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PHYS102

  • F5-03: THIN METAL SHEETS - COANDA EFFECT

    F5-03
    Demonstrate a fluid-flow model of the vocal folds.
    Blow down between the thin sheets. Air follows the curve of the sheets, according to the Coanda effect. The reaction force on the sheets pulls them together. When the sheets close, air pressure builds up, opening them and restarting the periodic cycle. We use an air gun for this demonstration so that we can keep the geometry uniform; it is easier during a lecture for the demonstrator to simply blow his or her breath into the region between the plates. This demonstration is often incorrectly explained using the Bernoulli principle. According to the INCORRECT explanation, the air flow is faster in the region between the sheets, thus creating a lower pressure compared with the quiet air on the outside of the sheets. This lower pressure causes the sheets to come together, whence the pressure builds up, forcing them apart, etc.

    This is demonstrably incorrect, as can be seen in the videos below, where one of the sheets is held away from the other and the air stream directed as in the above video. Note that the remaining sheet that is hanging moves toward the center even in the absence of the other sheet, so it does not form a narrow constriction. What is happening here is that the air moves along the surface of the sheet, according to the Coanda effect, leaving in a direction away from the center line of the two sheets. The reaction force on the sheet causes it to move toward the center line of the two hanging sheets. Click your mouse on the photographs below to see these two demonstrations of the Coanda effect.

    f5-03a f5-03b

  • F5-06 BEACH BALL - COANDA EFFECT

    F5-06
    Illustrates the Coanda effect.
    A beach ball can be floated in the air stream provided by an air blower or vacuum cleaner. The ball remains in the air stream even when the air stream is significantly tilted. As the air flows past the ball, the air flow curves around the surface of the ball, due to the Coanda effect. The reaction force on the ball levitates the ball in the airstream.

  • G1-01 EXAMPLES OF SIMPLE HARMONIC MOTION

    G1-01
    Illustrates simple harmonic motion

    This demonstration lets you compare three typical pendula: a simple pendulum (mass on string), a physical pendulum (swinging rod), and a mass on a spring. Any of these produce simple harmonic motion, with a variety of periods. Useful for showing that the same equation describes the motion of any type of oscillating body.
    You can also compare these real-world pendula with some simulated ones:
    1. Erik Neumann's Single Spring simulation
    2. Erik Neumann's Pendulum simulation
    3. PhET Masses on Springs
    4. PhET Pendulum Lab
    FS2

  • G2-01 MASS ON SPRING - HAND HELD

    G2-01
    Demonstrates resonance and phase shift at resonance
    The mass on the spring has a natural frequency, which can be demonstrated by simply holding one end of the spring a rest and allowing the mass to oscillate freely. Demonstrate resonance as follows: (1) With the mass hanging at rest, move your hand very slowly up and down. The mass follows your hand, showing that the mass and the driving force stay in phase for driving frequencies far below the natural frequency of the oscillator. (2) With the mass hanging at rest, move your hand very rapidly up and down. The mass moves opposite to your hand, showing that the mass and the driving force stay out of phase for driving frequencies far above the natural frequency of the oscillator. (3) Move your hand up and down at the natural frequency of oscillation; the phase relationship for resonance is that motion of the driver (hand) must be 90 degrees ahead of the motion of the oscillator. With an almost imperceptible oscillation of your hand, the resonance condition causes the mass on spring to begin to oscillate with a very large amplitude.
    G2

    G2-01A

  • G2-07: PSYCHOACOUSTIC VIBRATION TRANSDUCER

    G2-07
    Challenge your students to recognize pseudoscience while illustrating resonance
    A traditional explanation: "When a group of people concentrate on one of the pendula, held as shown by the instructor, their psychoacoustic brain waves rapidly become in phase, producing enough mechanical energy to make only that pendulum oscillate."

