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PHYS122

  • G4-14: SPOUTING BOWL

    G4-14
    Demonstrate resonance in a dramatic way

    Fill the bowl half-way with distilled water only. Make sure your hands are very clean, and wet them slightly before rubbing.

    Rhythmically rubbing the handles of this ornate Chinese bowl sets up an energetic standing wave that will propel the water in the bowl up to a half-meter into the air! Click the link below to see a video of the spouting bowl in action.
    G4

  • 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

  • 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

  • H2-26: 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

  • H2-41 DOPPLER BALL

    H2-41
    Demonstrates Doppler effect

    An electronic device making a loud squeal is turned on and placed inside a foam ball. The ball is then zipped inside a cloth cover hooked to the end of a cord, and whirled about the instructor's head or carefully tossed from person to person. The Doppler effect can easily be heard throughout even a large room.
    Engagement Suggestion:
    • Challenge students to describe other circumstances where they have heard this phenomenon
    Background:

    This is a classic illustration of the Doppler Effect. When a wave source is in motion, the wavelength of the emitted waves is observed to change by an observer along its direction of motion.

    It can be useful to present this in conjunction with an animation or simulation, to illustrate the effect visually; see the relevant page of our Directory of Simulations.
    H2

  • H2-42 DOPPLER EFFECT - TUNING FORK ON STRING

    H2-42
    Demonstrates Doppler effect

    A tuning fork is struck to activate the "clang tone" and whirled about the instructor's head on a string. The Doppler effect can easily be heard in a small classroom or a reasonably quiet lecture hall.
    Engagement Suggestion
    • Encourage students to listen closely to how the pitch changes, and compare it to other similar sounds. Where else do they experience this effect?
    Background
    As the source of the sound waves moves through the air, the wavefronts in the direction of motion are compressed, while the wavefronts in the opposite direction are extended, changing the pitch we hear. Because the fork is rotating, this causes a repeating pattern as the pitch is first higher, then lower, than the natural pitch of the tuning fork.
    H2a

  • H2-52: BEATS AND RESONANCE - TUNING BARS

    H2-52
    To demonstrate beats, and to demonstrate resonance between two identical tuning bar resonators.
    Two identical tuning bars are mounted atop resonators. Adding a small clamp onto one of the tuning bars reduces its frequency. Striking two tuning bars, one with a weight, then produces beats. The frequency of the beats can be adjusted by varying the position of the weight on the bar. Without weights on either bar, strike one of the tuning bars, then hold the other adjacent to the struck bar for a few seconds. If the struck bar is then damped, the sound continues. The second bar is in resonance with the struck bar, and some energy is transferred if they are physically near each other.
    H2

  • H2-53 BEATS - AUDIO OSCILLATORS AND SPEAKERS

    H2-53
    Hear beats
    To obtain beats, set the two oscillators to the same amplitude at very nearly the same frequency. Adjust the frequency of one oscillator to change the beats.
    ME3

  • H2-55: BEATS AND RESONANCE - TUNING BOXES

    H2-55
    Illustrate beats and resonance.
    Adding a small clamp onto one of the tuning bars reduces its frequency. Striking two tuning bars, one with a weight, then produces beats. The frequency of the beats can be adjusted by varying the position of the weight on the bar.
    Without weights on either bar, strike one of the tuning bars, then hold the other adjacent to the struck bar for a few seconds. If the struck bar is then damped, the sound continues. The second bar is in resonance with the struck bar, and some energy is transferred if they are physically near each other.
    This demonstration is similar to demonstration H2-52, except for use of open tuning boxes for resonance. It is a bit louder for use in the lecture halls, but perhaps a bit harder to explain because of the resonant boxes.
    h2

  • H3-61 BEAKER BREAKER

    H3-61
    Breaks a glass beaker with sound

    An audio oscillator and 100 Watt power amplifier are used to drive a heavy-duty horn driver which is mounted in the back of the plastic beaker cavity with the sound emerging through a hole, which can be seen in the photograph. The beaker is positioned on a foam pedestal in front of the speaker hole. A microphone is mounted at 90 degrees from the position of the speaker.

    The beaker is marked with its primary resonant frequency, found in advance using digital spectrum analysis of a recording of the beaker ringing after being tapped. Most beakers have two possible resonant modes 45 degrees apart, due to the weight of the spout; the most effective technique is to drive the resonance with the spout facing directly away from the speaker. Set the frequency of the oscillator as shown on the beaker, with an amplitude of around 140mVpp. The oscilloscope will show two waveforms, the input signal and the signal picked up by the microphone. You may need to adjust the frequency slightly to account for changes in temperature or age since the beaker was tested; slowly shift the frequency by tenths or hundredths of a Hertz to find the amplitude peak (do not try to tune by watching for a displacement in the phase relationship, as there is a time delay between the signals introduced by the hardware). This done, set the strobe around 3000 cycles per minute, and adjust it until you can see the sides of the beaker flexing.

    This can be used to show the resonance of the beaker. You can also, optionally, shatter it, by increasing the input voltage at resonance. Be careful not to exceed 1Vpp.

    After the resonant frequency is found and the amplitude turned up, the oscillation of the beaker can be caused to exceed its elastic limit and thus to shatter. See the video links below to view a slow-motion video of the beaker at the moment it breaks.

