The EJS Ideal Gas Expansion model simulates a two-dimensional ideal gas in a square box.   This simulation can be used as part of the activity described in "The Statistical Interpretation of Entropy: An Activity" by Todd Timberlake, to be published in The Physics Teacher.

In the model, the particles are initially all on the left (red) side of the box with otherwise random positions and random velocities (distributed according to a Maxwell distribution). Separate windows show the motion of the gas particles, a plot of the number of particles on each side versus time, and a histogram of the number of occurrences of a given number of particles on the left.  The user can modify this simulation if EJS is installed locally by right-clicking within the plot and selecting "Open Ejs Model" from the pop-up menu item.

EJS Ideal Gas Expansion model was created using the Easy Java Simulations (EJS) modeling tool.  It is distributed as a ready-to-run (compiled) Java archive.   Double clicking the ejs_entropy_IdealGasExpansion.jar file will run the program if  Java is installed.  EJS is a part of the Open Source Physics Project and is designed to make it easier to access, modify, and generate computer models.  Additional EJS models re available.  They can be found by searching ComPADRE for  Open Source Physics, OSP, or EJS.

Please note that this resource requires at least version 1.5 of Java (JRE).