How Things Work - Chapter 13 Demonstrations

To remind the students about resonant systems and resonant energy transfer, you might want to revisit the demonstration of resonant energy transfer to a pendulum in Section 7.2. To remind students about transverse waves, you might want to repeat the demonstration of transverse waves on a Slinky in Section 7.3.
Demonstration 13.1.1:  A Tank Circuit
Description: You charge a capacitor and then connect it to an inductor to form a tank circuit. As shown by an oscilloscope, the charge sloshes back and forth through the tank circuit in a natural resonance.
Purpose: To show that a tank circuit has a natural electronic resonance.
Supplies:
1 large non-electrolytic capacitor (a low-loss capacitor)
1 large inductor (a low-loss inductor)
1 battery
1 oscilloscope (preferably a storage oscilloscope so that you can view the trace for a long time)
1 single-pole double-throw (SPDT) switch
wires
Procedure: Connect the components as shown in the figure below. When the switch is in one position, the battery will charge the capacitor. When the switch is in the other position, the capacitor will be connected to the inductor to form a tank circuit. Keep the electric resistance of the components low to allow the charge to oscillate back and forth through the tank circuit for a long time.
To do the demonstration, first charge the capacitor and then flip the switch to form the tank circuit. The charge will oscillate in the tank circuit and the oscilloscope will display the changing voltages across the capacitor and inductor as they evolve in time. Remember to identify the two axes of the oscilloscope to the students, who will find the device unfamiliar.
Explanation: Charging the capacitor gives it electrostatic potential energy. This energy will become magnetic energy in the inductor when the capacitor sends its separated charges through the inductor. The inductor will use this magnetic energy to recharge the capacitor upside down. The process then repeats. The oscilloscope displays a history of this alternating current flow.
Demonstration 13.1.2:  A Radio Transmitter and a Nearby Antenna
Description: A small radio transmitter emits radio waves from its short vertical antenna. A nearby antenna receives those radio waves and uses their power to light a light bulb.
Purpose: To show that radio waves travel through empty space and carry power with them.
Supplies:
1 simple radio transmitter with a short vertical antenna (because of the short antenna, the transmitter must operate in the 100+ MHz range. Be sure not to violate any FCC regulations.
1 simple radio antenna that's tuned to receive the transmission (it's length should be twice that of the transmitting antenna). The antenna should consist of two halves and a light bulb should connect its lower half to its upper half.
Procedure: Turn on the radio transmitter so that charge begins to oscillate up and down its antenna. Hold the receiving antenna vertically, a meter or so away. The light bulb will glow. The moving charge on the transmitting antenna is causing charge to move on the receiving antenna. This moving charge deposits energy in the filament of the bulb and the bulb glows.
Now show that the effect diminishes as you move the receiving antenna away from the transmitting antenna—the electromagnetic fields from the transmitting antenna spread out and become weaker with distance.
Finally, hold the receiving antenna horizontally and show that the bulb doesn't light at all. That's because the electromagnetic waves from the vertically oriented transmitting antenna are vertically polarized and a horizontally oriented receiving antenna can only receive horizontally polarized waves. A vertically polarized radio wave will push charge up and down on an antenna, not sideways. Since the charges on the horizontal receiving antenna can't move up and down, no current flows in the receiving antenna.
Explanation: The electromagnetic fields from the transmitting antenna are causing currents to flow in the receiving antenna. These currents heat the filament of the bulb red hot.
Description: You turn on a radio transmitter and the static on an FM receiver suddenly disappears—the receiver is silent. When you then begin to FM modulate the transmitted wave, the receiver begins to emit sound.
Purpose: To show how radio waves are transmitted and received and to show how modulating those waves allows them to carry sound information.
Supplies:
1 radio transmitter that works in the normal FM band and that can be FM modulated by a small input signal
1 transmitting antenna
1 tape or CD player to FM modulate the radio transmitter
Procedure: Attach the transmitting antenna to the transmitter and turn on the transmitter. Tune the receiver until you find the silent transmission. Show that when you turn off the transmitter, the receiver begins to look for a transmission and you hear the hiss of "static." Now turn on the transmitter and attach the tape player or CD player to it (or even a microphone). Turn on the tape player or CD player. The receiver should begin to reproduce the sound.
