# GasLab Free Gas

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Uri Wilensky (Author)

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Model group CCL | Visible to everyone | Changeable by group members (CCL)
Model was written in NetLogo 5.0.4 • Viewed 262 times • Downloaded 54 times • Run 0 times

## WHAT IS IT?

This model is one in a series of GasLab models. They use the same basic rules for simulating the behavior of gases. Each model integrates different features in order to highlight different aspects of gas behavior.

The basic principle of the models is that gas particles are assumed to have two elementary actions: they move and they collide --- either with other particles or with any other objects such as walls.

This model is the simplest gas model in the suite of GasLab models. The particles are moving and colliding with each other with no external constraints, such as gravity or containers. In this model, particles are modeled as perfectly elastic ones with no energy except their kinetic energy -- which is due to their motion. Collisions between particles are elastic. Particles are colored according to their speed -- blue for slow, green for medium, and red for high.

## HOW IT WORKS

The basic principle of all GasLab models is the following algorithm (for more details, see the model "GasLab Gas in a Box"):

1) A particle moves in a straight line without changing its speed, unless it collides with another particle or bounces off the wall.
2) Two particles "collide" if they find themselves on the same patch (NetLogo's View is composed of a grid of small squares called patches). In this model, two particles are aimed so that they will collide at the origin.
3) An angle of collision for the particles is chosen, as if they were two solid balls that hit, and this angle describes the direction of the line connecting their centers.
4) The particles exchange momentum and energy only along this line, conforming to the conservation of momentum and energy for elastic collisions.
5) Each particle is assigned its new speed, heading and energy.

## HOW TO USE IT

Initial settings:

• NUMBER-OF-PARTICLES: the number of gas particles.
• TRACE?: Draws the path of one individual particle.
• COLLIDE?: Turns collisions between particles on and off.
• INIT-PARTICLE-SPEED: the initial speed of each particle -- they all start with the same speed.
• PARTICLE-MASS: the mass of each particle -- they all have the same mass.

As in most NetLogo models, the first step is to press SETUP. It puts in the initial conditions you have set with the sliders. Be sure to wait till the SETUP button stops before pushing GO.
The GO button runs the models again and again. This is a "forever" button.

Monitors:

• PERCENT FAST, PERCENT MEDIUM, PERCENT SLOW monitors: percent of particles with different speeds: fast (red), medium (green), and slow (blue).
• AVERAGE SPEED: average speed of the particles.
• AVERAGE ENERGY: average kinetic energy of the particles.

Plots:

• SPEED COUNTS: plots the number of particles in each range of speed (fast, medium or slow).
• SPEED HISTOGRAM: speed distribution of all the particles. The gray line is the average value, and the black line is the initial average. The displayed values for speed are ten times the actual values.
• ENERGY HISTOGRAM: the distribution of energies of all the particles, calculated as (m*v^2)/2. The gray line is the average value, and the black line is the initial average.

Initially, all the particles have the same speed but random directions. Therefore the first histogram plots of speed and energy should show only one column each. As the particles repeatedly collide, they exchange energy and head off in new directions, and the speeds are dispersed --- some particles get faster, some get slower, and the plot will show that change.

## THINGS TO NOTICE

What is happening to the numbers of particles of different colors? Why are there more blue particles than red ones?

Can you observe collisions and color changes as they happen? For instance, when a red particle hits a green particle, what color do they each become?

Why does the average speed (avg-speed) drop? Does this violate conservation of energy?

This gas is in "endless space" -- no boundaries, no obstructions, but still a finite size! Is there a physical situation like this?

Watch the particle whose path is traced in the drawing. Notice how the path "wraps" around the world. Does the trace resemble Brownian motion? Can you recognize when a collision happens? What factors affect the frequency of collisions? What about the "angularity" of the path? Could you get it to stay "local" or travel all over the world?

In what ways is this model an "idealization" of the real world?

## THINGS TO TRY

Set all the particles in part of the world, or with the same heading -- what happens? Does this correspond to a physical possibility?

