Sunday, 17 July 2011

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PURPOSE: To demonstrate "charging by friction."
DESCRIPTION: 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.
In fact, the electron charge is not transferred by "friction" but by contact through the process of quantum mechanical tunneling. The Demonstration Reference File contains a list, known as the "triboelectric series" which indicates the tendency of materials to become either positively or negatively charged when they are rubbed on (contact) each other.
See Question of the Week #128 for information on using this demonstration to enhance class involvement.
REFERENCES: Available. (PIRA 5A10.10)
Here are some web-based references:

Bill Beaty: The Amateur Scientist pages, good related commentary.

Ron Kurtus: The Triboelectric Series of Materials Causing Static Electricity, with lots of interesting commentary.

Thomas B. Jones, Professor of Electrical Engineering, University of Rochester: Triboelectric Series, shor list with appropriate limiting commentary.

, interesting list with link to the Electrical Discharge Association web page.

EQUIPMENT: Glass rod with silk, hard rubber rod with fur, projection electroscope with bright point source for projection.
SETUP TIME: 10 minutes.
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PURPOSE: To demonstrate Fourier synthesis of complex wave shapes.
DESCRIPTION: Complex waves may be formed using up to twelve harmonics with independently variable amplitudes and phases. Any individual harmonic such as the fundamental (in the photograph above) can be shown on one trace of the oscilloscope, while the sum is shown on the other trace. The wave can be simultaneously seen on the oscilloscope and heard using a loudspeaker with a separate volume control. Digital phase locking of all harmonics allows the frequency to be varied from below 100 Hz to above 1000 Hz while the wave shapes remain fixed, to show that timbre is primarily dependent on harmonic structure, and not on frequency or intensity. Some easily produceable wave shapes are square wave, sawtooth wave, triangular wave and pulse train.
Click on any of the wave shapes displayed below to see and hear an mpeg video containing the synthesis of that wave:
triangular wave square wave
sawtooth wave pulse train
Click here to see a video showing all of the above wave shapes as produced by a signal generator.

Click on the images above to see (and hear) the four basic waveshapes as created by a signal generator and as synthesized using our twelve-channel Fourier synthesizer. Each mpg contains about ten seconds of the wave as produced by a standard wave generator followed by ten seconds of the wave as synthesized using the first twelve harmonics. SUGGESTIONS: Practice for most effective use. Click here to view and print a copy of the Fourier Synthesizer instruction sheet in ".docx" format.
See Question of the Week #325 for information on using this demonstration to enhance class involvement.
REFERENCES: Available. (PIRA 3C50.00)
EQUIPMENT: Fourier synthesizer, four-trace oscilloscope, and loudspeaker on scope/TV cart, as photographed.
SETUP TIME: 10 minutes.
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PURPOSE: To demonstrate how sound waves can be focused by concave reflectors.
DESCRIPTION: Set the oscillator at about 3000 Hz for best results. Install speaker at the focus of one reflector and microphone at focus of second reflector; the oscilloscope views the microphone output. With the positions of all elements optimized the sound from the speaker reflects off its mirror to create a beam of sound, which is focused by the second mirror onto the microphone. All elements may be adjusted to verify the existence of foci.
REFERENCES: Available. (PIRA 3B35.30)
EQUIPMENT: Oscilloscope, oscillator, small speaker and microphone mounted on cross-carriages, two concave parabolic mirrors on fixed mounts, C-clamps to attach rail to cart.
SETUP TIME: 10 min.
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PURPOSE: To demonstrate that sound waves require a medium for propagation.
DESCRIPTION: 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.
SUGGESTIONS: This is actually not as simple as it seems. Although the conclusion is correct, it is not strictly speaking demonstrated using this setup. Rather, this demonstrates an impedance mismatch between the very low pressure air in the jar and the glass, which cannot be moved sufficiently to transmit sound under these conditions. I would use it anyway to make the physics point.
It is interesting to note that the "modern" centrifugal vacuum pump was invented by Joseph Boyle in the early sixteenth century to perform this experiment and determine whether sound propagated as waves (and would not pass through a vacuum) or particles (and would pass through a vacuum).
REFERENCES: Available. (PIRA 3B30.30)
EQUIPMENT: Jar with electric bell, vacuum pump and gauge, pre-assembled as photographed.
SETUP TIME: 10 minutes.
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PURPOSE: To demonstrate ripple tank phenomena.
DESCRIPTION: This ripple tank is small and therefore convenient to move. It has its own built-in stroboscopic light source and overhead projector, and comes equipped with accessories as shown in the photograph at the left; accessories are shown in detail at the right. Experiments which can be carried out with this apparatus include single slit diffraction, double slit interference, Huygens's principle, diffraction around an obstacle, and focusing of waves by a concave mirror.
SUGGESTIONS: This very small ripple tank is not recommended for Physics classrooms.
REFERENCES: Available. (PIRA 3B50.00)
EQUIPMENT: Ripple tank with accessories.
SETUP TIME: 10 minutes.
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PURPOSE: To demonstrate traveling waves.
DESCRIPTION: Make sinusoidal waves by moving the spines at one end of the machine up and down sinusoidally, either with your hand or using the wave generator. Vary the amplitude and the frequency and observe the wavelength. Show semi-quantitatively that the wave speed is the same for all frequencies.
SUGGESTIONS: Use absorb-o-matic device at right in photographs above to prevent reflection at the other end of the machine. The manual written by Shive contains excellent information on both the theory and the use of the wave machine; copies are available in the Lecture-Demonstration office.
REFERENCES: Available. (PIRA 3B22.30)
EQUIPMENT: Shive Wave Machine with absorber.
SETUP TIME: 10 minutes.
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PURPOSE: To demonstrate the concept of resonance and the idea of phase shift at resonance.
DESCRIPTION: 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.