applicrtionf and cmalogie~ Toy Flying Saucers and Molecular Speeds Reggie L. Hudson Eckerd College St Petersburg. FL 33733
I have lone noted a certain difficultv manv o f mv beeinnine chemistry siudents have i n undrrstanding;he e ~ " ~ e r i k e n t 2 determination ufgasspeeris and the verification o f the M a x well-Boltamann speed dis~ribution.In re>ponse to this Ihave developed an analoc)' which may be presented t r f o r e showing students the inw wheel or slotted-wheel sneed selector devices displayed i n k a n y t e x t s 3 (Fig. 1). T h e analogy begins w i t h something nearly every student recognizes, a Frisbee" flying saucer.' T h e Frisbee" serves as our molecule while a moving automobile w i l l be the speed selector. Suppose t h a t the car is moving forward a t a k n o w n soeed a n d has i t s r i e h t front a n d l e f t rear windows rolled down. s t a t i o n youseif level w i t h the car's path, with Frisbee" in hand, so t h a t the car i s advancine from vour left. With careful aim, and giving snme consideration w i h e car's speed, vott can toss the Frisbee* in the r i r h t front window and. due i o the car's forward motion, observe the flying saucer exit the
edited by: RON DELORENZO Middle Georgia College Cochran. Georgia 31014
left rear window. If y o u t h r o w too h a r d (high speed), the Frisbee" w i l l hit t h e car's driver a n d if y o u t h r o w t o o l i g h t l y (slow speed), t h e Frisbee" w i l l n o t move fast enough t o exit through the l e f t rear window but w i l l be trapped in the back seat area (Fig. 2). Students readily see t h a t a fast car requires a h a r d t h r o w for t h e Frisbee" t o pass through the t w o open windows while a l i g h t t h r o w is needed for a slowly moving car. T h u s there i s a definite connection between the car's speed (a k n o w n quantity) and the Frisbee% speed (an unknown quantity) in the same way t h a t there i s a connection between the k n o w n
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Tom, Ernest R. and Ellis, George, "Foundations of Chemistly," Holi. Rinehart, and Winston, inc., New York, 1973, p. 118. Diliard. Clyde R. and Goldberg. David E., Themisby: Reactions. Structure, and Properties," Macmillan PublishingCo.. Inc.. New York. 1978, p. 433. Campbell. J. Arthur, "ChemisW: The Unending Fromier," Goodyear Publishing Co., Inc., Santa Monica, California, 1978, p. 217. "Frisbee" " is a registered trademark Of WhamO Mfg. Co., Sari Gabriel. California.
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Figure 2. Bird's eye view of a Frisbeemmoving through a car at four differem times The Frisbee% path is to me right while the car is moving toward ~e boltom of figwe. In (a) F r i s b d entm Me tmnt window on Me passenger side, in (b) and (c) a mminw mmqh me car and me rear window on me&iver's side, and in Id) it passes out the window clearing the car.
A
'Source of molecules
single beam
B
Beam is c h o ~ & + ~ J ~ o l e c u l &distribute into "ramoles." themselves asthose ~heinstdnta with nigner velocit es sample passu move anead and those through the with slower velocities lag behind. slit in disc A, the molecuies are together in one tightly grouped arrangement.
Figure 1. This apparatus can be used to measure the molecules' velocity distribution in a sample of gas. Discs A and B are rapidly rotated on a common axle in the direction shown. The slower moving molecules take longer to reach disc 8 l b n the taster Ones. The relative numbers of moiecules sbiking each section of disc 8 may be determined. M data can be relaw to me dattive velmitii and plotted to give a graph. (This figure is reprinted with permission of Dr. E. R. Twnand Hoit. Rimhart, and Winston.) Volume 59
I
The first molecules to arrive at disc 13 fall in ~ i c t o r ~ i . These are the fastest molecules. The majority strike sector 2.3. and 4 with sector 3 catchina more than any other sector. The slowest moving molecules strike sector 5.
Number 12
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December 1982
1025
motion of a slotted speed selector wheel and the unknown speed of a gas molecu1e.s I have found that nearly all beginning chemistry students are able to follow this analogy with the aid of a simple model or a blackboard drawing and that my subsequent explanation of molecular speed determinations is made easier by this
1026
Journal of Chemical Education
analogy. To raise student interest a story may be attached to the analogy such as that one must toss the Frisheea to stun a fleeing bank robber running on the opposite side of the car. For the curious student, one can easily show that the needed ~ ~ i speed ~ &equals the car's speed muhiplied by me car's widm and divided by the distance between adjacent frontand rear windows.
