UniServe€Science News, Vol. 4, July 1996

Boltzmann

A kinetic molecular theory demonstrator

Hugh Powell is a lecturer in the Department of Chemistry at the University of Adelaide

The kinetic molecular theory of gases provides an excellent opportunity for the presentation of computer graphics, as it is based on the motion of molecules, and is simple enough to allow calculations of molecular trajectories on a reasonable time scale. Boltzmann simulates and displays the random motion of up to 200 hard-disc atoms in a two-dimensional box, or arena. From random starting positions and directions, and a single initial velocity corresponding to the selected temperature, the atoms move following Newton's laws of motion with no attractive interactions between atoms and perfectly elastic collisions. One can choose the number of atoms, their hard sphere radius and their temperature (mean kinetic energy). The program calculates the pressure generated by collisions of the atoms with the walls of the arena, as well as displaying the distribution of one of a number of quantities, such as the instantaneous speed and kinetic energy of the atoms. The volume of the arena is fixed, but an optional dividing wall allows simulation of the effect of doubling the volume, as well as simulations of effusion of the atoms through a small hole.

The most important part of the display, the square arena, rightly takes up a large portion of the screen, aiding visibility in lecture demonstrations. However, the choice of colours (red atoms against a black background) does not give very good contrast. The rest of the display is rather cluttered, containing all the various option buttons, as well as calculated and predicted properties. With 50 atoms, the simulations run sufficiently quickly for a demonstration only if rather high temperatures (ca. 1500 K) are chosen. This is presumably a problem with the chosen step length that could be easily remedied.

I trialed some demonstrations of the gas laws and diffusion suitable for our first-year course with a group of second year students. The response of the students was enthusiastic, with written comments including `I wish we could have had this last year', `The visual aid is like a second dimension for the lecture' and `I think it is a great way to learn - almost entertaining!'. There were some reservations concerning the visibility of both the simulation box and the calculated pressure, mean speed, etc.

Similar simulations can be performed using the competitive package, GASSIM, from IME Software (for Windows). In this program, the colour of the atoms can be chosen so as to produce maximum contrast for lecture demonstrations. The program is also faster than Boltzmann, and allows more complex simulations including the effect of gravity and a variety of wall types. However, the fact that the temperature cannot be specified by the user is a considerable disadvantage for the sort of simulations appropriate to a typical first-year chemistry course. In addition, I found Boltzmann (US$125) much easier to use, so I would strongly prefer it over GASSIM (AUS$30) despite its higher price. Although the information sent by the developer stated lecture demonstrations were the primary purpose for Boltzmann, I felt that it would be equally suited to student-use in either tutorial or dry-lab situations. I intend to use Boltzmann in my first-year Chemistry lectures this year, and would certainly recommend anyone teaching first-year physical chemistry to give it a trial.

Hugh Powell

hpowell@pene.science.adelaide.edu.au

Requirements: 386 CPU; Windows 3.1; 430K RAM; colour VGA monitor.
Cost: US$125
Supplier: Trinity Software, PO Box 960, Campton NH 03223-0960 USA
Fax: 603-726-3781 Email: trinity@hfk.com URL: http://206.14.56.63/trinity.html