depression that will
keep the ball from rolling off. Thump the edge of the card to send it rapidly out from under the
ball. The ball will fall into the well. One version of this device has, beside the pedestal, a piece of
spring steel that does the "thumping."
Pulling Dollar Bill from between Coke Bottles
Use a fairly massive block of wood that has hooks at opposite ends. Hang
the block by a fairly weak string, e.g. kite string. Connect a piece of the
same string to the bottom of the block. Tell the students you will pull down
on the bottom string; ask which string will break first and why. They will
usually use he weight of the block in arguing that the top string will break.
Instead of pulling slowly as they expect, snatch down on the bottom string
and use the inertia of the block in explaining why that lower string is the
one to break.
Inertia of Four Eggs
Balance one old-style glass pop bottle on another with their openings
together. Raise the upper bottle and insert one end of a dollar bill between
lips of the two bottles. Lower the bottle and balance it again. Bet that you
can remove the bill without upsetting the bottles. The trick is to hold the
other end of the bill firmly while bringing the fingers of your other hand
down very rapidly onto the center of the bill. If you do this rapidly enough,
the bill will come out leaving the bottles balanced one above the other.
This demo is dramatic but has a rather elaborate
setup. Above four beakers partly filled with water
place a strong, light weight aluminum pan. On this
pan, above each beaker place a cardboard center from
a roll of toilet tissue. At the top of each such tube
place an egg. The idea is to quickly snap the pan
away horizontally so that, because of their inertia, the
eggs remain above the beakers and fall safely into the
water. The way to accomplish this is to position the
beaker-pan-tube-egg stack close enough to the edge
of the table so that the edge of the pan will be hit by
the handle of a broom positioned as shown at right.
Bend the bristles of the broom against the floor and
hold them with your foot so that they will act as a
spring to propel the handle toward Cuicchi Crash Dummy Whiplash
the table when you
release the handle. The handle should hit the edge of
the pan before being brought to a stop by the edge of
the table.
Large Bed of Nails-Cinder Block Smash
Crash your hand or something else horizontally into the base of the
dummy. Because the head is supported only by a piece of spring steel
the inertia of the head makes it stay behind briefly. Get rid of the
student misconception that the head goes backward by having some
fixed object touching it before the crash. This nice demonstration made
and donated by Starkville, MS physics teacher Paul Cuicchi.
Although this popular and dangerous demo is often said to demonstrate inertia, it really involves
lots of other physics and is usually poorly explained. The idea is to have a victim in a nail bed
sandwich. He lies on one bed of nails with another on top of him. Above his chest, on the back of
the top bed is placed a large cinder block. Someone smashes the cinder block with a large
hammer. The victim remains unharmed. Both participants and anyone else who is close should
wear eye protection. One famous textbook author claims to have had a bone broken in this demo.
Projectile Motion
Walking Super Ball Drop
To illustrate the fact that gravity does not affect the horizontal motion of a projectile, walk across
the room at constant velocity while holding a superball in clear view about waist high. Drop the
ball as you continue walking. The ball should come back almost to your hand so that you can
catch it with almost no movement of your hand relative to your body. The ball continued to have
the same horizontal motion that it had before you dropped it.
Spring Gizmo That Drops One Ball and Shoots Another Horizontally
Blow Gun Monkey Shoot
This piece of commercial apparatus has one solid ball and one
with a hole through it. After the "gun" is cocked the hollow ball
is placed on the back end of the plunger and the solid ball is
placed in a well just in front of the other end of the plunger.
When the plunger is released the hollow ball is dropped and the
solid ball is shot horizontally. The two balls should hit the table
top simultaneously making only one sound. It is important to
have the apparatus very level and firmly attached to a rod about
2 ft above the table top. Use trial and error to make sure the shot
ball will not clear the edge of the table before hitting.
The blow gun is aimed at a can (substituted
for the monkey to avoid problems with the
animal rights folks) held by an electromagnet
that is in series with a switch at the 
opening to
the gun. When the blown ball bearing
becomes a projectile it trips the switch and
the can becomes a falling object. No matter
the speed or distance involved, the ball should
hit the can. The gun should be secured to a
rod clamped to the table corner. Use two
clamps and a short rod to give the gun some
freedom of movement for aiming . Place the
electromagnet several meters from the gun
and somewhat above it. Connect the supply
lines for the magnet to a six volt battery. Hang
the lines so that they will not be in the path of
the ball. With the
can suspended from
the magnet sight through the unloaded gun to aim it at the can. Tightly
clamp the gun in this position. Load the gun; this may cause the can to
fall. With the target can suspended from the magnet, blow the ball
from the gun. It may take some trial and error to learn to blow with the
right force to hit the can well above the floor and well below the
magnet.
Store the gun and wire neatly as shown at right by using the gun and
rods and clamps to form a frame.
Caution: store the gun with the exit end outward so that it will
not be damaged by the wall or other objects on the shelf or in the storage box.
Circular Motion
Cut String on Swinging Ball
Ball through Slot in Pie Pan Cover on Overhead Projector
The idea is to swing a ball in a horizontal circle at the
end of a string held above your head. Slowly lower
your hand so that the string swings across a sloped
razor blade and is cut. The ball continues tangent to
the circular path rather than radially outward as some
students expect. The balls used have slits that allow
the string anchor inside to be removed for easy replacement of strings. The razor blade is held in
a slot in a wooden dowel that is clamped to the top of a tall stand. Try positioning the stand and
the center of the circle so that the released ball flies across the front of the classroom near the
front row of students. Warn the students to protect themselves because sometimes a partially cut
string does not separate until farther around the circle from where it hit the blade.
Use a transparent plastic pie pan cover (of the type that comes Tumbler of Water on Board Swinging in Vertical Circle
with
graham cracker crusts) that has a pie slice shaped piece cut away.
With the cover upside down on the stage of an overhead projector
send a ball bearing around the cover. The centripetal force will be
provided by the sloped edge of the cover until the ball reaches the
missing section. From then on the ball's path will be tangent to the
original circular path.
A board about one foot square is swung in a large vertical circle by means of four strings attached to its corners. Before swinging the board adjust your grip on the strings so that they hang with the board horizontal. With practice you can produce this vertical circle even with a tumbler or goblet of water "sitting" on the board. Explain how the water and goblet inertia keep them against the board.
Distortion of Spinning Circle of Spring Steel
The device (5A) used consists of two circular pieces of
spring steel mounted on a rod. When the rod is spun (by
means of a drill motor or belt driven rotator (12E,H240))
the steel pieces are no longer circular. They must distort
for the steel to provide the centripetal force to keep each
particle of the band in its circular motion. This can be
related to the distortion of spinning astronomical bodies.
This is the classic experiment that shows that all objects have the same acceleration due to gravity in the absence of air resistance. First, with the tube vertical, flip it 180 several times and observe that the coin falls more rapidly than the small pieces of cloth that play the role of the feather. Then use a hose to connect the tube to the portable vacuum pump. Evacuate until the pump becomes much quieter. Close the valve at the end of the tube, turn off the pump, and remove the hose. Again switch the tube between vertical orientations to show that the coin and cloth now fall with the same acceleration.
Weightlessness of Leaky Bottle
The main device is a plastic pop bottle that has several small holes in the sides not too far from the bottom. There is also a hole in the screw-on cap. Also needed is a bucket or pan to catch most of the water that runs from the bottle. Fill the bottle with water, put on the cap, and hold your finger against the hole in the cap. The water should not run out. With the bottle over the bucket or pan show that the water comes out when you raise your finger from the cap. Stop the flow again. Now drop the bottle into the bucket or pan. The students should be able to see that the water does not come out while the bottle and the water in it are in free fall.
A dramatic variation involves the fact that projectiles are in free fall. You can throw the bottle
over the heads of some students without getting them wet. Let a student use a bucket to catch the
bottle.
Cavendish Balance from Advanced Lab
Shown at left is one cavendish and at right is a newer type that can be interfaced with a computer.
Impulse and Momentum
Air Track Collisions
With a portable air track use two carts of equal mass to show all the simple cases: (1) elastic
collision with one at rest initially, (2) inelastic collision with one at rest initially, (3) completely
elastic collision with two cars moving in opposite direction with same speed initially, (4)
completely inelastic collision with two cars moving in opposite direction with same speed
initially, etc. In addition to momentum conservation, this demonstration illustrates the fact that,
in each case, the forces that the carts exert on one another are equal in magnitude and opposite in
direction.
Billiard Ball Newton's Cradle
Egg Drop
This nice large cradle was made and donated by
Starkville, MS physics teacher Paul Cuicchi.
This demo is meant to show that, for a given impulse, the force is smaller when the time is
longer. Drop an egg onto a foam pad (4A). If you manage to catch the egg on the rebound, it will
not be broken. For contrast you can drop the egg into a transparent, hard-bottom, bowl or beaker
or you can just use the student's understanding of what would happen. Variations might involve
using a rubber egg (6D-dr2).
Catching and Egg in a Sheet
Happy and Sad Balls
Hold a bed sheet or table cloth so that it has a "J" profile. Have someone
throw an egg at fairly high speed toward the sheet. The sheet will give to stop
the egg and then catch it; the egg should be unharmed. Caution: don't miss
the sheet. A few years ago the MSU baseball pitcher who still holds the
school record for wild pitches did miss the sheet in a physical science class.
Track and 
This demo (involving what are sometimes called happy and
unhappy balls) illustrates that twice as much impulse
(momentum change) is involved in reversing the velocity of and object as in just stopping it. First
show with the two free balls that one rebounds elastically and the other sticks where it hits.
Position the two hanging balls so that they are next to the top of the 2X4 block. Pull back the sad
(non-elastic) ball so that the string is horizontal. Release the ball and let it swing down to hit the
block. The block will be moved some but not knocked over. Repeat the collision with the happy
(elastic) ball. The block should be knocked over showing that the impulse was larger.
Two-Ball Drop: Planetary Slingshot Analog
This apparatus accomplishes about the same thing as an air
track without the need for a blower. There are two low friction
carts which can make elastic or inelastic collisions. It is easy to
load more mass on a cart. There are accessories such as
propellers, force sensors, and sonic-ranger motion detectors.
Some devices can be connected to a computer interface for
easy display of parameters.
Be careful: this demo could cost your some
teeth or could wipe out an overhead light.