Try different settings, especially the extremes. Are the histograms different? Does the trace pattern change?

Are there other interesting quantities to keep track of?

Look up or calculate the real number, size, mass and speed of particles in a typical gas. When you compare those numbers to the ones in the model, are you surprised this model works as well as it does? What physical phenomena might be observed if there really were a small number of big particles in the space around us?

We often say outer space is a vacuum. Is that really true? How many particles would there be in a space the size of this computer?

## EXTENDING THE MODEL

Could you find a way to measure or express the "temperature" of this imaginary gas? Try to construct a thermometer.

What happens if there are particles of different masses? (See "GasLab Two Gas" model.)

How would you define and calculate pressure in this "boundless" space?

What happens if the gas is inside a container instead of a boundless space? (See "Gas in a Box" model.)

What happens if the collisions are non-elastic?

How does this 2-D model differ from the 3-D model?

Set up only two particles to collide head-on. This may help to show how the collision rule works. Remember that the axis of collision is being randomly chosen each time.

What if some of the particles had a "drift" tendency -- a force pulling them in one direction? Could you develop a model of a centrifuge, or charged particles in an electric field?

Find a way to monitor how often particles collide, and how far they go, on average, between collisions. The latter is called the "mean free path". What factors affect its value?

In what ways is this idealization different from the one used to derive the Maxwell-Boltzmann distribution? Specifically, what other code could be used to represent the two-body collisions of particles?

If more than two particles arrive on the same patch, the current code says they don't collide. Is this a mistake? How does it affect the results?

Is this model valid for fluids in any aspect? How could it be made to be fluid-like?

## NETLOGO FEATURES

Notice the use of the `histogram` primitive.

Notice how collisions are detected by the turtles and how the code guarantees that the same two particles do not collide twice. What happens if we let the patches detect them?

## CREDITS AND REFERENCES

This model was developed as part of the GasLab curriculum (http://ccl.northwestern.edu/curriculum/gaslab/) and has also been incorporated into the Connected Chemistry curriculum (http://ccl.northwestern.edu/curriculum/ConnectedChemistry/)

This was one of the original Connection Machine StarLogo applications (under the name GPCEE) and is now ported to NetLogo as part of the Participatory Simulations project.

## HOW TO CITE

If you mention this model in a publication, we ask that you include these citations for the model itself and for the NetLogo software:

• Wilensky, U. (1997). NetLogo GasLab Free Gas model. http://ccl.northwestern.edu/netlogo/models/GasLabFreeGas. Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL.
• Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL.

This model was created as part of the project: CONNECTED MATHEMATICS: MAKING SENSE OF COMPLEX PHENOMENA THROUGH BUILDING OBJECT-BASED PARALLEL MODELS (OBPML). The project gratefully acknowledges the support of the National Science Foundation (Applications of Advanced Technologies Program) -- grant numbers RED #9552950 and REC #9632612.

This model was developed at the MIT Media Lab using CM StarLogo. See Wilensky, U. (1993). Thesis - Connected Mathematics: Building Concrete Relationships with Mathematical Knowledge. Adapted to StarLogoT, 1997, as part of the Connected Mathematics Project. Adapted to NetLogo, 2002, as part of the Participatory Simulations Project.

This model was converted to NetLogo as part of the projects: PARTICIPATORY SIMULATIONS: NETWORK-BASED DESIGN FOR SYSTEMS LEARNING IN CLASSROOMS and/or INTEGRATED SIMULATION AND MODELING ENVIRONMENT. The project gratefully acknowledges the support of the National Science Foundation (REPP & ROLE programs) -- grant numbers REC #9814682 and REC-0126227. Converted from StarLogoT to NetLogo, 2002.