    Of course, this is actually a demonstration of driven resonance - with a bit of practice, via small movements of your hands you can drive any one of the pendula you choose. Encourage your students to analyze pseudoscientific explanations for real phenomena.
    G2

  • G2-12: BARTON'S PENDULUMS

    G2-12
    Demonstrate driven resonance.
    A set of non-coupled pendula are placed on a platform that rocks at the same frequency as one of the center pendula. The rocking motion drives the motion of the pendulum with which it is resonant, but only partially drives the others, showing systematically how a vibrating system responds when the natural frequency is below, at, and above the driving frequency, as seen in the photograph at the right.

    g2-12a

  • G3-28 SUSPENDED SLINKY

    G3-28
    Shows longitudinal and transverse traveling waves & standing waves
    Transverse or longitudinal pulses can be created by appropriate motion of your hand at one end of the SLINKY. Using your hand you can also create transverse standing waves and discuss the overtone series. Gently vibrating one end of the spring (either by hand or using the motor) at the appropriate frequency creates longitudinal standing waves.
    FS1

  • G3-43 WHIP

    G3-43
    Illustrates transverse wave motion.
    A wave started down the whip increases its velocity as the whip decreases in diameter toward the tip. By the time the wave reaches the tip of the whip, the velocity of the whip motion can become greater than the speed of sound in air. The "cracking" of a whip is believed by many physicists to be a result of the sonic boom thus created.

    Please consider carefully how to appropriately present this device in class if used.
    G3

  • G3-51 ROPE WAVE GENERATOR - FREQUENCY VS. WAVELENGTH

    G3-51
    Shows the relationship between frequency and wavelength for fixed tension cord
    Keeping the tension in the rope fixed (same weight on hook) and raising the frequency creates standing waves with shorter wavelength (more loops).
    FS1

  • G4-03: RIPPLE TANK - DOPPLER EFFECT

    G4-03
    Show how wave fronts crowd together in front of and spread out behind a moving source.
    The single point source can be moved by rotating the support arm on a lazy susan. Moving the source uniformly in one direction demonstrates the Doppler effect in a clear and understandable way.
    OS7

    g4-03a g4-03b

  • G4-22: CHLADNI FIGURES - OSCILLATOR DRIVEN

    G4-22
    Show two-dimensional standing waves in a metal plate
    The Chladni plate is a system for creating and illustrating two-dimensional standing waves in a surface. A variety of flat plates can be mounted on the oscillator (including square, circular, and violin-shaped plates). As the plate vibrates, fine white sand is shaken about and traces out the nodal lines of the vibrations of the plate. The system operates by means of magnetostriction. A thin-walled annealed nickel tube is used to drive various Chladni plates. The nickel tube is threaded into the center of the plate, and inserted through a coil under the plate, which rests on a thick felt surface. An oscillator in the 10-30 kHz frequency range drives a 20-Watt audio amplifier to provide the current creating the magnetic field. The field is biased by a small horseshoe magnet to avoid frequency doubling in the tube. A mirror allows larger groups to view the plate easily.
    FS1

  • H1-01 BELL IN VACUUM

    H1-01
    Demonstrates sound wave requirement for a medium

    An alarm-style electric bell is mounted inside a large glass bell jar, with external switches to control both the bell and the pump. This enables the instructor to compare the propagation of sound and light.

    Start the bell, then pump the air out of the jar. Air pressure in the jar is read by the large gauge. As the air is removed, the sound intensity decreases, ultimately to nearly zero. Turn off the vacuum pump when the jar is evacuated and crack the valve open, allowing air to re-enter the jar. As the pressure increases the sound of the bell comes back, but without the noise of the pump.

    Engagement Suggestion
    • Consider asking the students to make predictions before each step - how will removing the air change what they hear? What they see? What will happen as the air returns?
    • Compare this to videos the see of people working in the vacuum of space, in real life and in the movies. What do you see and hear in real life? How is this presented in fiction, and why?
    Background
    There are subtleties to this effect. The pump is not creating a true vacuum within the chamber. The vast majority of the air has been removed, reducing the environment’s ability to transmit sound; but the other (perhaps more important) effect in play is the difference in density between the interior of the chamber and the glass and the external atmosphere; this creates a major change in impedance, causing what little sound can be transmitted within the chamber to reflect back. Also, off course, the bell is not floating in free space, and some vibrations can always be transmitted through the supports and wires.

    For small groups, also consider H1-04, a more portable version of this demonstration.
    FS1

  • H1-03: BELLS

    H1-03
    Use bells as a sound source.
    This is simply a couple of fixed brass bells. Strike the bell with the hammer and listen to the interesting sounds. Talk about vibrations of the bell being transferred to the air and then to your ear. This can be a fun way to get the class's attention for the beginning of a lecture on sound propagation.
    H1

  • H1-11: MICROPHONE AND OSCILLOSCOPE

    H1-11
    Show the wave shape of various sounds.
    This setup can be used to look at the wave shape of the speaking voice, singing, whistling, playing musical instruments, musical synthesizers, oscillators with speakers, etc. The oscilloscope trace shown is that of a baritone voice singing the vowel "ee" with a frequency of about 160 Hz. The photographs from the oscilloscope above compare the sounds of a clarinet (top, resembling a square wave), a crumhorn (center, resembling a sawtooth wave), and a recorder (bottom, resembling a triangular wave).

