When a length of rubber tubing is streched and one end is oscillated, standing waves can be generated. Using a mechanical oscillator, stable standing waves up to the sixth harmonic can be generated. DUE TO STROBOSCOPIC EFFECT, NEITHER THE STILL PHOTOS NOR THE MPEGs OF THIS DEMO CAN ACCURATELY DEPICT IT. You've just got to see it.
This demo should be used to introduce the concept of electrons in an atom as standing waves when discussing the Bohr model of the atom and quantum states. This demo takes about ten minutes to perform.
A good discussion of standing waves in a string can be obtained at http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/waves/u10l4d.html
Bohr obtained values for the relative energies levels of the hydrogen atom by assuming that the electron was, in effect, a string standing wave wrapped into a circular orbit around the nucleus.
- 3 m length sand-filled rubber tubing wih loops on both ends
- vertical oscillator
- switched power strip
- Two demonstrators (or demonstrator and volunteer) hold onto tubing and stretch it tight between them.
- One demonstrator moves the end of the tubing up and down to introduce the fundamental mode of vibration into the tube.
- By increasing the rate of oscillation the second and third harmonic can be induced in the system.
- To demonstrate higher harmonics one end of the tubing is looped around the vertical oscillator arm, the oscillator is turned on, and the speed control is adjusted to obtain stable standing waves.
Demonstrators should wear goggles incase one end of the tubed gets accidently released and recoils.
The oscillator is obtained from the physics demo room.