Hold a large ball, such as a basketball, about
two feet off the floor. In contact with this ball
and just above its center hold a much smaller
ball. Drop the two balls simultaneously so
that they fall together. Stand back because
the small ball will rise fast and to a much
greater height than it had when you dropped
it. You can approximate this as an elastic
collision between the large ball going up and
the small ball going down at the same speed.
In the limiting case of small ball mass negligible compared to large ball mass the small ball
should rise three times as fast as it fell and therefore should reach nine times its original height. It
is a similar, although three dimensional, gravitational "collision" with a planet that is used to
speed up some space craft on their way much farther out into space.
Center of Mass
Styrofoam Object with LED's
Box of Various CM Demos
At right are shown two versions of a styrofoam object
that has one LED in front of the center of mass and
another well away from the center of mass. The
irregularly shaped object at left is convenient for
throwing and catching. With the lights low, it should
be thrown across the front of the lecture room with
spin so that the axis of rotation and LED's are toward
the audience. The CM LED will be seen to follow a
smooth projectile path while the other will have a
much more complex path. The L-shaped object at right
can be thrown in a similar way. It is also convenient for showing the connection between the CM
and stable equilibrium. In the position shown it is stable because CM LED is above the base of
support. If the object is rotated 90 counterclockwise and placed down again, it will not be
stable.
Pendulum Cart
There are several items including a few flat objects whose
CM can be determined by hanging each from several
different point on the object. There are also several that
balance on a single point because the CM is actually below
that point. There is a round object tapered in such a way
that it appears to roll uphill on a ramp. There is a leaning
tower toy that becomes unstable when more mass is added
to the top end.
The cart rolls with little friction on its three roller-skate
wheels. Hanging from a frame on the cart is a pendulum with
a fairly massive bob. With the pendulum oscillating the cart
will move in such a way that the CM stays at rest, if it is
initially at rest, or moves with constant velocity. This motion
also illustrates Newton's laws in that the force put on the
pendulum by the cart is "equal-but-opposite" the force put on
the cart by the pendulum. The net force on the system is zero
and so is the acceleration of CM. Newton's laws can be
illustrated in another way. With a rigid obstacle at one end of
the cart, pull the pendulum toward that obstacle. While you
hold the pendulum in that position, you cause the cart to put a force on the obstacle and the
obstacle to put a force on the cart in the opposite direction. The system is in equilibrium, not
because the two forces just mentioned are equal in size, but because the force you exert on the
pendulum is the same size and in the opposite direction form the force exerted on the cart by the
obstacle. When you release the pendulum the obstacle force accelerates the system forward. After
losing contact with the obstacle, the center of mass moves at constant velocity.
Third Law
Electric Vehicle on Sheet of Paper
Propeller-Driven Cart
Place the drive wheels of a "tether-controlled" electric toy car
on a loose sheet of paper. When the car accelerates forward the
force on the paper will be backward and it should move
backward.
There is a homemade battery-powered propeller cart (4D).
There are also propeller attachments for the low friction
carts. (7C)
Energy Conservation
Bowling Ball Pendulum
Connect the hook of the chain to the ring in the ceiling in front of the lecture table. While
standing upright pull the ball back until it is against your chin. Release the ball and let it swing
away and return. Show your faith in energy conservation by not flinching. Have a student test
his/her confidence in energy conservation. Ask what would happen if the ball were given a little
kinetic energy when it is released.
Chain Reaction of "Dominoes" of Increasing Size
Conversion of Gravitational Energy to Kinetic Energy for Descending Spools
A sequence of properly placed "dominoes" will fall down
one after the other when the smallest is pushed over. There
is a conversion of gravitational potential energy to kinetic
energy as each block falls. Make sure the largest block does
not fall off the lecture table and become damaged.
A string or ribbon is wound around the inner part of a spool. With the outer
end of this string or ribbon attached to a rigid support the spool is allowed
to fall. During its decent gravitational potential energy will be converted to
translational and rotational kinetic energy. For any distance fallen the speed
of decent of the center of mass will depend on the ratio of the inner spool
diameter to the overall moment of inertia of the spool. Consider a race
between two spools that are identical except that one has much larger inner
spool diameter than the other.
Rotational Motion and Angular Momentum
Moment of Inertia Rods
This demonstration can serve as an introduction to the concept of moment of inertia. It utilizes two PVC pipes that look the same and have the same mass. One has most of the mass (two metal plugs) near the center and the other has most of its mass near the ends. Without explaining the difference between the two rods, have two students each hold a rod near the center with one hand. Ask them to give the rods a large angular acceleration by twisting their wrists back and fourth as rapidly as possible. Of course, the student with the rod with the mass at the center will have an easier time of it.
Speed Up of Spinning Student When Moment of Inertia Decreases
This large gyroscope has a built in motor that will spin the rotor
at fairly low speed. However, many demonstrations work just as
well when the rotor is spun by hand and allowed to coast. There
is a box of attachments. The weights on the rotor axis screw on and off. For example, one of the
weights can be removed or repositioned to allow the torque due to gravity to cause the spinning
rotor to precess.
Have a student stand on the lazy susan or sit on the stool on the turntable. (6E) Hand the student
two weights (7E) and ask him/her to hold the weights straight out. Start the student to spinning
slowly. Ask the student to pull the weights in against his/her body. As the moment of inertia of
the student/weights decreases the angular velocity increase to keep the angular momentum the
same.
Angular Momentum of Bicycle Wheel and Spinning Student
Have a student stand on the lazy susan or sit on the stool on the turntable (6E). Hand to the
student a spinning bicycle wheel (7E). The total angular momentum about a vertical axis will be
conserved. You can illustrate the vector addition of the spinning student and bicycle wheel
angular momenta for a variety of cases depending on the orientation of the wheel when you hand
it to the student and how the student changes that orientation. For example, you could have the
wheel axis vertical when you hand it to the student. If the student then flips the wheel axis to
horizontal the student will begin to spin in the same direction that the wheel was spinning
initially. If the student flips the wheel axis 180, he/she will spin twice as fast as would be the
case if he/she had only flipped the axis 90 to horizontal.
Hoop and Cylinder Race Down Incline
Direction of Travel of Pulled Spool
Use the long board ramp (hanging under 3)
with a very gentle slope. Putting a couple of
books under one end is probably steep
enough. Start the hoop and solid cylinder (6C)
at rest at the top end and discuss why the
same one always wins the race. Be sure to
catch them at the bottom end to prevent them
from being damaged by a fall to the floor.
In this demonstration a pull on the ribbon around the
spool can cause it to rotate clockwise, roll
counterclockwise, or slide without rotating depending on
the angle between the pull and horizontal. The easiest
analysis involves considering torques about the point of
contact with the desktop. Torques due to gravity, normal
force, and friction will all be zero about this point. The
net torque will be the torque due to the pull, giving the
three possibilities shown below.
Oscillations and Resonance
Inertia Balance Oscillations
Resonance of Mass on Spring
The inertia balance has a platform supported by two
pieces of spring steel. Mass sitting on the platform will
oscillate back and forth horizontally. It is an easy matter
to change the mass on the balance and observe the effect
on the period of oscillation.
Inverted Coupled Pendulum Resonance Device
Hang from a spiral spring a mass that will give a period of oscillation on the
order of a second. Drive the oscillations by means of a very weak periodic
force. For example, you can push down periodically with a drinking straw.
The straw will bend and provide a very weak force. Show that when the force
is in step with the natural oscillations, a large amplitude eventually occurs.
For a more elaborate driving mechanism uses weak rubber bands to attach the
bottom of the spring (no added mass necessary) to an eccentric pulley (14D-dr6) attached to a belt-driven rotator (12E,H240). Adjust the rotator speed for
maximum amplitude of the oscillations of the spring.
Tuning Fork Resonance
This device has on each end inverted pendula of three lengths. Each
"pendulum" consists of a mass attached to the top of a vertical spring
steel rod. The pendula on one end match those on the other. When a
pendulum of a particular length on one end is set into motion, with
coupling through the base, there will be a resonance oscillation of its
twin on the other end but there will be little oscillation of the pendula
of different lengths.
Use the two large tuning forks with attached sound boxes (resonant air columns) sitting on a
foam pad. One of the forks should have a clamp which can be moved along the fork to change its
frequency. Move the clamp toward the fork's tip to lower the frequency. With the two forks
sitting on a foam pad, strike one fork and listen for a few seconds before using your hand to stop
its oscillations. You should hear nothing from the other fork. Because the frequencies do not
match there is no "sympathetic vibration." Adjust the clamp to make the frequencies of the two
forks match. (You can use trial and error or you can use beats.) Now strike one fork. Listen for a
few seconds and then stop this fork's oscillations. You should now hear a faint sound from the
other fork which has been set into oscillation by resonance.
Wilberforce Pendulum Resonance
Connection between SHM and Circular Motion
The Wilberforce pendulum is a combination of a mass on a spring and a torsional
pendulum. If the frequencies of these two oscillators match, oscillations of one of
them will transfer energy to the other. For example, if you start the mass to bobbing
up and down on the spring, soon the mass will be twisting back and fourth. You can
adjust the frequency of the torsional oscillator by using the two weights on screw
arms to change the moment of inertia. When there is no frequency match there will
be little coupling between the two modes of oscillation.
One simple way to show this is just to wedge a tennis ball in the spokes of the bicycle wheel (7E) and let the students view the wheel edge on as it rotates. The ball will appear to bob up and down in SHM.
For a slightly more elaborate demonstration use a stiff wire of the
shape shown at right (14D-dr6) to attach a ball to a belt-driven
rotator. Hang a mass 
from a spiral spring (6D-dr6&7).
Adjust the amplitude of a mass on a spring to match the radius of
the circle in which the ball moves. Adjust the period of rotation
of the ball to match the period of oscillation of the mass on the
spring. With these two moving objects side by side make the
point that the vertical component of the balls motion matches the
motion of the mass on the spring. You may wish to use a
light source (with rays of light parallel to the plane of the
circular motion) to cast shadows of the two objects on a
screen.
As an alternative to the method above you can use a device
that converts rotational motion to linear reciprocating
motion (13A) or vice versa.
FLUIDS
Ping-Pong Ball Cannon
Water Manometer
Tilt the cannon with the end opposite the vacuum connection
upward. Place a ping-pong ball in the lower end near the connector.
Seal both ends with aluminum foil that is smoothed against vacuum
grease on the ends and secured by means of thick rubber bands.
Alternatively, with no vacuum grease on the ends, seal them with
wide plastic packaging tape that laps over and sticks to the sides.
