In the Spring of 2002, Paul Hewitt and Chris Chiaverina teamed up to present one of the most memorable sessions in a long time; The Best Physics Demo Show. Paul Hewitt is the Author of Conceptual Physics and Conceptual Physical Science. Chris Chiaverina is former President of the American Association of Physics Teachers and co-author of Light Science. Both are award winning educators that have advanced the cause of effective science education across the country.The following are demos presented by Chris and Paul at the NSTA 2002 National Convention in San Diego.
Using a shoebox and any compact disc, students may construct a no-cost spectroscope. Functioning as a diffraction grating, the disc’s pitted surface separates incident light into its component colors. Both emission and absorption spectra are observable using this easily constructed optical instrument.
As the figure indicates, the spectroscope consists of a CD mounted diagonally at one end of a shoebox. The CD may be held in place with either a small piece of tape or by inserting it in a slit cut in the bottom of the shoebox. A second narrow slit, approximately 5-cm long and 5-mm wide, is cut in the shoebox lid. Situated over the CD and parallel to the width of the shoebox, this aperture admits the light to be analyzed.
At the end opposite the CD a viewing window, roughly the size of a postage stamp, is cut in the shoebox’s end panel. Trial and error is used to obtain the optimal spectral display when positioning the CD.
Placing a plastic petri dish over the slit in the top of the shoebox permits the examination of absorption spectra (see figure). When placed in the petri dish, virtually any colored solution will produce dark stripes on a continuous incandescent spectrum. Wakabayashi, et al 1. suggests trying solutions of chlorophyll, cobalt chloride and potassium permanganate.
“We recently used the CD Spectroscope as an extra credit assignment. The evaluation rubric used is as follows:”
- Spectroscope construction – 2 pt.
- Correct spectra key showing spectra of five different light sources (You might consider incandescent sources, standard fluorescent lights, various neon lights, sodium and mercury street lights, and new high intensity car headlamps.) – 2 pt.
- Spectra of additional sources – 1 pt/each (up to 3 pt)
- Attractive exterior (e.g. decorative covering, clever spectra key display, spectroscope label, etc.) – 2 pt.
- Signature of parent(s) or guardian(s) verifying that you have demonstrated your spectroscope and explained its operation to them – 1 pt.
The reaction by students and their parents to this device has been amazing! Students enjoyed observing the spectra of light sources found outside the classroom. Parents, recently visiting school for annual parent-teacher conferences, were clearly thrilled about their child’s involvement in physics.
Chris Chiaverina, New Trier High School
1. Wakabayashi, F; Hamada, K.; Sone, K.; J. Chem. Educ. 1998, 75, 1569
12 Volt Electricity
My favorite demo with electric circuits is the car battery with extended terminals. The extended terminals are simply a pair of rigid rods (welding rods is what I use). They are easily inserted and removed when female connectors are permanently fastened into the battery terminals. Alligator clips at the ends of lengths of wire allow bulbs to be connected to the terminals, showing series and parallel circuits. It is easy for students to see the usefulness of circuit diagrams that look so much like the demo itself. In the sketch to the right we see how an ammeter can be connected to read the line current.
Three identical lamps of resistance 12 ohms are connected to the 12 volt battery as shown below.
- 1. What is the current in each lamp?
- 2. What is the voltage across each lamp?
- 3.What is the power dissipated in each lamp?
- 4. How does the power dissipated in Lamp C change if Lamp A is unscrewed?
- 5.What happens to the power dissipated in Lamp A if Lamp C is unscrewed?
- 1.By Ohm’s Law, Lamps A and B have 0.5 A in them. Lamp C has 1.0 A.
- 2.Voltage across Lamps A and B is 6v each (12v across both). Voltage across Lamp C is 12v.
- 3. Power in A is 3w, and likewise, 3w in B. Power in C is 12w.
- 4.No Change
- 5.No Change
~ Paul Hewitt
Simple Standing Wave
For little over a dollar, you can construct a standing wave device that may be used for demonstrations or quantitative measurements. The standing waves produced by this simple apparatus must be witnessed to be believed.
- 1 – 1 ½ v electric motor
- 1 – AA battery
- 1 – ~1 ½ length of 3/8″ dowel
- 1 – ~1 meter length of string or cord
- 3 – 2″ lengths of insulated wire
- tape (electrical, masking, etc.)
Form vibrator unit by attaching battery to motor with tape.
Solder 2″ leads to the ends of the battery. Solder one of these leads to an electrical contact on the motor. Solder third lead to the other contact on the motor.
- Drill ~1/16″ hole close to end of dowel rod segment. Attach dowel rod segment to motor shaft. Friction should keep dowel rod on motor shaft.
- Tie string or cord to convenient point on vibrator unit.
- To activate vibrator, twist free ends of leads together. You can also use a AA battery holder as an option. Then you can just insert the battery to make the connection.
I. Demonstrating Standing Waves
- Activate vibrator unit.
- Hang vibrator from string. Adjust length of string until a standing wave is obtained. To change number of anti-nodes in standing wave pattern, simply vary the length of the string.
II. Determining Wave Speed – Method I
- Establish standing wave on the string.
- Measure the distance between adjacent nodes. Multiply this distance by two to obtain wavelength of disturbance.
