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inventory control A Teacher-Built Device to Demonstrate Molecular Motion Norman W. Hunter Western Kentucky University Bowling Green. KY 42101 Dale A. Holder Russell County High Schodl Russell Springs, KY Devices for the purpose of demonstrating molecular motion are available from several science supply wmpanies; however, the purchase price (approximately $275) cannot usually be justified due to the meager funds available to most high school chemistry teachers. Many high schools budget less than this for their entire science department! Several methods of constructing a device to show molecular motion have been described in the past by Alvea.' Slabaueh.' Wmllev and McLachlan3 and Von Arx.' '?heeasy-to-guild device described here &st the builder (D. A. H.) absolutelv nothing. I t is constructed entirely from basement stora& room Gems. It consists largely of scnip of rlrur I'lrxi~las,parts from a n old Erector Sot, and from a discaaed d o k a r set. These materials, combined with a n overhead projector, give the chemistry teacher the ability to demonstrate such concepts a s random motion, pressure, temperature, dfision, Brownian motion, and mean free path. The device (see Figs. 1and 2) is basically a 30 x 30 cm Erector Set frame desimed to fit on a n overhead oroiector. . " The frame surrounds a clear Plexiglas center, 15 x 15 cm. A"mo1ecular chamber." 11.5 x 11.5 cm, is wnstructed from four pieces of flexible'plastic from the Erector Set. Each side of the chamber is 11.5 x 3.5 em. When marbles or beads, representing atoms or molecules, are placed in the
' Alyea, H . N . J. Chem. Educ. 1964, 41,A518.
Slabaugh, W . H. J. Chem. Educ. 1953,34 68. 3Woolley, R. H. and McLachlan, D. J. Chem. Educ. 1950.27,187. Von AM, W. S. J. Chem. Educ. 1945,22,57.
Blackhawk Christian School 7400 East State Boulevard Fort Wayne, IN 46815 chamber, the images can be projected onto the screen for viewing by the entire class. The drive system for the device is a DC motor and rear axle from a toy slobcar. One wheel is left on the axle, with a small hole drilled through the wheel slightly off center. Attaching the "molecula; chamber" to this holc should produce an oscillating or camxhafl effect when the motor is ooeratcd tscc lower left of oicturel. The stroke can he adjisted easily, but one of adout 1.25 cm produces vigorous action in the chamber. A small soft s ~ r i n is e used to secure the chamber to the frame. The ~ ~ r i ~ ~ i s ~ oonc the a t eopd ~ o s i t side e of the drive svstem (see Fies. 1 and 2). The motion is geueratld through a small toy transformer (power supply) t h a t permits the operator to use the original hand-held speed control from the slot-cai- set. The speed control is used to vary the speed of the motor, which in turn controls the oscillations of the "molecular chamber". In this way v a r v i n ~amounts of "energy" can be supplied to the "molecules" to represent changing temper& ture effects and other concepts. An important feature of the device is the flexible plastic walls of the "molecular chamber". This chamber somewhat reduces the sound generated during operation, though it still is noisy a t high speed. Also, the plastic allows the chamber to flex during the back and forth stroke giving much more action to the "molecules". Many of the older Erector Sets have metal pieces, rather than plastic. The metal will be a little noisier, but it should provide enough motion to the "molecules".
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Some Concepts Demonstrated by this Device Zkmperahre Effects: Marbles or beads are placed in the chamber. The motor is run at various speeds to simulate different temoeratures. .Random Motion, Meon Free Path, Prenaure: T w o or three mnrhlrs or heads nrr plnerd in the chamber and the device is run at some desired speed. Random motion is seen easily and the mean free path is noted. Pressure is also noted as due to collisions per unit area on the walls of the chamber. As more particle; are added, the mean free path is diminished and the pressure increases.
Frame (30 cm x 30 om)
Molecular Chamber
DC ~0101.
Figure 1. General layout for molecular motion device
Figure 2. Picture of molecular motion device. Volume 69 Number 1 January 1992
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