Pump out the tube. Puncture the bottom seal and the ball will be
propelled at high speed through the top seal. Cautions: make sure no
one is near the muzzle and shut off the pump just before the
puncture or just after it. The pump will be damaged by pumping for
an extended period against atmospheric pressure and also by being
left under vacuum while not running.
Add a little food coloring to the water for visibility. The pressure in the natural gas line us usually
low enough to measure with this device.
Three Hole Bernoulli Column (19E)
The speed with which the water leaves a hole, and therefore the water's
projectile path, can be related to the distance of the hole from the top of the
water.
Box of Bernoulli's Principle Demos
Contents include two different types of
water-jet aspirator.
There are also two different types of bulbs
that can be hung close together. Blow
between identical bulbs and they will be
pushed together by the greater air pressure
outside the air stream. There is a ping-pong
ball which can be suspended on a stream of
air blown from your mouth at a
considerable angle below vertical; this
takes practice and a steady blow.
Magdeburg Hemispheres
Pressure and Bed of Nails
With the two hemispheres joined together (with a little vacuum
grease on the mating surfaces) evacuate the space. Close the valve
and remove the sphere from the pump. Invite a strong looking guy to
pull the hemispheres apart. When he fails, open the valve to admit
air and then ask a slight girl to separate the two hemispheres.
This demo requires only the small bed of nails. Have a student wearing durable jeans sit on the
bed of nails. Alternatively press a blown up balloon against the nails.
Air Resistance on Coffee Filters
Establish the dependence of the force of air resistance on speed by noting the higher terminal velocity of two stacked filters over the terminal velocity at which one filter falls. Note the
continued increase in terminal velocity as more filters are added.
Smoke Ring Cannon
This large box (12A) has a circular hole in one side and a plastic membrane pulled inward by
rubber bands on the opposite side. When the membrane is pulled outward and released, a large
vortex ring shoots from the hole. It will remain fairly stable as it travels many meters. It can be
used to extinguish an candle or move someone's hair across the room from the cannon. You can
make the ring visible by filling the box with smoke or fog (11A) before firing. Unfortunately, this
activity is likely to set off the super sensitive smoke detectors in our classrooms resulting in a
clearing of the building and a visit from the cops and fire department. Therefore, we must restrict
the visible vortex demo to other buildings.
HEAT AND THERMODYNAMICS
Crooke's Radiometer
Hero's Engine
Inside the glass envelope mounted on a needle point are rotating vanes
with one side dark and the other highly reflecting. Shine a light on the vanes and they turn with
the reflecting sides leading because the residual gas in the envelope is heated more by the highly
absorbing dark sides. There is more momentum transfer to gas molecules from this side. If the
pressure inside were low enough and the pivot more nearly frictionless, the vanes would turn in
the opposite direction because the reflected light would have its momentum changed more than
the absorbed light.
By means of a tall ring stand suspend the half-filled flask above a bunsen burner. When the water starts to boil jets of steam from the tips of the two tubes will cause the flask to spin.
Stirling Engine
A small (denatured) alcohol burner heats the cylinder.
PVT Surfaces
Box of Thermal Expansion Demos
Included in this box is the famous ball and
ring demo. When both are at the same
temperature the ball is too large to pass
through the ring. Heat the ring over a burner
and the ball will pass. Never pause with the
ball surrounded by the ring or the ring will
cool and clamp the ball in such a way that it
will be very difficult to remove.
There are several bimetallic strips, some with
and some without handles. The strips are
straight at room temperature but take on
considerable curve when heated over a burner.
There are two bimetallic strip switches, one
connected to two light bulbs in an interesting
way. The small metal disks in the box will jump into the air if they are placed on the table while
hot.
Thermal Expansion of Long Wire
Attach the test tube clamps
holding the two insulators to
the tops of two similar ring
stands. Plug the power cord
into a variac. With the wire
cold adjust the ring stands so
that so that the wire has very
little sag. Use the variac knob
to slowly increase the voltage
to the wire. The wire should
sag. Eventually, the wire
should glow and should have considerable sag.
For an additional interesting effect bring a magnet near the hot conducting wire. The magnetic
force on the wire will be periodic. By trial and error you may be able to get an interesting
standing wave pattern.
Adiabatic Expansion-Cloud Formation
Have a milliliter or so of water in a clear bottle. With the bottle capped the liquid and vapor will come to equilibrium. Increase the air pressure in the bottle and then suddenly release the pressure. The air should cool nearly adiabatically. With the reduced pressure the air should be supersaturated with water vapor. Condensation will occur. To get enough cloud to see easily you need to seed the air with condensation nuclei in advance. Burn a match near the bottle opening letting a little smoke into the bottle. You won't need enough smoke to see. When the expansion occurs this smoke should provide enough condensation nuclei to provide an easily visible cloud. There are several ways to produce the pressure increase followed by the expansion: (1) Just blow into the bottle with your mouth maintaining the pressure a few seconds before releasing it. (2) Attach a large plastic hypodermic syringe to be bottle by means of tubing and stopper. Push in the plunger and hold the pressure a few second before pulling out the stopper. (3) Use a pump up Fiz Keeper designed to use with pop bottles.
A slightly more high tech version of this demo uses a cylindrical plexiglass
chamber. Use a large hypodermic syringe to change the pressure. Make the cloud visible by
shooting a laser beam through the cylinder in a dark room.
Boiling Water at Room Temperature in Bell Jar
Fill a small beaker about one-third full with water and place it inside the bell jar. Pump out the
jar. It will take a while but the water should eventually begin to boil. Repressurize the bell jar
and remove the beaker. Let a student feel the water to see that it has gotten cooler rather than
hotter.
Freezing by Boiling
Place the plexiglass vacuum chamber on the overhead projector. Place water to a depth of 2 to 4
mm in a very light weight, transparent plastic container. (Such a container can be constructed by
taping a 1 cm high, 2 cm diameter circular wall of acetate transparency to a >3 cm diameter disk
of the same acetate. Support the container on an insulating ring (perhaps cut from a styrofoam
cup) inside the vacuum chamber. If you wish to monitor the temperature of the sample, install the
lid of the chamber in such a way that the thermocouple is in the sample and connect the thermocouple to an appropriate digital thermometer. Watch the image of the
sample on the screen as you pump out the chamber. You have to concentrate on this image for a
minute or more or you will miss the main event. The water will quickly boil. Keep watching and
eventually the image will become opaque as the sample suddenly freezes. At the instant that the
super cooled sample freezes, its temperature will suddenly jump from well below 0 up to
precisely 0. Display the temperature by means of the Flex Cam Video Camera (H240) focused on the thermometer display or
by means of a thermometer output voltage graph drawn by an interfaced computer.
Electrical Conduction by Glass at High Temperature
This demo illustrates that at high temperature even glass will
have enough charge carriers to become a conductor. By
means of binder clips slipped over glass rods clamp the two
ends of a small rectangle of window glass. Use the clip leads
of the wiring harness to put the glass in series with a light
bulb. Plug the circuit into a wall outlet. The harness has two rocker switches: one a "dead man"
switch to make it less likely that you will electrocute yourself and the other a switch that short
circuits the light bulb. Use a bunsen burner on an asbestos pad to heat the glass. With the "dead
man" switch closed and the other open, when the glass gets hot enough the bulb will begin to
glow because the glass is becoming a conductor. Short circuit the bulb and turn off the burner.
The current in the glass will continue to heat it and eventually the glass will melt from this
resistive heating and separate in a bright flash.
ELECTRICITY AND MAGNETISM
Electrostatics
Classic Pith Ball Experiment
Charged Balloons and Glass Flask
Two small pith balls hanging from a stand (8C) are observed to
repel one another after they are touched by a charged object
(8D). The charged object might be a negative hard rubber rod
that has been rubbed by fur or it might be a positive glass rod
that has been rubbed by silk. You can also show that a ball
charged by one of these rods is repelled by the rod that charged it
and attracted by the other rod.
In this variation on the classic pith ball experiment, two inflated balloons (21F-dr1) are hung by
strings from a horizontal bar. When both balloons are rubbed with fur (8D), they become
negatively charged and repel one another.(left below) A glass flask about the same size as the
balloons is rubbed with silk and then is found to attract the balloons. (right below)
Uncharged Objects Attracted by Charged Objects
Various Electroscopes
In the classic pith ball experiment we begin with a ball that is attracted to the
charged object brought near it. A charged balloon can be stuck to a wall or,
perhaps, to the ceiling. Small objects (slivers of paper, grains of salt, bits of
aluminum foil, etc.) on a table top can be attracted to a dry drinking straw
that has been rubbed with a dry napkin. The most dramatic version of this
phenomenon is the attraction of a thin stream of water by a charged object
held near it. The water will be diverted considerably from its usual vertical
fall.
Testing for Conduction
The photo shows several electroscopes. Just as
effective as the commercial models is the one at
right that was constructed of some scrap acrylic,
some strips of very thin aluminum foil, and a
binder clip to hold the swinging strip of foil
against the other at the top.
Charge (8C) an easily visible electroscope (8C) and then show that you can discharge it by touching it with your finger. Test various materials to see which can form a conducting link between your hand and the charged electroscope. Test various metal, plastic, glass, and wooden rods. Test a small fluorescent tube.
For a dramatic finish show that current makes the fluorescent bulb light. Use the electrophorus
(8B) as the source of charge. Hold the metal contacts at one end of the bulb in one hand and
touch the charged electrophorus to the other end of the bulb. With the lights out the bulb should
be seen to flash. Insulate yourself from the floor (with some kinds of shoes you may already be
sufficiently insulated) and repeat this flashing exercise several times in succession. Ask the
students where all the charge went. Answer: onto you. Touch the far end of the bulb to a water
pipe to discharge yourself. You should get an extra bright flash and a bit of a jolt. The students
will particularly like to see you flinch.
Conducting Spheres and Other Shapes on Insulating Stands
Charging by Induction
The small sphere (second from right) can be
placed inside the separable sphere (left).
Use the small metal sphere (8C) on the insulating stand as the object to be charged. Use a
hanging pith ball (8C as a detector. Use a hard rubber rod (8D) that has been rubbed with fur as
the source of charge. First, for contrast, do charging by contact. Touch the charged rod to the pith
ball and then to the sphere. Bring the ball near the sphere and show that it is repelled. "Ground"
the sphere by touching your finger to it. Charge the sphere by induction by holding the rod near
the sphere while "grounding" the opposite side of the sphere. Remove the "ground" and then take
the rod away. Bring the pith ball near to show that it is now attracted.