    A variety of sound sources can be requested separately.
    H1, ME2, ME3

  • H1-21: SPEED OF SOUND - PHASE CHANGE

    H1-21
    Determine the speed of sound.
    A sine wave from a standard function generator is sounded by a small loudspeaker and picked up by a microphone. The original sine wave triggers the oscilloscope, which displays the signal from the microphone. Motion of the microphone away from the loudspeaker, measured by a ruler or the optical rail scale, is accompanied by a phase or time delay, measured by motion of the oscilloscope trace against the underlain gridlines. The speed of sound is the distance the microphone is moved divided by the additional time lag. This works best at frequencies of at least 3000 Hz.

    This is one of our most reliable demonstrations for showing an accurate measurement of the speed of sound in class, but does require a bit of explanation. This can also be an opportunity to discuss measurement equipment and the mathematics behind the process.
    H1, ME2, ME3, OM1, OM2

  • H1-32: WAVETEK AND AUDIO CART - EQUAL SOUND LEVEL STEPS

    H1-32
    Illustrate the effect on the ear of successive changes of exactly 10 dB.
    Setting the generator to a sine wave in the 100-1000 Hz region, the intensity can be changed up and down by 10 dB steps, covering about a 50dB range. Be careful not to exceed the maximum of either the loudspeakers or your ears.

    Invite students to compare their experience of different levels to phenomena they are familiar with, such as conversations and concerts.

  • H2-11: SOUND LENS

    H2-11
    Demonstrate focusing of sound by refraction in a sound lens
    A balloon filled with carbon dioxide acts as a focusing sound lens, due to its convex shape and the smaller velocity of sound in the carbon dioxide. When the lens is inserted between the loudspeaker and the microphone, the sound wave is focused, increasing the sound level at the microphone, as seen on the oscilloscope. The source is either a small chunk of dry ice in a flask or a cylinder of carbon dioxide.

    For comparison, air (very little focusing) and helium (defocusing) balloons can also be provided upon request.

    For good results, position the microphone and the loudspeaker about 40 cm apart, inflate the balloon to about 20 cm diameter, and use a frequency of about 2-4 kilohertz.

    If you use additional balloons of different gases, as mentioned above, have students make predictions about what effect density will have before showing the result.
    H2, OM1, ME2. ME3, I0, FS1

  • H2-21 AUDIBLE YOUNG'S EXPERIMENT - GROUP LISTENING

    H2-21
    Demonstrates interference of sound waves with two coherent sources
    The oscillator-amplifier is set to approximately 3000 Hz, with identical signals being applied to both loudspeakers. Rotating the loudspeakers past the listeners allows them to observe the interference pattern by hearing the alternating maxima and minima in the intensity pattern.
    OS2

  • H2-23: INTERFERENCE - KLINGER TRANSPARENT SLIDES

    H2-23
    An optical analog to the interference pattern from two identical sound sources.
    Small transparencies consisting of a series of equally-spaced concentric rings are superimposed on each other. The interference pattern from these two identical sources can be observed as the distance between the two sources is varied.

  • H2-26: PHASE REVERSAL BETWEEN STEREO SPEAKERS - MUSIC

    H2-26
    Demonstrate interference of sound in a dramatic way.
    Two loudspeakers are connected in the monaural mode to the power amplifier and positioned close together as shown in the photograph at the left above. A switch box in the leads of one of the speakers allows reversal of the phase of that speaker. When music with lots of bass is played, flipping the phase reversal switch causes huge reduction in the amplitude of the bass frequencies. This is a very dramatic effect.

    A nice experiment shows the relation of phase to physical position. Play an 80 Hz tone into the two speakers, then reverse the phase to reduce the sound to virtually nothing. Uncoil the wire from the back of one speaker and move the speaker 12 or 15 feet across the front of the room; the loud bass tone returns! The waves from the two speakers are no longer out of phase. Can easily be combined with H2-27.
    FS1

    h2-26a