    Engagement Suggestion
    • Show the students that there are two different resonant frequencies, and challenge them to develop theories of why this is.
    • Consider using this in conjunction with H3-62 to illustrate the effects of the beaker's spout in a more obvious (and quieter) manner.
    Background
    This process of driven resonance potentially leading to mechanical failure can be related to many engineering problems. This is an excellent opportunity to discuss how physics applies to real-world problems, like the Tacoma Narrows Bridge collapse.
    Also, be sure to explore our directory of oscillations and waves simulations to show other examples of complex mechanical oscillations.
    FS1, LS2, SU5

  • H3-71 STROKED ALUMINUM ROD

    H3-71
    Illustrates longitudinal standing waves in an aluminum rod.
    Apply powdered violin rosin to your fingers or wear a rosined glove and stroke the aluminum rod firmly while holding it at a nodal point. Holding it in the center produces the fundamental, holding at 1/4 of the way from one end produces the second harmonic, holding at 1/6 of the way from one end produces the third harmonic, etc. The rod is about 6 ft long, and the speed of sound in aluminum is about 16,700 ft/sec, so the frequency of the fundamental is about 1400 Hz. The sound is very loud and lasts a long time; the Q for this system is around 100,000!
    Alternatively, request an (optional) mallet to use with the rod. Use the mallet to strike the rod on one end; by holding the rod at a node or antinode, all or some modes can be excited or damped.

  • H4-04 FOURIER ANALYSIS - DIGITAL OSCILLOSCOPE

    H4-04
    Demonstrates the Fourier spectrum of complex waves
    This experiment uses a digital oscilloscope with a fast Fourier transform module to determine the Fourier spectrum, simultaneously displaying the wave shape and the Fourier spectrum on its monitor. Any periodic wave from a wave generator or sound, such as a musical instrument or the singing voice, can be analyzed. A variety of waves can be input from wave generators, such as the standard wave shapes, and a microphone, such as steady-state instrumental or vocal sounds.

    Invite student musicians to bring in their instruments for analysis.
    H4, ME2, ME3

  • H4-34: GUITAR AND OSCILLOSCOPE

    H4-34
    Illustrate how a guitar works
    Play notes or chords on the guitar to see their wave shapes on the oscilloscope. Notice that as the notes decay their wave shapes change, a result of different decay times for different harmonics.
    OS5, ME2, ME3

  • H5-21: THREE DECIBELS

    H5-21
    Demonstrate the logarithmic nature of dBs
    The decibel scale measures sound pressure level logarithmically against a base value. As such, a doubling of sound intensity (such as by doubling the number of approximately equivalent sound generators) creates a 3 decibel increase. This scale was developed as a result of Weber's Law: that a noticeable difference in a stimulus is an increase in a constant fraction.

    To demonstrate that doubling the sound intensity creates a change of 3 decibels in the sound intensity level: Divide the class in two, ask one half to make some noise (eg clapping), and note the sound intensity level. Then ask the whole class to make the same noise, raising the sound intensity level by 3 dB. For large groups, a camera may be provided to display the sound level meter.
    H5

  • J1-01 TRIBOELECTRICITY - CHARGING BY FRICTION

    J1-01
    Demonstrates "charging by friction"
    Rubbing silk on a glass rod makes the glass positive and the silk negative. Rubbing fur on a hard rubber rod makes the hard rubber negative and the fur positive. This effect is known as "triboelectricity," from the Greek "tribein," or to rub. The positively charged glass rod and the negatively charged hard rubber rod can then be used (1) simply to illustrate that electrical charge exists using an electroscope or (2) to perform other electrostatics experiments.
    J1b

  • J1-05 CHARGED BALLOONS

    J1-05
    Demonstrates "charging by friction"
    Rub a balloon on your clothing to give it electrical charge, then stick it on the blackboard, wall, etc. This is essentially the same triboelectric effect as J1-01, but with materials more familiar to students.

    Note that the backplane shown in the photo is for illustrative purposes; most classrooms have walls that work just fine.
    J1b

  • J1-06: FUN-FLY-STICK

    J1-06
    Demonstrates electrostatic fundamentals
    This is a battery operated static electricity generator that allows you to float tinsel shapes above the electrically charged stick. Since like charges repel each other, the negatively charged tinsel floats above the negatively charged stick.
    J1b

  • J1-13: ELECTROSTATIC INDUCTION

    J1-13
    Illustrate charging by induction.
    Run the Van de Graaff for a couple of seconds to develop a small charge on the dome, then turn it off. Hold the two plates touching each other near the dome (but do not draw a spark) with one closer to the dome and one further away. While the plates are near the dome, separate the two plates and then remove them from the area of the dome. The two plates will be equally but oppositely charged, as can be verified using the electroscope.
    J1a, J1b

  • J1-21 ELECTROSTATIC ATTRAC AND REPULS - CHARGED CYLINDERS

    J1-21
    Demonstrates electrostatic attraction and repulsion
    Charge the glass cylinders positive by rubbing with silk, and charge the hard rubber cylinder negative by rubbing with fur. The two positive glass cylinders repel each other, but both are attracted to the negative hard rubber cylinder.
    J1b

  • J1-24 ELECTROSTATIC HAIR RAISING

    J1-24
    Demonstrates electrostatic repulsion
    While standing on a large styrofoam insulating block, touch your hands to the top of the Van de Graaff dome, then have someone turn it on. The fact that your hair stands on end is a result of the repulsion between charges of the same sign that collect on your hair.
    J1a, OS2