Electrophorus
Whimshurst Machine
This device is a good source for more charge
than can be gotten from a rubbed rod. An
electrophorus consists of a metal base, a
dielectric (wax or plastic), and a conducting
plate on an insulating handle. The plate is
charged by induction from the dielectric. First,
charge the dielectric by rubbing vigorously it
with fur. Place the plate on the dielectric. In
essence, you are only placing the plate near the
dielectric because they contact in only a few
places. Ground the top side of the plate. Remove
the ground and then lift the plate. The plate will
be charged opposite the dielectric. As little, if any, charge is removed from the dielectric, it can
be used to charge the plate many times without the need to rub it again. The grounding can be
accomplished in many ways. You could touch the top of the plate with your finger. In practice,
the easiest thing to do is to slide the plate over so that it touches the metal base. Then slide the
plate back away from the base before lifting it from the dielectric.
This device is a good source of charge and fairly high voltage.
Use it to illustrate the breakdown of air. The size of the spark
gap between two small metal spheres is controlled by means of
two large insulating handles. The smaller handles nearby are
used to connect the electrodes to two leyden jars. With these
connected the capacitance is greatly increased as evidenced by
the much greater cranking time it takes to get the voltage high
enough for a spark. These sparks should be hot enough to poke a
hole in a card held between the balls.
To literally add a little spark to your presentation you can hold
one of the balls and let the spark jump from your finger to the
other ball. Unless you want this to hurt make sure the leyden jars are not connected.
Van der Graff Generator
This well known device can produce voltages higher than those from the whimshurst. Get an idea of the maximum voltage by the length of the sparks you can draw to the small grounded sphere. The generator works best when very clean. If it is performing poorly, use alcohol to clean the dome inside and out. If you do the standard hair standing on end demo, seek out a girl with fine clean hair. Have her stand on a plastic box (9A). Be patient; it may take a while for the hair to stand up much. With this small machine you are unlikely to come obtain the dramatic effect often seen in textbook photographs. On days when this demo does not work very well salvage things by putting a cup of foam peanuts on top of the generator. They should float away as the dome charges.
One former professor jazzed up his demonstration by holding the dome with one hand while he
brought a finger of his other hand near a bunsen burner which had gas flowing to it. A spark
jumped from his finger and lit the burner.
Charge Confined to Surface of Conductor
Charge a pith ball and the small sphere on the stand and show that they repel. Place the small
sphere concentrically inside the sphere that comes apart into two halves. Only after the outer
sphere is closed move the small sphere over to contact the larger one on the inside. Pull the small
sphere back to the center and separate the halves of the bigger sphere. Show that the pieces of the
big sphere are charged and that the small sphere is not.
Electrostatic Bells
Dissectable Leyden Jar
Touch a charged electrophorus plate to the top of the center post
of the bell assembly. This charges the outer bells. The bells will
ring as the clappers are attracted to the outer bells, share some of
the charge and then are repelled, move to the inner bell, share
charge with it and then get repelled back to the outer bells. The
process repeats for some time.
RC Circuit
The photo shows the disassembled leyden jar along
with some accessories. Connect the outer can and
center electrode of the assembled leyden jar capacitor
to the terminals of the whimshurst by means of ball
chains. With the whimshurst's two leyden jars
disconnected turn its crank to charge the leyden jar.
Use the ball tipped curved conductor on the insulating
handle to discharge the leyden jar to show the spark. It
may take some trial and error to know how much
charge to put to get a good spark. Charge the capacitor
again. Use the hook on the insulating handle to
remove the chains without discharging the capacitor. Then use the hook to lift the inner
conductor out of the insulator. Use the tips of your fingers to remove the insulator from the outer
conductor. Touch the two conductors together. You should observe almost no spark. You may
then pass the two conductors to one or mor students for inspection. Reassemble the leyden jar. In
the last step be sure to use the insulated hook to reinstall the inner conductor. Then discharge the
capacitor and note the large spark that comes from what seemed to be a discharged capacitor.
This reveals the importance of polarization of the dielectric. There is controversy about the
complete explanation.
On a breadboard we have a series circuit consisting of a 9v battery, a large capacitor, and a
flashlight bulb. The current through the bulb will decay exponentially whether the capacitor is
charging or discharging. The time constant is such that the changes in bulb brightness are easy to
see. Use a small wire on the breadboard to change between charging and discharging.
Circuits
Batteries and Bulbs
Rather than a demonstration, this exercise is often done by the students at their desks. To
establish the idea of a complete circuit and to build familiarity with batteries and bulbs the first
task is to light one bulb by using it with only one wire and one D cell battery. Then progress to
up to 4 batteries and several bulbs in various combinations of series and parallel.
Large Cenco Lecture Table Galvanometer
Various Projection Galvanometers
This large meter can be used as a galvanometer, a voltmeter, or
an ammeter. Resistors stored on the back are required for all
but galvanometer operation. In galvanometer mode the meter
is quite sensitive and will be damaged if connected directly to
any battery or power supply. The meter is most useful for
showing currents induced in coils of wire by changing
magnetic fields. The open construction of the meter makes it
useful for showing how such a meter works.
Electric Pickle
Two of these meters can be used with the overhead
projector and the other (at far right) uses an optical lever to
project a spot of light on a scale. All are very sensitive and
should be used very cautiously to avoid damaging them.
The small Pasco meter (left front) can be used as a
voltmeter and has several voltage ranges.
Stick a fork in each end of a large dill pickle and connect
clip leads of the wiring harness to the two forks. Make
sure the forks are not close to touching one another.
With the pickle over a dish Parallel and Series Bulbs
to catch any dripping juice,
plug in the harness and
depress the rocker "dead man" switch. This connects the
pickle to 120v ac. The pickle will conduct and eventually
glow brightly. Spectroscopic investigations indicate that
much of the glow is from excited sodium atoms. It may
be necessary to sand the forks a bit in advance if they are badly tarnished. Please clean up the
forks and plate before putting them back in the box.
Swinging LED Demonstration of AC Current
The wiring of these two sets of three sockets is easy to see.
Besides showing both arrangements with identical bulbs it is
interesting to show the parallel case with three different bulbs,
e.g. 40w, 75w, and 120w, and then ask the students which bulb
will be brightest if these three bulbs are put in the series set of
sockets.
This demo has a bicolor LED (in series with an appropriate resistor) at the end of an ordinary electrical cord. When the LED is swung in a circle in a darkened room it draws a dashed circle of alternating red and green light. Begin the demonstration by showing that such an LED (also in series with an appropriate resistor) lights red for one direction and green for the other direction of current supplied by 9v battery. The dashed circle then provides evidence that the current from an ordinary outlet is alternating in direction.
A similar demonstration that uses a neon bulb at the end of a cord draws a dashed circle of only
one color.
Chassis Ground: Lighting a Bulb with Cart as Return Lead
Household Ground: Lighting a Bulb with Water Pipe
Use one of the unpainted metal carts. First show the lighting of
a #47 bulb connected by means of two leads to a 6v lantern
battery. Then remove the negative battery lead from the bulb
and connect that lead to the cart. Touch the bare terminal of the bulb to the cart at another
location to show the wiring pattern used in automobiles.
CAUTION: This one can kill you if you aren't careful. Use an extension cord that has a grounded
metal outlet box. The cord that is provided for this demo has a "dead man" switch that must be
held depressed for the outlets to be live; this should make it harder for you to electrocute
yourself. Plug the bulb into the hot (short) and neutral (long) slots of the outlet by means of test
probes. To make good contact the probes need to be near the centers of the slots. The bulb should
light. Remove the probe from the hot slot and plug it into the ground (round) opening. The bulb
should not light. Replace this probe in the hot slot and move the probe from the neutral slot to the
ground opening. The bulb should light. If the extension cord is plugged into a ground fault
interrupter (GFI), as it will be if it is plugged into the lecture table, the GFI should click off
before the bulb lights. The GFI will have to be reset; under the lecture table, press the button on
the face plate of the outlet. To avoid this situation plug the cord into an outlet that does not have
a GFI. (There should be such outlets in the walls of the room.) Repeat the operation with the
probe moved from the neutral to the ground slot. The bulb should light. Now move this probe so
that it is touching the metal box. The bulb should light. Finally, move this probe so that it is
touching the water pipe. Again, the bulb should light.
Pasco Interface Display of Voltages in RLC Circuit
You will need to use the Pasco
Science Workshop 750 Interface
(21C) and a computer that has the
"Data Studio" software installed.
Connect the computer to the video
projector. Connect the interface to
the PC by means of the USB cable
and to power by means of its
adaptor. Use the Blue Pasco Circuit
board (21C). The output terminals
on the right front of the interface
will supply the ac signal. Using two
banana leads connect a series RLC
circuit with R= 100, L= 8.2 mH,
and C = .1 µF. Connect three voltage probe leads from the three analog channels of the interface
to measure the voltages across the three components. For the phase of your voltage traces to
make sense you will have to pay attention to the polarity of your connections.
To activate your display do the following: (1) Double click the "Data Studio" icon. (2) At the
main menu select "Create Experiment." (3) In the "Sensors" window double click on "Voltage
Sensor" three times. This will place A, B, and C sensors on the picture of your interface. (4) In
the "Signal Output" window (below sensors window) double click on "Output." This activates
your sine wave signal source. That is, it makes the output of the interface the input to your
circuit. Set the amplitude for 1 volt and the frequency at 2500 Hz. Drag the output control box to
the lower right of your screen. (5) In the "Displays" Window, in the bottom left corner of the
screen, double click on "Scope." This makes your PC act as an oscilloscope. Accept "Voltage
A." Enlarge the oscilloscope screen as much as possible without covering your frequency and
amplitude control box. (6) From the top left "Data" window drag channels B and C onto your
oscilloscope screen. Also drag the output onto the screen. You now have a four channel
oscilloscope. (7) By clicking the appropriate arrows below and to the right of your oscilloscope
display change the sweep to .1 ms/div and the sensitivity of all four traces to .5 v/div. (8) Click
the "Start" button.(9) To get a stable display click the trigger button at left just above the
oscilloscope display.
You should now have four traces displayed. You may find it easier to observe their relative
phases if you click the "Stop" button to freeze the trace. Click "Start" again before changing
parameters. It will be interesting to see the effect of increasing frequency. By using the frequency
+ button of the output control window scan the frequency up to about 5500 Hz, the approximate
resonance frequency for this circuit. Note the large and approximately equal capacitor and
inductor voltages. Insert the iron core (stored in a clip at the left of the circuit board) into the
inductor. The effect of the increased inductance should be dramatic. Scan the frequency down to
find the new resonant frequency.
