applications and analoqies Simple Analogies in General Chemistry Thomas McCullough, C.S.C. St. Edward's University
Austin. TX 78704 Great Plains Indians and Dalton's Law
Long before Europeans arrived in America, the native Indians of the Great Plains roamed freely and hunted buffalo, never bothering to stake claims to the land. Their philosophy was that the land belonged to all and not to one individual or small group. Everyone had free access to the entire range. Similarly in a mixture of gases, the total volume is available to all the molecules without restriction. No special portion of the volume is reserved for any particular gas. Therefore, we write Dalton's law of partial pressures as @ , + p 2 + p3
. . .
) V = ( n l + n z + n3 . . . ) R T
where Vis the total volume and available to all the gaseous species just as the Great Plains were available to all the Indian tribes.
I ~ O NDELORENZO edited by
Middle Georgia College Cochran, GA 31014
Bullets, BB's and the Photoelectric Effect
If you shoot a BB gun at a man in a suit of armor, the pellet will bounce off harmlessly If a beam of low energy light strikes a metal surface the light will be reflected with no change in the metal. To make.it more interesting let's shoot at the armor with dozens of BB guns. As before the pellets still rebound harmlessly. In like manner let's increase the intensity of the light or use many light sources but keep the original wavelength and frequency. Again the light will be reflected completely off the metal surface. High Velocity Bullets and High Frequency Light
The next step is to replace the BB guns with a hunting rifle and high velocity bullets. One shot will pierce the armor easily and damage anyone inside. Shifting from low frequency to high frequency light and shining it on the metal will cause a very different reaction. The light will be absorbed by the metal which, in turn, will eject electrons. This analogy makes use of both intensive and extensive properties. The number of BB's or bullets is extensive as is the amount of light. The velocity of the bullet, like the frequency of the light, is the intensive property.
Volume 69 Number 7 July 1992
543
