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Ideal Gas Expansion
written by Todd Timberlake
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).
1 source code document is available
Subjects Levels Resource Types
Thermo & Stat Mech
- Models
= Ideal Gas
- Second and Third Law
= Entropy
- Statistical Physics
- Lower Undergraduate
- High School
- Instructional Material
= Activity
= Interactive Simulation
Intended Users Formats Ratings
- Learners
- Educators
- application/java
  • Currently 3.0/5

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Access Rights:
Free access
This material is released under a GNU General Public License Version 2 license.
Rights Holder:
Todd Timberlake
EJS, Easy Java Simulations, OSP, Open Source Physics, entropy, equilibrium, free expansion, ideal gas
Record Creator:
Metadata instance created June 17, 2010 by Todd Timberlake
Record Updated:
June 9, 2014 by Andreu Glasmann
Last Update
when Cataloged:
June 17, 2010
Other Collections:

ComPADRE is beta testing Citation Styles!

Record Link
AIP Format
T. Timberlake, Computer Program IDEAL GAS EXPANSION (2010), WWW Document, (
T. Timberlake, Computer Program IDEAL GAS EXPANSION (2010), <>.
APA Format
Timberlake, T. (2010). Ideal Gas Expansion [Computer software]. Retrieved July 13, 2024, from
Chicago Format
Timberlake, Todd. "Ideal Gas Expansion." (accessed 13 July 2024).
MLA Format
Timberlake, Todd. Ideal Gas Expansion. Computer software. 2010. Java (JRE) 1.5. 13 July 2024 <>.
BibTeX Export Format
@misc{ Author = "Todd Timberlake", Title = {Ideal Gas Expansion}, Month = {June}, Year = {2010} }
Refer Export Format

%A Todd Timberlake %T Ideal Gas Expansion %D June 17, 2010 %U %O application/java

EndNote Export Format

%0 Computer Program %A Timberlake, Todd %D June 17, 2010 %T Ideal Gas Expansion %8 June 17, 2010 %U

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The AIP Style presented is based on information from the AIP Style Manual.

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Ideal Gas Expansion:

Is Based On Easy Java Simulations Modeling and Authoring Tool

The Easy Java Simulations Modeling and Authoring Tool is needed to explore the computational model used in the Ideal Gas Expansion.

relation by Mario Belloni

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