Magnetism
Variety of Magnets
Magnetic Deflection of Electrons-Discharge Tube
Shown at right are some of the magnets available,
ranging from the large magnetron magnet at left to the
small ceramic magnets at front. There are also several
high field neodymium magnets used with specific
demonstrations.
Magnetic Deflection of Electrons-Oscilloscope
Shown at right is a very low pressure tube (11A) in
which a beam of electrons is accelerated to high speed.
The electrons pass through a slot and then leave a streak
on a phosphor. A magnet (11F) can be used to deflect
the beam. The high voltage is supplied by an induction
coil (11C) that is powered by a 6v battery. It may take
trial and error to get the appropriate polarity so that the
electrons leave their streak on the phosphor.
More convenient than the discharge tube demo above is the use of an old oscilloscope (9F) to
supply the electron beam. With no voltage on the oscilloscope's deflection plates only a spot will
be seen on the phosphor at front. A magnet (11F) can be used to move this spot around.
Magnetic Force on a Current
Use a clip lead several meters long. Hold the wire so that a loop of it dangles between the poles
of a large strong magnet such as a magnetron magnet. Very briefly connect the ends of the wire
to the terminals of a 6v storage battery. The wire should deflect. Reverse the polarity and the
deflection should be in the opposite direction. Don't leave the circuit connected for more than a
second. Rather than a clip lead, you may find it convenient to use one of the rectangular wire
frames in the box for this demo. In any case do not leave the battery connected very long.
Audio Speaker
Make intermittent very brief contacts of the leads of a speaker to a small battery and note the movement of the paper cone in response to the magnetic force on the coil glued to that cone.
Also note the sound made.
Magnetic Field of a Current
Use a compass on the overhead projector to detect the field of a clip lead connected briefly to a
6v storage battery.
Collection of Coil Shapes for Overhead Projector
Magnetic Force Between Two Currents
Use these current carrying conductors with a low voltage
battery or power supply. Use a compass or iron filings to
reveal the field directions.
Thermocouple-powered Electromagnets
This device (in front of the shelves just under 10A,11A,
12A) uses long wires clamped to the two ends of a piece of
metal conduit. When the conduit is near horizontal the two
wires dangle well below the it. If a current is sent through
the wires they will exert a magnetic force on one another.
Rather than a dc source, use a step-down transformer (10B)
to supply ac current to the wires. The two wires are
electrically connected to the conduit at the end far from the
transformer. The transformer can be connected to the wires
in two ways. The current can go out one wire and back the
other giving currents in opposite directions or the current
can go out both wires and return in the conduit giving
currents in the same direction. With the conduit pointed
toward the students or above their heads, there will be a
noticeable deflection of the wires while there is current. Do
not leave the transformer connected for more than a second
or two.
Strong Electromagnet
The left magnet in the photo has a large thermocouple
that can be heated with a bunsen burner. With the
magnet securely mounted on a rod, the iron end plate
will support several kilograms. At right in the photo is a
water heater gas valve with a thermocouple-powered
magnet to keep the valve open when the pilot light heats
the thermocouple.
Box of Solenoids
This magnet has a coil surrounded by an iron housing. When this part
is combined with the load bearing iron end plate there is a closed
magnetic circuit which enables the magnet to support a load much
larger than the 1kg shown in the photo. A 6v lantern battery provides
adequate current. Don't leave the battery connected any longer than
necessary for the demonstration.
Among the coils in this box is an automobile solenoid that is an electromagnet that both
electrically connects the battery to the starter motor and mechanically applies the starter gear to
the flywheel gear.
Suspended Button Magnet Stack Compass
Various Electric Motors
Suspend a stack of four Neodymium button magnets by a monofilament thread
about half a meter long with its lower end trapped between magnets. As the person
holding the thread rotates slowly 360 the magnet stack will be seen to keep its
same orientation in space. To make this orientation more apparent you can trap
between the center two magnets a card with the two sides labeled differently. (The
card will also damp the, otherwise long lasting, torsional oscillation.) One end of
the stack will be seen always to point north. This end can be labeled the N-pole. It
is, of course, pointing toward the Earth's geographical north and toward the Earth
magnet's S-pole.
Shown at right are some of the demonstration dc electric
motors. Most have delicate parts that easily get out of position.
You may have to do some minor repairs before using these.
Power them briefly with a 6v battery. They may overheat if
operated for more than a few seconds.
Faraday's Law
Box of Earth Inductors
Jacob's Ladder: Step-Up Transformer
These coils were originally designed to use with ballistic
galvanometers to show voltages induced by the Earth's
magnetic field when the coils changed orientation. The Circular
coil was meant to be used with a mechanical rotator. It has a
commutator that can be contacted by means of two spring steel
brushes. The rectangular coil is useful as a prop in discussions
of torque on a current in a magnetic field. This coil can be
connected to a large galvanometer and moved near a strong
magnet to show induced voltages.
The high voltage transformer is connected to two nearly vertical wires
inside a plexiglass shield. Rising air currents cause the arc between the
wires to rise. A new arc always strikes at the bottom where the wires are
closest together.
For your protection: the power cord has a "dead man" switch that must be
held closed for there to be current to the primary of the transformer. Even
with this switch you must exercise caution to avoid getting near bare
electrodes. The heavy apparatus is equipped with casters to allow you to
roll it slowly to class. Pull the apparatus by means of the string attached at
bottom front rather than pushing it by means of the delicate upper part.
Small Tesla Coil and Bulbs
Large Tesla Coil
Screw in the knob on the end to activate the tesla coil. Hold a
bulb in your hand and then let the tesla coil spark to the metal of
the bulb. There should be an interesting glow. Because of the
high frequency and resultant very small skin depth you will
hardly feel anything.
Flashbulb Faraday's Law Demo
This coil is about a meter high.
LED's on Pipe Lighted by Falling Magnet
A coil wound around one end of a piece of PVC pipe (standing to left of
19) is connected in series with a flashbulb from an old Kodak Flashcube
(11C). The picture shows the bottom end of the pipe. With the pipe
vertical, a neodymium magnet is dropped into the pipe. The current that
is induced in the coil by the passing magnet sets off the flashbulb.
This is similar to the flash bulb demo above except in this case there are a number of coils along
a PVC pipe (standing to left of 19). Each has attached to it an LED. As a Neodymium magnet
(11C) falls through the pipe it lights the LED's one after the other.
Eddy Currents in Pipe Due to falling Magnet
In this demonstration a Neodymium magnet (11C) falls through a copper or aluminum pipe
(standing to left of 19). Eddy currents induced in the pipe exert a retarding force on the falling
magnet. It takes several seconds to fall through the pipe. For contrast, drop an aluminum plug
through the pipe.
Eddy Currents in Swinging Copper Plate
The copper plate is the "bob" of a pendulum and swings between the poles
of an electromagnet. Adjust the pendulum and magnet to make sure that the
plate swings between the magnet poles without hitting. Show how the plate
swings with no power to the magnet. Then, with the magnet connected to a
6v battery, let the pendulum swing again. The damping should be obvious.
Do not leave the battery connected any longer than necessary. Replace the
solid plate with a slotted plate to show the decreased damping.
Eddy Currents in Spinning Disk (11F)
Jumping Ring-Heated Ring
Use a pizza-cutter-like copper disk. Give the disk a spin and
show how long it will coast. Then spin it again and hold it
between the poles of a strong magnet. The eddy current damping
should be evident.
Use the large vertically mounted 120v ac solenoid with the
movable iron core. Extend the core upward until its bottom
is about even with the bottom of the coil. Place the
aluminum ring on the core. When the power is switched on
to the coil the ring will jump well above the top of the core.
Be prepared to catch it. Superficially, the explanation is
that the induced current in the ring is opposite the current in
the coil giving a repulsive force. More careful analysis of a
whole cycle will lead to the conclusion that the force
should be attractive for half the time with an average force
of zero. Even more careful analysis would have to take
into account the resistance of the ring and the effect that it
has on the phase difference between the coil current and the
ring current.
If you hold the ring and do not let it jump off the core, it
will quickly become quite hot. You can let a student do the
holding or you can toss the hot ring to a student who can
tell the rest of the class about how hot the ring gets. What we have, of course, is a step-down
transformer with huge current in the ring secondary.
When you are finished store the ring on the bottom of the lowered core.
Turn-by-Turn Transformer Demo
This demonstration uses the same vertical solenoid as does the jumping ring demo. Also needed
are a 6v flashlight bulb in a socket and a clip lead several meters long. Mount the bulb on a stand
so that it is easily seen by the students. Attach the ends of the clip lead to the two electrical
connectors of the bulb socket. With the solenoid connected to 120v ac and with its iron core
extended upward at least six inches, wind the clip lead wire one turn at a time around the core.
As more turns are added the bulb will begin to glow and will increase in brightness with each
added turn.
Hand-crank Generators
The old generator, at upper right in the photo, has a coil
that is rotated by the crank. The crank is quite easy to
turn when there is no load but will become noticeably
harder to turn when the switch button is pressed to light
the bulb.
In the "Y2K" hand-powered flashlight, at left in the photo, a magnet rotates inside a fixed coil. To show the effect of the load, note how long the magnet will coast with the bulb in the circuit. Then slightly unscrew the bulb/lens assembly to remove the bulb from the circuit and again note how long the spinning magnet coasts.
The Genecon at the bottom of the picture works as either
a motor or generator. Connect two of them together and have one act as a motor and the other as
the generator that supplies the current.
Cenco Motor/Generator
Tri-color LED Lenz Law Demo
Depending on the arrangement of the brushes and
connections to a 6v battery the device at right can be used as
a dc motor, dc generator, or ac generator.
This demo utilizes the two-in-one kind of LED that lights red for current in one direction and green for the other direction of current. Begin by connecting such and LED (in series with an appropriate resistor) to a 9v battery. Use both directions of current to familiarize the students with the LED.
Connect another such LED (without resistor) to the two ends of a coil that has thousands of turns.
Push the coils between the poles of a large magnetron magnet. Then pull the coil away. The LED
will have a different color when leaving the magnet than it did when approaching. Repeat with
the coil or magnet turned over.