- Use a strobe light to measure frequency of wave.
- Calculate wave speed from v=f·λ
III. Determining Wave Speed – Method II
- Use balance to find mass of vibrator unit and mass of a string sample.
- Calculate weight of vibrator in newtons. This equals tension in string.
- Find linear density of string (µ= mass/length).
- Calculate wave speed from v= √T/µ.
~ Chris Chiaverina
The long plastic bag demo nicely illustrates Bernoulli’s principle. If you were to blow it up by placing it firmly to your mouth, many lung-fulls of air would be needed. But when you hold it in front of your mouth and blow, air pressure in the stream you produce is reduced, entrapping surrounding air to join in filling up the bag. So you can blow it up with a single breath! This is especially effective after your students have counted many of their own breaths in attempting to fill up the bag!
~ Paul Hewitt
The Talkie Tapes demonstrated at the Physics Demo Show were created specifically for the 2002 NSTA National Convention. Additional tapes are available and can be ordered here. All the tapes say “Science is Fun!”.
How to use them:
- Attach the notched end of the Talkie Tape to a sheet of paper, plastic cup, balloon, etc. with a piece of tape. You can also punch a small hole in the bottom of the plastic cup and then tie the pointed end in a knot.
- Slide your thumbnail down the grooved side of the strip and listen carefully. The strip will talk!
- That’s it. No Player. No Batteries. Plays for years. Plays anywhere!
What’s going on:
When a conventional phonograph record is made, a metal cutting needle cuts a squiggly groove in the plastic. The wavy groove corresponds, or is analogous, to the sound wave making the needle vibrate. That is why this technology is called analog recording. When a phonograph record is played, the needle, or stylus, vibrates as it passes through the grooves on the record’s surface. These vibrations correspond to the original sound that made the cutting needle move. The vibrating playback needle is connected to a small electrical generator (a cartridge) to produce an electrical signal. This signal is then amplified and sent to a loudspeaker. With Talkie Tapes, the recorded information is spread out along a 24″ red plastic strip. On one side of the strip is a series of ridges and pits. These high and low points contain information about frequency and loudness of the sound that was used to produce them. During “playback”, your thumbnail takes the place of the needle. The sound is amplified by forcing something with a larger surface area (a cup, paper, balloon, etc.) to vibrate. This vibrating surface, in turn, moves the air molecules that carry the sound to your ears. In this case the cup is used to amplify the sound wave. You can also bite the end of the tape and your head will act as the sound box. Make your own record play-back device:
Get a large piece of poster paper and roll it into a cone. (like a dunce cap). Then take a needle (or straight pin) and push it through the pointed end of the cone. You will also need an old turntable or kid’s record player. Get an old record (you can find these at garage sales or second hand shops) and place it on the moving turntable. Carefully set the needle on to the record, gently holding the cone. You’ll have no trouble hearing the recording. You can also simulate this sound reproduction by flicking the needle with your finger. Your students will be amazed how loud it is. Be sure you tell your students not to try this with their CD’s! That’s a completely different process. Don’t laugh, you’d be surprised!
~ Chris Chiaverina
This apparatus consists of a sphere that represents a raindrop, along with three colored dowels. One dowel is white, representing a ray of sunlight. Another is red, representing a ray of red light, and another blue for a ray of blue light. The angles between the dowels, show the angles between incoming sunlight and outgoing refracted and internally reflected red and blue rays of light. A volunteer crouching in front of your chalkboard shows the class how the only drops that cast light to him or her originate in drops along a bow-shaped region. Hence the geometry of the rainbow’s bow shape.
~ Paul Hewitt
We ended up not showing this demo at the show because the turn out of teachers was so large it would have been very hard to see. But here’s the diagram for setting up dominoes in such a way as to see Einstein’s face.
~ Chris Chiaverina
Atmospheric Pressure Mat
The rubber mat with the 50-gram hook is my favorite for demonstrating atmospheric pressure. It originates with John MacDonald of Boise State University. A sheet of soft rubber with any handle at its center will do. The one John devised consists of a square sheet about a foot on each side, with a 50-gram mass hanger poked through the center. Toss the rubber sheet on any perfectly flat surface—best on the top of a lab stool. Picking the rubber up by a corner is an easy task, because the air gets under it as it is lifted. But lifting it by the middle is another story. As the middle is raised, a low-pressure region is formed because air cannot get in. The rubber sheet behaves as a suction cup, and the entire stool is lifted when the handle is raised.
~ Paul Hewitt
Color Mixing with “More Power”!
Red, blue and green chemical light sticks are used to produce white light by spinning them on an electric drill. A three-inch bolt is used to attach the sticks to the drill. The head on the bolt is first removed with a hacksaw. The bolt is then secured to the drill just as a bit would be. A “sandwich” consisting of alternating nuts and light sticks is then assembled.
That is, a nut is placed on the bolt, followed by a light stick, followed by a nut, etc. The light sticks should be separated by roughly 120 degrees. This assembly is held snugly together by tightening down on the last nut. As the red, blue and green light sticks spin, the eye-brain system melds the three colors and white is perceived. If a band of opaque tape, such as electrical tape, is attached to one of the sticks, the complement of the obscured color will be seen.
~ Chris Chiaverina