Explanation: The unmodulated radio wave represents a silent period in an FM transmission. But when you begin to shift the frequency of the radio wave back and forth, the FM receiver recognizes these shifts and uses them to shift the air pressure at its speaker back and forth. You then hear sound.
Follow-up: Try a similar experiment with AM modulation. However, the wavelengths involved in normal AM transmission are very long and the antennas become more complicated. Still, you can work with antennas that are too short if you don't care about perfection. If you AM modulate a radio transmission in the AM band range, an AM receiver will pick it up and produce sound.
Demonstration 13.1.4:  The Wavelength of a Radio Wave (Actually a Microwave)
Description: You move one arm of a Michelson Interferometer and determine the wavelength of the microwave emerging from a microwave source.
Purpose: To show that electromagnetic waves really do have wavelengths and that these wavelengths can be measured.
Supplies:
1 microwave source
1 microwave detector
2 microwave mirrors (actually just aluminum plates with bases so that they stand on a table)
1 semi-transparent microwave mirror (50% transmission and 50% reflection)
Procedure: Assemble the components to form a Michelson interferometer (see the figure below). The microwave source and the microwave detectors should be 90° from each other around the central semi-transparent mirror.
In operation, half of the wave emerging from the microwave source will bounce off one mirror and half will bounce off the other mirror. When the two partial waves recombine on their way to the detector, they can interfere constructively or destructively, depending on the relative lengths of their trips on the different arms of the interferometer.
If you slowly change the length of one of the legs, the strength of the microwave at the detector will vary up and down. Moving a mirror half a wavelength of the microwave will cause one complete cycle of variation (e.g. from strong to weak and back to strong). If you measure the distance you must move the mirror to complete one full cycle, and double that distance, you have the wavelength of the microwave.
Explanation: Adding half a wavelength to one arm of the interferometer causes the partial-wave in that arm to travel one wavelength further (it covers the added distance twice). Adding a full wavelength to the travel of a partial-wave leaves that partial-wave unchanged.
Demonstration 13.1.5:  The Polarization of a Radio Wave (Actually a Microwave)
Description: You insert a collection of isolated, parallel metal rods into the microwave traveling from a source to a detector. When the rods are oriented in one direction, the microwave is unaffected, but when the rods are turned 90°, the microwave no longer reaches the detector.
Purpose: To show that radio waves and microwaves are typically polarized.
Supplies:
1 microwave source
1 microwave detector
1 microwave polarizer (a collection of thin metal rods mounted in an insulating holder so that they are all parallel to one another and about 1 cm apart)
Procedure: Place the microwave source and the microwave detector about 50 cm apart and point them toward one another. Turn them on and detect the strong microwave traveling from the source to the detector. Now insert the polarizer between the two. Observe that when the rods are oriented in one way (either vertically or horizontally), they permit the microwave to travel unimpeded. But when the rods are rotated 90°, they prevent the microwave from reaching the detector. They reflect it. The microwave is polarized in the direction of the rods in this second orientation.
Explanation: When the rods are oriented along the microwave's polarization, charge can move along the rods in response to the microwave's electric field. This movement of charge reflects the microwave.

# Section 13.2 Microwave Ovens

Demonstration 13.2.1:  Heating a Fluid of Molecular Dipoles with a Changing Field
Description: An array of magnetic arrows (tiny compasses) jitters back and forth as you move a magnet nearby.
Purpose: To show that you can add energy to a fluid of dipoles by causing those dipoles to turn back and forth with a changing electric or magnetic field.
Supplies:
1 array of magnetic arrows (available from a scientific supply company)
1 bar magnet
Procedure: Place the array of magnetic arrows on the table and allow it to settle. Point out that this array represents a fluid that's at low temperature. Each arrow is a molecule in that fluid. To "heat up" the array, wave the bar magnet back and forth near its surface. The arrows will twist back and forth. As they do, they will interact with one another and some of them will start spinning and jittering wildly, even when you take the bar magnet away. The array now represents a fluid that's at a higher temperature.