Super Sensitive LED Lenz Law Demo
This demo is similar to the tri-color LED version but uses a high gain op-amp amplifier to make
it much more sensitive to small flux changes. There are in this case separate red and green LED's
to indicate the direction of induced current. A sensing wire or coil can be connected to the
amplifier/indicator by means of banana plugs. With a coil with thousands of turns the LED's will
light even if the coil moves very slowly near a magnet. The amplifier/indicator is sensitive
enough to display flux changes due to the Earth's field. You can use one of the so-called earth
inductors that were originally used with ballistic galvanometers. Connect one to the amplifier.
Flip the coil over in the Earth's field and you can see the red and green LED's alternately light.
You can use a long wire as a sensing "coil". Swing the wire jump-rope fashion in the Earth's
field.
WAVES AND SOUND
Waves
Transverse and Longitudinal Waves on Long Spring
Use one of the springs that can be stretched to several meters in length. Secure one end to a wall
or stand. At the other end make a transverse pulse by plucking upward and releasing a small part
of the spring next to the hand that stretches it. Have the students note the reflected pulse that
comes back. Repeat this several times until they clearly see that the pulse is tuned upside-down
on reflection. To make a longitudinal pulse stretch or compress a small region of the spring and
release it. The longitudinal oscillation of the spring will be more evident if you have white tape
markers as several points.
Antique Hand Crank Transverse Wave
Simulator
Torsion Wave Device
This device works because of eccentrically mounted
disks that support the rods which are tipped by
white balls. As the crank is turned the harmonic
wave shape moves across the machine. Point out the
simple harmonic motion of each ball. This machine
is very good for showing that a particular wave feature, e.g. a crest, moves one wavelength in one
period of oscillation.
This device has equally spaced dumbbells mounted on a spring
steel band about three meters long. You send a wave along it by
giving the end dumbbell a twist around the band. The pulse will
propagate slowly and will reflect. The kind of reflection will
depend on whether the far end of the band is rigidly mounted or
is held by a long string and is free to rotate.
Sound
Tuning Forks with Sound Boxes
These two tuning forks are nearly matched; one has an
movable clamp for adjusting its frequency. One fork can be
used as a vibrating sound source. It can also be used in
showing the resonant coupling to the attached column of air in
the sound box. The box is somewhat less than one-fourth
wavelength long. Cover the opening to show that the air
column does, indeed, amplify the sound from the fork.
The two forks can be used together to demonstrate resonance
or beats.
Nail Violin-Nail Guitar
This small guitar-shaped board has a row of nails with exposed lengths that make them vibrate at frequencies close to those of musical notes. The nail sequence for Mary Had a Little Lamb is written on the board. The nails can be struck with a pencil or bowed with a violin bow.
Double Sonometer
This is a very large two-string instrument with built-in spring balances to indicate the tensions in the "strings."
Eight Pipe Octave Organ
Matched Vibrating Bars
Barphone
These two bars have attached sound boxes. They can be used
in a way similar to the two matched tuning forks. To produce
beats you can change the frequency of one bar by sticking a
small piece of modeling clay on it.
Transmission of Sound Through Long Spring
Martin's Spinning Speed of Sound Measurement Device
Sound made at one horn is transmitted to the other end though
the spring.
This apparatus consists of a spinning disk, with a white index
mark, and two photographic flash units with attached sound trigger
devices. The flash units are positioned to illuminate the spinning
disk. One sound sensor is placed close to the disk and the other
farther away. In a darkened room, someone near the sensor that is
farther from the disk claps his hands. As this sound reaches the two
sensors at different times the two flashes illuminate the disk at
different times. The angular distance between the two white index
positions seen can be related to the speed of sound.
If necessary, use a strobe light in determining the rotation speed of
the disk.
Sound Spectrum with Audio Oscillator and PA System
Connect an audio oscillator to the lecture room PA system by means of banana plug wires
inserted into the blue box mounted under the center of the lecture table. Scan through frequencies
throughout the range of hearing.
Speaker Baffle Interference
Attach a two inch speaker to a small radio by means of the ear phone jack. Tune the radio to a local station. Let the students hear the poor quality and volume of the sound. Place in front of the speaker a piece or cardboard or other material with a hole just large enough to leave the speaker cone uncovered. The sound will greatly improve. One explanation is that without this baffle the waves from the front and back of the speaker cone interfere destructively. They are out of phase in the sense that when the speaker cone is raising the pressure in front of the speaker it is lowering the pressure behind. Both of these pressure waves propagate to the audience. With the back wave impeded by the baffle, you get the full effect of the front wave.
Two Speaker Interference
The sound sources are two inch speakers
that have styrofoam cup baffles in front.
The speakers are supported on a
horizontal rod. Make the rod free to
swing about a vertical axis by clamping
onto a ring stand a close fitting ring that
will prevent the clamp holding the
horizontal rod from sliding down. The
clamp then need not be tightened onto
the ring stand. Connect the speakers to an
audio oscillator that is putting out a
sound wave of 1-3 kHz. Ask the students
to cover one ear. Rotate the rod
supporting the speakers so that the distance difference for each student will change. Ask if they
hear the changes in loudness. Because of the non linear response of the ear many of the students
may be uncertain. Because of complications, including reflections from the walls, you will not
get complete destructive interference for anyone. A change by a factor of two or more in intensity
may be hardly perceived by listeners. For a clearer indication of the changes in intensity use a
small crystal microphone some distance from the speakers to pick up the sound. Connect this
speaker to a high-gain amplifier and the amplifier output to an oscilloscope. The sound signal
will then be seen on the screen. As the speakers are moved, changes in intensity of the displayed
signal should be evident.
Standing Waves-Resonance
120 Hz Standing Waves on a String
This involves the same apparatus used by the students in experiments in several of our labs.
A 120 Hz vibrator moves one end of a string. The other end passes over a pulley to a weight
hanger. The mass on the hanger can be adjusted to produce various harmonics. The frequency is
always 120 Hz but you are adjusting the sound speed, and therefore the wavelength, to make
different numbers of half wavelengths fit on the string.
Computer Controlled Standing Waves on a String
This makes use of a Pasco Mechanical Vibrator, a
Pasco Science Workshop 750 Interface, and a PC that
has "Data Studio." Connect the interface to the PC by
means of the USB cable and to power by means of the
power adaptor. Connect the output terminals of the
interface to the input terminals of the vibrator by means
of banana leads. Using no more than a few newtons of
tension and making sure that the vibrator drive post is
locked in position, stretch an elastic band from the
vibrator post to some other support.
Release the drive post. Run "Data Studio."
Select "Create Experiment." Double click on "Output"
in the small "Signal Output" window. This should give
you an output control box that lets you control a sine
wave signal fed to the vibrator. Set the amplitude for no
more than two volts. Use the arrows to decrease the
frequency interval from 100 to .1 or 1 Hz. If the "Auto" button is selected, turn it off so that you
can use the "On" and Off" buttons. Use the + button
to scan the frequency upward until the string vibrates in its fundamental mode. Scan higher for
other modes.
Standing Waves on a Long Spring
Attach one end of a long string to a support and stretch the spring across the front of the room.
By moving your hand that holds the end of the spring up and down periodically with small
amplitude you should be able to produce large amplitude transverse standing waves of the
fundamental and several other harmonic frequencies.
Singing Rod
Air Column Resonance
The source of sound in this case will be an aluminum rod or brass pipe
(standing to left of 19) that is vibrating longitudinally in a standing wave
pattern. These vibrations are excited by friction. Spray the sides of the rod near
one end with baseball pitchers rosin or rub it with violin bow rosin (14D-dr1).
Grip the rod tightly at its (marked) center with thumb and forefinger of one
hand. With the other hand on the tacky portion of the rod, pull toward you. It
will take some trial and error to find out how much pressure to put with this
pull. When you get it right you will cause the rod to squeal at high frequency.
You can add to the intensity with each additional pull until the sound is very
loud. The mechanism for setting up the vibrations is a grab-and-slip resonance
similar to that of a violin bow on a string.
Connect an audio oscillator (13E) to the small speaker (14E). Hold the speaker near the top of an
air column such as that in a drinking glass or piece of pipe. Vary the frequency until the sound is
amplified by the air column. Continue to scan the frequency until you find higher harmonics.
Caution: not every loudness is air column resonance. The speaker itself may have a resonant
frequency at which its sound output is greater.
Twirl-A-Tune
Heat-Driven Singing Pipes
This is a flexible corrugated tube which gives off sound when one end is
held and the other is swung rapidly in a circle. A sequence of more than
one harmonic will be heard as the tube slows down.
With these devices sound is generated by the turbulence in hot
air rising through a mesh. The length of the pipe determines the
frequency that will be amplified and heard. In one kind of pipe
(shown at right) there is a wire mesh closer to the end that
should be used as the bottom end. Hold the pipe above an ordinary bunsen burner and lower
it so that the mesh will be heated. Raise the pipe above the
flame and the air that rises through the mesh will vibrate at
many frequencies. The pipe's fundamental will be amplified
and easily heard. You can ham up your presentation, and make
the students think, in the following way: Have the pipe in one
hand and a tumbler or other container in the other. After the pipe starts to sing tilt it over to
horizontal as if you are pouring something from it into the container. While you are doing this the
sound stops. Tilt the pipe back to vertical as you pretend to pour something back into it from the
container. Ask the students to explain or start them thinking by giving a ridiculous explanation
such as that the heat puts sound into the pipe and that the sound can be stored temporarily in the
container.
The mesh in the pipe is unnecessary if you use the Mecker type of burner that has many holes or a grill at the top. In this case you will get the sound while the pipe is down around the top of the burner. With a little trial and error you will be able to find the correct position to get a loud low note from a fairly long piece of PVC drain pipe.
Caution: In either version of this demonstration the pipe can get
hot!
LIGHT
Reflection and Refraction
Various Water Tanks
To make a laser beam show up in the water tank add a tiny pinch of powered coffee creamer to
the water. To make the beam outside the tank show up use chalk dust.
Blackboard Optics
Bending Light Beam
This is a set of reflection, refraction, and
focusing demonstrations in a box. The
light paths from a single-ray projector
(lower right in the picture) or multi-ray
projector (lower left in the picture) are seen
as the light scatters from the surface of the
chalk board. The projectors are attached to
the board by means of strong magnets and
they are powered by 12 volts from a power
supply or battery eliminator (8F,12F).
(There is a long white extension cord that
makes it unnecessary to have the power
supply near the ray projector.) There are
clear plastic pieces in the shape of a block,
a prism, a semicircle, and several lenses. There are magnet-secured brackets for holding these
pieces against the board. For some demonstrations it is easiest to just hold these refracting pieces
with your hand.
