Al Tobias (wat4y) - Office: Rm. 201, (434) 924-0538 - Lab: Rm. 202, (434) 924-6800
Demonstrations are cataloged according to PIRA Bibliography
PLEASE REQUEST DEMOS WELL IN ADVANCE
Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.
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Soap Film Standing WavesProcedure:Don't try this one for the first time in front of a class, it takes a little practice. There are two attachments that go on the mechanical vibrator. One has the driving force offset from center and is useful for observing angular vibrational modes. Start with the oscillator at approx. 10Hz and adjust the amplitude so it is about 1/4 inch or so. For the wire attachment, the 2nd, 3rd and 4th radial modes are at around 25Hz, 50Hz, and 100Hz and for the disk attachment at about 14Hz, 30hz and 60Hz. Other modes can be found in between these frequencies as well. The resonant frequency of the modes is shifted slightly as the film evaporates and thins so adjustments are needed to stay on a resonance for an extended period of time.
Once a resonance is found, turning the knob on the box connected to the strobe and oscillator will change the phase of the triggering signal sent to the strobe. This will allow you to "freeze" the film at any point during its vibrational cycle and is a good aid in the visualization of the wave shape.
Hints:Drive the mechanical vibrator with the low impedance output of the pasco oscillator. Connect input of the phase shift box to the TTL output on the oscillator and the output of the box to the strobe.This demo is best viewed with the naked eye, but for a large class a camera output to a television gives the best results. Make the room as dark as possible.
Equipment:
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