Explanation: In a real fluid containing molecules that have dipoles (normally electric dipoles), a changing field will cause those molecules to turn back and forth and rub against one another. This rubbing causes them to become hotter and raises the temperature of the fluid.
Demonstration 13.2.2:  Boiling Water in an Ice Cup
Description: A small amount of water is poured into a depression in a large ice cube and the ice cube is placed in a microwave oven. After a few moments, the water begins to boil, even though the ice is still largely intact.
Purpose: To show that water is a much more efficient absorber of microwaves than is ice.
Supplies:
1 large plastic container - a cube about 20 cm on a side
1 small plastic container - a bowl about 8 cm in diameter
1 heavy weight
water
Procedure: Boil some water and allow it to cool to room temperature (boiling eliminates dissolved gases that would otherwise form white bubbles in the ice cube). Put the water into the large container, filling it about ¾ of the way and place the water in the freezer. You may need to insulate it so that it freezes slowly and doesn't crack during the freezing process.
When frozen, remove the container from the freezer and allow it to warm until it reaches its melting temperature. Place the small container on top of the ice, in the center of the container, and place the weight inside the container. Add about 2 cm of ice water to the large container. The weight in the small container should keep that container pressed against the ice and should keep the ice from floating up in the large container.
Return the container to the freezer until the whole ice cube is frozen. Remove the ice cube from the large container and remove the small container from the ice cube. Place the ice cube on a ceramic plate and return it and the plate to the freezer so that they're cold.
When you're ready to do the experiment, transfer the plate and the ice cube to the microwave oven, pour hot water into the depression in the ice cube and turn on the microwave oven. The water will heat much faster than the ice will melt and, if you're fortunate, the water will begin to boil before the ice cube cracks or melts. Even if something goes wrong, it will be clear that the water absorbs far more microwave power than does the ice.
Explanation: The water molecules in the ice are held rigidly in place and can't rotate in response to the microwave electric fields. The water molecules in the liquid water respond easily and absorb most of the microwave power. The water thus heats up while the ice does not. Putting the ice on the cold plate allows its bottom surface to remain solid, since contact with the bottom of the oven would normally melt the ice there.
Demonstration 13.2.3:  Burning Up a CD or DVD in a Microwave Oven
Description: A CD placed in a microwave oven sparks beautifully after about 2 seconds of cooking.
Purpose: To show that the microwave oven contains fluctuating electric fields..
Supplies:
1 microwave oven.
1 CD or DVD .
Procedure: Simply prop the CD or DVD in the microwave so that it is visible through the window. Start the microwave oven and watch for about two second. The aluminum layer will then start to spark wildly. When the sparking stops (after about 1 second), immediately turn off the oven so that it doesn't overheat the CD or DVD and make it smell any worse than necessary.
Explanation: The microwave oven's fluctuating electric fields propel charge through the ultrathin conducting layer(s) in the CD or DVD. The layers cannot handle the current well and overheat quickly. The plastic expands faster than the layer, shreading the layer. The sudden appearance of sharp edges on the fragmented layer allows for sparking.
Demonstration 13.2.4:  Magic Lights in a Microwave Oven
Description: An indicator called "Magic Lights" is placed in a microwave oven and glows brightly while the oven is on.
Purpose: To show that the microwave oven contains fluctuating electric fields..
Supplies:
1 microwave oven.
1 "Magic Lights" -- available on-line from Mr. Microwave and other suppliers for about \$10 .
1 beaker containing a few ounces of water.
Procedure: Prop the Magic Lights inside the oven and turn it on. The neon lamps in the magic lights will glow brightly. If you now add the water, the electric field intensity will be reduced and the Magic Lights will glow less brightly or intermittantly. If there is a rotating platform on the bottom, the Magic Lights will fluctuating in brightness as it moves through regions of strong and weak intensity..
Explanation: The microwave oven's fluctuating electric fields propel charge through the wires inside the Magic Lights and then through the neon gas in its bulbs. The stronger the local electric fields, the brighter the glow. Since the pattern of electromagnetic waves inside the oven exhibits interference structure, the glow internsity varies with location.