Into the water tank that is about 10 cm long and 50 cm long put enough water to fill it about a
third to half way. Add a tiny amount of coffee creamer to scatter light. Position a laser at the end
of the tank so that the beam can be seen passing through the tank a centimeter or so from the
bottom. It may be necessary to put the tank up on blocks. Now pour into the tank some sugar
solution. The solution can be made by mixing about a cup of sugar with about two cups of hot
water. Add a little coffee creamer to this also. The solution should be cool before it is used in the
demonstration. About a cup of the solution should be sufficient for one demonstration. As you
move along the tank from the laser end to the center pour the solution in gently trying not to stir
the water up too much. It may work best if you pour the solution in through a long stem funnel
with the tip at the bottom of the tank. After the turbulence settles down, the beam will be seen to
curve downward dramatically. In the case shown at right the beam reflects off the bottom of the tank and is bent downward again. It is easy to use Fermat's principle to explain the bending. You
can relate this bending to similar bending of sunrise and sunset light in the Earth's atmosphere. It
is the reverse of the kind of bending that gives mirages.
Total Internal Reflection (TIR)
TIR in Blackboard Optics Semicircular Block
With the single ray projector attached to the chalk board hold
the semicircular plastic block so that the beam is normal to
the curved surface. Keeping this condition move the block so
that the angle of incidence on the flat face increases past the
critical angle.
TIR in Blackboard Optics Prism
With the single ray projector attached to the chalk
board position the plastic prism to demonstrate
both of the cases shown at left.
Rigid Plastic Light Pipes
Water Stream Light Pipe
Frustrated TIR
In this demonstration light is trapped by TIR in a stream of water exiting
a plastic bottle through a hole in its side. Stopper the hole and fill the
bottle. Add a tiny amount of coffee creamer to scatter light. With the
laser on a tripod, position the beam to hit the stopper. With the room
lights off, remove the stopper. The stream will be red because of the
light that scatters from the trapped beam. To ham up your presentation
catch some of the water in a container into which you have secretly put
a little red food coloring. When the lights are back on pour out some of
the red water and make some ridiculous comment about the light
trapped in it.
Magic Drawing Board or Glo-Doodler
Use two 45-45-90 prisms with their long
faces together. Send laser light nearly
perpendicularly into one of the short faces so
that the light will be totally reflected at the
long face. Squeeze the prisms together until
some of the light leaks through the very thin
layer of air between the prisms. At right is
shown an assembly that is useful for this
demonstration. A short ring stand has a clamp that holds two suitable
prisms.
This child's toy, manufactured under several names, exhibits several
physics principles. It consists of a flexible fluorescent sheet of plastic
over a white background. When a writing stylus is pressed against the
sheet, bright fluorescent writing is seen. The sheet absorbs room light
of short wavelengths and emits light at longer wavelength. Most of
the emission directions are such that the light is trapped in the sheet
by total internal reflection (TIR). Note the bright edge of the sheet. When the sheet is pressed close to the white backing, light escapes into the backing by frustrated
TIR. From the backing some of the light is scattered back through the fluorescent
sheet making the scattering locations glow brightly with fluorescence radiation.
Image Formation
Plane Mirror Flying Trick
Stand on a table with a large plastic mirror between your legs in such a way that the reflecting side is toward the students. One of your legs will be hidden but the students will see the front leg and its reflection making it seem that they see both legs. Lift the foot that is in front of the mirror off the table. It may be useful to have a block of wood behind the mirror under your back foot. It will appear to the students that both of your feet are off the table. You are flying. Ham it up by flapping your arms or by reaching back and grabbing the back of your belt to appear to be lifting yourself.
Illusion Box
In this small toy a small model airplane seems to be suspended in space. Actually there is just
half a plane attached to a diagonal mirror.
Real Image of Light Bulb
A large concave mirror
forms a real image of a
light bulb. (See a photo
of the image at left.) The
bulb is hidden from view
inside a box. (See the
photo at right.) There is a
bulb socket on the
outside of the box above
the bulb. If the bulb is
positioned one mirror
radius from the mirror
the bulb's image will be
at the socket. Students looking into the mirror will seem to see a
bulb in the socket. It takes a little trial and error and a little shimming up of the mirror to get the
image just right. Briefly place a sheet of paper at the image to show how a real image can be
formed on a screen. Consider positioning the arrangement so that the students will see it when
they arrive for class. You can move the bulb closer to the mirror to get a larger image perhaps on
the projection screen. A projection cart is not big enough to hold this demo. There is an
appropriate piece of plywood (stored in the 16A-17A corner) that can be placed on top of a cart.
As an alternative to the image by a mirror, form an image by means of a large lens. A suitable
lens is mounted in a board on a stand that can be used with the light bulb in the box.
Slide Projector Lens Replacement
Blackboard Optics Ray Tracing Through Lenses
Remove the usual lens from a slide projector (16A) and hold various other
converging lenses (18D-dr5&8) between the projector and the screen to
show how they can form real images. Include in this demonstration a half
lens or half-covered lens to emphasize that it is the curvature of the refracting
surfaces and not the shape of the rim of the lens that determines the imaging
properties. Shown at right is an old slide/film strip projector with its usual
lens folded aside. Two possible imaging lenses are shown.
Use the multi-ray projector attached to the chalk board along with one of the plastic lenses.
Adjust the output mirrors of the projector for five parallel rays. Note the focal point where the
rays meet. Both spherical and chromatic aberration may be evident.
High Resolution Vision Only at Center of Retina
While you stand at one side of the front of the classroom have the students stare at something on
the other side. Hold up several fingers and ask the students to tell you how many without moving
their gaze. Move toward the side that they are staring at until they are able to tell you the number.
This illustrates that we have densely packed visual cells that give perception of image details
only near the center of the retina.
Spectrum and Color
Color Vision Only at Center of Retina
While you stand at one side of the front of the classroom have the students stare at something on
the other side. Hold up a colored object and ask the students to tell you what the color is without
moving their gaze. Move toward the side that they are staring at until they are able to tell you the
color. This illustrates that we have densely packed cone cells for perceiving color only near the
center of the retina.
Spectrum Formed by Prism
Spectrum of Overhead Projector Light Formed by
Grating
We have a Blackboard Optics single-ray projector mounted on a stand
with a prism (see at right). Connect the single-ray projector to a 12v
supply (8F,12F). Place this stand on a table or cart no more than a
couple of meters from the projection screen and adjust the positions so
that light goes from the single-ray projector through the prism and on
to a spectrum on the screen. Room lights should be off.
Hand Out Gratings
This method forms a much more useful spectrum than does
the prism. Attach a piece of holographic diffraction grating
to the output lens of an overhead projector by means of a
single piece of tape. Position the grating so that the "lines"
are vertical. Use two pieces of cardboard to form a slit on
the overhead projector stage. The slits image on the screen
should be vertical. With the room lights out and the
projector on, the slit's image will be the zero order spectrum. On each side of it you should see a
very bright first order continuous spectrum. (An interesting variation is to replace the slit with an
anti slit; remove the two pieces of cardboard and put a thin strip of cardboard where the slit was.
Try to interpret the "spectrum" that you now see on the screen.)
Pass out to the students the holographic diffraction gratings in 2x2 slide mounts. Position several
light sources at the front of the room for them to view through their gratings. Use and
incandescent bulb and several gas discharge tubes to show the difference between discrete and
continuous spectra. There is a three-source stand that can be used for viewing three spectra
simultaneously.
Colored Shadows
Color Mixing Box
The apparatus has red, green, and blue aquarium lights. With these
lights on, use them to cast shadows of your hand on the projection
screen. Explain the color combinations seen. For example, where
only the green light is casting a shadow you will see magenta, the
sum of red and blue.
This device has red, green, and blue light sources controlled by switches and brightness controls.
The sum light appears on a frosted screen at the front of the box
Tinker Toy Color Mixing Device
With the overhead projector pointing away from the
screen, place the base of this device (with the three
holes and color filters) on the projector stage. Unfold
the device and position it so that each of the three
small mirrors intercepts one color of light coming
through the filters. Use the Tinker Toy mounts to
adjust the mirrors until you have three colored disks of
light on the screen. Adjust the mirrors to make
different colors overlap so that you can show how the
primary colors add.
Colored Filters on Overhead Projector
Use red, green, blue, cyan, magenta, and yellow plastic filters on the overhead projector. Ask the
students to predict which color will be seen where two filters overlap. Remind them that a filter
looks the color(s) that it does not absorb. For example, where red and green overlap black will be
seen because red absorbs green and blue and green absorbs red and blue. However, where red
and magenta overlap red will be seen and where yellow and cyan overlap green will be seen.
Spectrum of Light Transmitted by Color Filters
Make use of the spectrum formed by a grating attached to the overhead projector. Note what
is missing from the spectrum when a colored filter is placed over the slit formed by the two
pieces of cardboard.
Colored Object in Spectrum
Make use of the spectrum formed by a grating attached to the overhead projector. Place colored objects in the spectrum. For example an orange painted object in the blue part of the spectrum should look dark. However a fluorescent orange object in the blue part of the spectrum will look orange.
Alternatively, rather than form a complete spectrum, use a sodium lamp as your light source and
examine the appearance of various colored objects.
Projected Spectrum of Mercury
Caution: do not view the mercury lamp directly.
The spectroscope consists of a mailing tube with a
hole in the side for the mercury lamp. At one end
the tube is covered by a sheet of holographic
diffraction grating and a thin fresnel lens. The
distance from the lamp hole to the lens is a little
more than the lens focal length. Use this lens to
form a spectrum on the screen as shown at right.
To reveal some of mercury's ultraviolet lines place
a fluorescent card just inside the violet line. If you
have someone in the class who has had cataract
surgery, it is likely that this person will see the uv lines without aid of a fluorescent card.
Electromagnetic Spectrum-Ultraviolet
Use and ultraviolet light to illuminate various fluorescent objects such as starch and fluorescent
cards.
Electromagnetic Spectrum-Infrared and Microwaves
With the room lights off show how the TV remote control
(17C) causes the orange patch on the IR detector card to
flash. Aim the control's LED at the card from a few cm
away and press one of the buttons. The card must be
"charged" first by exposing it to room light for a few
minutes. The microwave detector is of the type used to
detect leaks from microwave ovens. You can also use the
klystron microwave source and the diode detector (17F)
with matched horns.
