Activity Based Physics Thinking Problems in Thermodynamics: Kinetic Theory |
The next two problems use the program THERMO
*. This program simulates the motion of gas molecules
in a two-dimensional box. The molecules have a size equal to 1 pixel (point)
on the screen. You can do many things with this program. To control it:
Note the menu bar at the top of the screen. You can select any of the items
from the menu bar by pressing the highlighted letter. A menu will pop down.
Continue selecting using the highlighted letter. When you have selected
the item you want, press <ENTER>. Then press <ESCAPE> until
all the menus disappear except the main menubar. You will know you have
pressed it enough times if it says "Spacebar to toggle run/stop"
in the upper left. Then use the spacebar to start and stop the program.
Note the status bar at the bottom of the screen. This tells you which forces
and view options are turned on. (The ones turned on are displayed in white.)
1. Reset the parameters in THERMO by selecting Data/Read settings/Settings
from the menu bar. A box should appear that says "Settings.dat".
Press <ENTER> to accept this and then press <ESCAPE> until
you are back in the simulation. This should reset the parameters as follows:
number of particles = 500 reset type = mono square box size = 100x100 histogram
= on histogram type = velocity forces = none You can reset these values
yourself one at a time using the menubar or you can do them all by selecting
<KeyIn> and following the instructions there. (a) Start the program using the spacebar and observe both the histogram
and the distribution of the particles in the box as the particles move.
Describe your observations at each step in a few sentences. 2. Start with the conditions you finished the last problem with. (a) Turn on Trace by selecting Screen/Trace and pressing <ENTER>.
This will display one of the particles' paths and hide the rest. Watch the path of the particle for at least four or five bounces. Describe
the paths. Use your observations to discuss the question: If the particles
fall in a gravitational field, and if they lose energy by colliding with
other particles, why doesn't the whole gas "run down"? That is,
why don't all the particles eventually lie at the bottom of the box at
rest with all their energy dissipated? a. Suppose that the molecules of water are close-packed. From the density
of water and Avogadro's number, estimate the approximate linear dimensions
of a water molecule (i.e., how big is it measured from one end to another). * To download an executable of the program, click on
the word THERMO. This program is a DOS program. (It can be run from Windows,
but you will probably need to create a PIF file to do so.) To run it you
must also have a "Borland Graphics Interface" (BGI) file appropriate
for your graphics screen in the same directory as the program. For most
computers today, the appropriate file is "EGAVGA.BGI". Both the
program and the BGI file are contained in a "zip" file. Unzip
them into the same directory using PKUNZIP or WINUNZIP. These problems written and collected by E. F. Redish. These problems
may be freely used in classrooms. They may be copied and cited in published
work if the Activity Based Physics Thermodynamics Problems site
is mentioned and the URL given. Web page created and edited by K. A. Vick. To contribute problems to this site, send them to redish@physics.umd.edu.
Maintained by
University of Maryland PERG
Activity Based Physics Thinking Problems in Thermodynamics: Kinetic Theory
(b) Let the program run for a few minutes until the particles are reasonably
uniformly distributed in the box.
(c) Turn on collisions and let the system continue to evolve.
Were the particles in equilibrium at the end of step (b)? At the end of
step (c)? Explain.
(b) Watch the motion of the particle until it travels at least 5 times
the size of the box. How often does it collide in this time?
(c) Turn on gravity.
b. The program THERMO simulates a gas of molecules in a two-dimensional
box. In H&R the derivation of the equation of state of an ideal gas
was given, but in three dimensions. Reconstruct this derivation in two
dimensions and indicate whether or not PV = NkT is true in 2D as well as
in three.
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Last modified June 21, 2002