Demonstration 13.2.5:  A Clear Incandescent Lightbulb Glows in a Microwave Oven
Description: A clear incandescent lightbulb is placed in a microwave oven and glows brightly while the oven is on.
Purpose: To show that the microwave oven contains fluctuating electric fields..
Supplies:
1 microwave oven.
1 incandescent light bulb, ideally with a clear rather than frosted interior surface.
1 beaker containing a few ounces of water.
Procedure: Prop the bulb in the beaker with the electrical end pointing down into the water. Place it in the oven and turn the oven on. The bulb will glow brightly. If it doesn't, reduce the amount of water. However, be careful not to run the bulb very long with too little water -- it will overheat, overpressure, and explode violently. A few seconds is safe, but after that you should be extremely careful.
Explanation: The microwave oven's fluctuating electric fields propel charge through the wires inside the bulb and into the bulb's gas through the sharp ends of the wires. A glowing plasma forms in the bulb. Depending on the gas content of the bulb (usually nitrogen and/or argon), you'll get different colored glows.
Demonstration 13.2.6:  A Burning Match Launches Plasma Balls in a Microwave Oven
Description: A burning match is proped up in a microwave oven and produces glowing balls of plasma in the air while the oven is on.
Purpose: To show that the microwave oven contains fluctuating electric fields..
Supplies:
1 microwave oven.
1 wooden match .
1 nonconducting support that doesn't contain water--such as oil-based modeling clay.
Procedure: Prop the match vertically in the support and place them in the middle of the microwave oven. Light the match, close the door, and turn on the oven. The carbon of the match will begin to glow with white incandescent heat and will soon begin to emit plasma balls into the air. These balls will rise quickly to the top of the oven and (hopefully) disappear. If one lingers on the ceiling of the oven, immediately turn of the oven or you will scorch its ceiling. The magnetron has to work hard to power those plasma balls, so you will hear the unit strain under the load. You can sometimes trap the plasma balls inside glass or plastic domes briefly, but they will soon melt or break through the domes -- they are extremely hot.
Explanation: The incandescent carbon emits a great deal of electric charge into the air and eventually converts the air into a self-sustaining plasma. Heated by the microwaves, the plasma floats upward due to its buoyancy. This demonstration puts a heavy burden on the microwave oven and can kill its magnetron. But you can probably do it dozen or maybe hundreds of times before the magnetron fails. Just keep things brief.
Demonstration 13.2.7:  Resonant Cavities - For Sound
Description: You hold a vibrating tuning fork in front of an acoustic cavity that's resonant at the same frequency as the tuning fork. The air in the cavity begins to vibrate loudly.
Purpose: To demonstrate the existence of resonant cavities for sound waves. (This is as an analogy to resonant cavities for electromagnetic waves.)
Supplies:
1 resonant acoustic cavity (available from a scientific supply company)
1 tuning fork (resonant at the same frequency as the cavity)
1 tuning fork mallet
Procedure: Strike the tuning fork and note that it doesn't emit much sound—it doesn't compress or rarefy the air very effectively. Now hold the vibrating tuning fork in front of the resonant cavity so that it can begin to transfer its energy to the air in the cavity. The cavity will emit a much louder tone.
Explanation: The tuning fork's vibrational energy moves to the air in the acoustic cavity by resonant energy transfer. In a microwave oven's magnetron, resonant energy transfer continuously adds energy to the electromagnetic waves inside the magnetron's cavities.
Demonstration 13.2.8:  Resonant Cavities – For Microwaves
Description: A cut-off microwave oven magnetron has many resonant cavities in it.
Purpose: To show the tank circuit cavities that define the microwave oven’s frequency and help it produce its microwaves.
Supplies:
1 discarded microwave oven magnetron, cut in half so that its microwave cavities are visible end-on.
Procedure: Simply allow the students to look at the cavities and, hopefully, at the antenna that couples energy out of one cavity and into the cooking chamber. Each cavity is a split ring and behaves as a tank circuit: the arc of the ring is the inductor and the two open ends are the capacitor.
Explanation: During operation, charge moves back and forth between two adjacent cavity-ends. As current flows around each split ring, a magnetic field develops inside that ring, so energy goes back and forth between