Interference and Diffraction
Near Point Source-Concentrated Arc Lamp
This source is about as close to a point source as you can come although a Mini Mag Lite with the lens housing removed does pretty well. Use the point source to see diffraction patterns through slits between fingers or slits in sheets of paper. The concentrated arc lamp lights only after momentary application of the start switch.
Below at right is the design for a diffracting sheet to pass out to the students or have them construct for themselves.
Double Slit Interference
At the center of the dashed rectangle is a cut made by a razor blade or box knife. As a student
looks through the slit toward the point source, he/she can tug gently at the edges of the paper to
increase the slit width. Inside the dashed circle is a tiny pinhole made by the tip of a straight pin
or needle. If the edges of the hole are smooth enough it will produce a nice Airy disk with rings.
Use an optical bench, laser, and slits from the elementary
laboratories. Also use a couple of lenses to make a low power
telescope to increase the size of the interference pattern on the
screen. For example, a +127mm focal length lens followed by a
-22mm focal length lens will form a galilean telescope that can
be focused to give enlarged fringes on the screen. To show the
effect of blocking one of the slits use a razor blade held by a
large binder clip.
Effect of Slit Width on Width of Diffraction Pattern
Use an arrangement similar to that above with single slits of various widths. Alternatively, have
the students view some concentrated source of light through a slit cut in a piece of paper. Let
them note what happens when the sides of the slit are pulled farther apart.
Soap Film Interference Projection
Use the device that has a projection bulb mounted on a frame above a black board. Turn this
device on its side with the bulb closer to the screen than the
board and with the board approximately parallel to the
screen. Near the board place a small open bottle of bubble
solution and above the bottle opening suspend (ring down) a
small bubble blowing ring of the type that comes in small
bottles of bubble solution. Position the converging lens
(mounted on a black wooden block) so that it will image the
ring on the screen. Raise the bottle to put a film of solution
on the ring. Reposition the ring so that the reflection from the
film hits the image on the screen. It may take a few tries to
get this right. As the film drains (up as seen in the real
image) and gets thinner, colored interference fringes will be
seen. As the film gets still thinner at the top (bottom on the
screen) the fringes will disappear and the film will appear
dark. This destructive interference for nearly zero path
difference gives evidence the phase difference caused by
one internal and one external reflection.
Caution: The lamp gets very hot. Don't get anything very near it and don't touch anything that has been near it.
You
may need to use a cardboard screen to keep the bright light
from blinding the audience.
Gasolene Film on Water Projection
This is a difficult demonstration to get right but some folks like it.
Use the projection bulb on the frame that is mentioned in the
previous demonstration. Place the black board on the overhead
projector and position the lamp a short way from the projector's
lens. ("Short way" has to be at least 10 cm or you will melt the
projector.) Do not turn on the projector. Position a small black
saucer of water under the lens so that the lamp light reflects from the
water into the projector lens so that the water surface is imaged on
the screen by that lens. Put a small drop of gasolene on the water. If
you have things set up right, after the film becomes thin enough you
will see a colored interference pattern on the screen.
Thin Air Film Between Pieces of Plate Glass
At right is shown one version that has two large
pieces of plate glass clamped together to make the
air film between them quite thin. Note the dark
places where the negligible film thickness gives
destructive
interference
because of the reflection phase difference.
You can use smaller free glass plates or microscope
slides under filtered green light from a mercury lamp
to get visible interference even when the film is thick
enough to exceed the coherence length of white light.
The Flex Cam Video Camera (H240) can be used to display this interference to the whole class at once.
Polarization
Linear Polarizer for Microwaves
Plug in the source and aim its horn into the horn of the detector. Let the source warm up for at least a minute before adjusting any controls. If the detector indicates no signal adjust the source control slightly. Once you have a signal slowly adjust the source and detector controls until the detector needle indicates near full scale. Point out that the source produces vertically polarized waves and that the detector consists of only a vertical diode and a microammeter.
Tilt the source or detector to show the reduced detected signal when the electric field of the
waves is not parallel to the diode. Show the slotted aluminum plate and indicate that it acts as a
polarizer for microwaves. Ask what orientation of the plate will best transmit the vertically
polarized radiation. The students usually say that the slots should be vertical to transmit and
horizontal to block the waves. Show them that the opposite is the case. Point out the role in
absorption played by the currents caused in the aluminum.
Polarization by Scattering
Put a tiny amount of powered coffee creamer into a tank of water. Send through the water a beam of unpolarized light, for example from a slide projector with a slide with a small hole in it.
With a large sheet of polaroid show that the scattered light is polarized perpendicular to both the
incident and scattered directions. Alternatively, place a polaroid between the source and tank to
show that more light is scattered when the transmission axis of the polaroid is perpendicular to
the scattered direction. If you use an internal mirror He-Ne laser as a source, the intensity of
scattered light will change as the polarization of the incident light changes. If you use a red laser
pointer, your source is polarized.
Polarization by Reflection
The idea is to use a piece of polaroid to analyze the polarization of light scattered from a piece of
glass. One method is to pass out microscope slides and small polaroids to students. Alternatively
use a piece of glass to scatter overhead projector light onto the projection screen and show the
effect of a polaroid between the glass and screen for various angles of incidence. Point out that
polaroid sunglasses are used to reduce glare from horizontal surfaces.
Random Polarization of Internal Mirror He-Ne Laser
Place a polaroid between a He-Ne laser and the screen. The spot on the screen should change in
brightness with time. The changes will be more rapid when the laser is first switched on.
Typically, such a laser emits most of its light in a superposition of two longitudinal modes that
are polarized perpendicular to one another. As the laser cavity changes length the amplitudes of
the two modes change. The total intensity in the two modes changes little with time; hence you
don't notice much intensity fluctuation without the polaroid. By trial and error you may be able
to find the polarizer postion which gives little change in transmitted intensity with time. This
position of the polaroid transmission axis bisects the two mode polarization directions. The
changing polarization of these He-Ne lasers makes them poor sources for many polarization
demonstrations.
Polarization of Laser Pointer
As you can verify with a polarizer the output of red laser pointers and other red diode lasers is
nearly linearly polarized. The output of green laser pointers, however, appears to be unpolarized.
Calcite Crystal
Place the crystal over a mark on a transparency on the overhead projector. Two images of the
mark will be seen. Rotate the crystal about a vertical axis to show that one of the images rotates
about the other. Use a polaroid to investigate the polarization of the light giving each image.
Stress Birefringence of Squeezed Lucite
Use two wooden bars connected at one end by a bolt to squeeze
on a round lucite plug that is between crossed polarizers on the
overhead projector. There are other shapes of clear plastic that
can also be stressed between crossed polarizers.
Stress Birefringence of Cellophane and Clear Plastic
Many clear plastic materials including many cellophane package covers and most clear plastic
boxes are stressed during their manufacture. Crumple or fold a piece of cellophane and place it
between crossed polarizers on the overhead projector. You should see brilliant colors. At any
location the color seen will be the compliment of the color that is removed because for that color
the cellophane acts as a full wave plate and maintains the plane polarization of the light allowing
it to be blocked by the second polarizer. Similar colors will be seen for clear plastic boxes such
as those used to store CD's or cassette tapes.
MODERN PHYSICS
Fluorescence
Use a black light to illuminate various fluorescent objects. Note that the color seen is not present
in the illuminating "light." You can also use blue or violet light to illuminate a fluorescent orange
or yellow card showing that the wavelengths seen are longer than those of the illumination. One
possible source of this blue light is from the spectrum of white light formed by a grating
attached to an overhead projector. Among the interesting fluorescent materials are starch,
teeth, fluorescent chalk, and laser dye. You can also point out the fluorescence going on in the
Glo Doodler.
Cloud Chamber
Tracks of high speed charged particles become visible when the charges along the path provide
condensation nuclei for a super saturated vapor. You will need to prepare the chamber (in the
location where it will be viewed) about 20 minutes beforehand.
Remove the glass top from the chamber and apply anti fog
solution to its top surface. With the chamber removed from its
base, prepare the cloud chamber by pouring several ounces of
denatured alcohol on each of the two felt wicks at the sides of
the top of the chamber. Then pour enough alcohol on the floor
of the chamber to barely cover it.
(Caution: The alcohol is flamable and its vapor may be explosive! Avoid flames or sparks.)
Place the base of the chamber on a level surface and, if
necessary, use the leveling screws to make sure the chamber will
be level. Fill the aluminum tray in the base with liquid nitrogen
to within about one-half inch of its top.
Place the chamber onto the base
and the glass top onto the
chamber. Being careful not to
block either the brass-fitting
source holder in the side of the
chamber or the thick Plexiglas
window at the lower back,
surround the bottom of the
chamber with the black cloth
skirt to help keep air currents
from the liquid nitrogen area.
Place the slide projector about 18 inches from the back of the chamber so that the light will enter
through the thick block of Plexiglas.
It will take about 20 minutes for the alcohol vapor to get established so that the super saturated region near the bottom of the chamber will reveal particle tracks. The chamber should continue to work for about a half hour before new liquid nitrogen is needed. When you are ready to view tracks turn on the slide projector and dim the room lights.
Once it is dry, rubbing the outside of the top cover with silk will establish an electrostatic field that will help sweep charged droplets to the bottom of the chamber and provide a clear region for new tracks.
With no source in the chamber and the plastic plug rod in the source holder, tracks due to cosmic
rays, radon in the air, and other natural sources should be evident. For frequent alpha particle
tracks, remove the plug from the source holder and insert the thorium-alloy rod. The rod will be
held in the proper position when the plastic rod in which it is imbedded is snug in the source
holder. An alternative source of alpha particles is 241Am from a smoke detector. Such a source is
available on a small stainless steel button mounted in the tip of a modified gray Euro Ball pen.
Remove the tip cover and insert the source into the chamber through the source holder. A dense
spray of alpha particles will enter the chamber. (Actually, the source has been covered with clear
tape in which there is a small hole. Without this alpha-limiting tape too great a flux of alphas
would enter the chamber and after about a second the tracks would no longer be visible because
the appropriate vapor environment would not be maintained.) For electron tracks remove the
plug and other sources and place the 14C disk source in the recess in the outer part of the source
holder and secure this source with the set screw.
Either have the students come by one at a time to view the tracks in the chamber or use the Flex Cam Video Camera (H240) to project the image of the bottom of the chamber onto the screen.
When you are finished with the chamber wipe off alcohol and condensed water. Leave the glass cover off so that the alcohol can evaporate from the wicks. When the chamber is dry and clean, return it and the accessories to shelf (18F)
.