Click to Run Model

```globals
[
tick-delta                      ;; how much we advance the tick counter this time through
max-tick-delta                  ;; the largest tick-delta is allowed to be
init-avg-speed init-avg-energy  ;; initial averages
avg-speed avg-energy            ;; current averages
fast medium slow                ;; current counts
percent-fast percent-medium     ;; percentage of the counts
percent-slow                    ;; percentage of the counts

]

breed [ particles particle ]

particles-own
[
speed mass energy          ;; particle info
last-collision
]

to setup
clear-all
set-default-shape particles "circle"
set max-tick-delta 0.1073
make-particles
update-variables
set init-avg-speed avg-speed
set init-avg-energy avg-energy
reset-ticks
end

to go
[ if collide? [check-for-collision] ]
ifelse (trace?)
[ ask particle 0 [ pen-down ] ]
[ ask particle 0 [ pen-up ] ]
if floor ticks > floor (ticks - tick-delta)
[
update-variables
update-plots
]
calculate-tick-delta

display
end

to update-variables
set medium count particles with [color = green]
set slow count particles with [color = blue]
set fast count particles with [color = red]
set percent-medium (medium / count particles) * 100
set percent-slow (slow / count particles) * 100
set percent-fast (fast / count particles) * 100
set avg-speed  mean [speed] of particles
set avg-energy  mean [energy] of particles
end

to calculate-tick-delta
;; tick-delta is calculated in such way that even the fastest
;; particle will jump at most 1 patch length in a tick. As
;; particles jump (speed * tick-delta) at every tick, making
;; tick length the inverse of the speed of the fastest particle
;; (1/max speed) assures that. Having each particle advance at most
;; one patch-length is necessary for them not to jump over each other
;; without colliding.
ifelse any? particles with [speed > 0]
[ set tick-delta min list (1 / (ceiling max [speed] of particles)) max-tick-delta ]
[ set tick-delta max-tick-delta ]
end

to move  ;; particle procedure
if patch-ahead (speed * tick-delta) != patch-here
[ set last-collision nobody ]
jump (speed * tick-delta)
end

to check-for-collision  ;; particle procedure
;; Here we impose a rule that collisions only take place when there
;; are exactly two particles per patch.

if count other particles-here = 1
[
;; the following conditions are imposed on collision candidates:
;;   1. they must have a lower who number than my own, because collision
;;      code is asymmetrical: it must always happen from the point of view
;;      of just one particle.
;;   2. they must not be the same particle that we last collided with on
;;      this patch, so that we have a chance to leave the patch after we've
;;      collided with someone.
let candidate one-of other particles-here with
[who < [who] of myself and myself != last-collision]
;; we also only collide if one of us has non-zero speed. It's useless
;; (and incorrect, actually) for two particles with zero speed to collide.
if (candidate != nobody) and (speed > 0 or [speed] of candidate > 0)
[
collide-with candidate
set last-collision candidate
ask candidate [ set last-collision myself ]
]
]
end

;; implements a collision with another particle.
;;
;; THIS IS THE HEART OF THE PARTICLE SIMULATION, AND YOU ARE STRONGLY ADVISED
;; NOT TO CHANGE IT UNLESS YOU REALLY UNDERSTAND WHAT YOU'RE DOING!
;;
;; The two particles colliding are self and other-particle, and while the
;; collision is performed from the point of view of self, both particles are
;; modified to reflect its effects. This is somewhat complicated, so I'll
;; give a general outline here:
;;   1. Do initial setup, and determine the heading between particle centers
;;      (call it theta).
;;   2. Convert the representation of the velocity of each particle from
;;      speed/heading to a theta-based vector whose first component is the
;;      particle's speed along theta, and whose second component is the speed
;;      perpendicular to theta.
;;   3. Modify the velocity vectors to reflect the effects of the collision.
;;      This involves:
;;        a. computing the velocity of the center of mass of the whole system
;;           along direction theta
;;        b. updating the along-theta components of the two velocity vectors.
;;   4. Convert from the theta-based vector representation of velocity back to
;;      the usual speed/heading representation for each particle.
;;   5. Perform final cleanup and update derived quantities.

to collide-with [ other-particle ] ;; particle procedure
;;; PHASE 1: initial setup

;; for convenience, grab some quantities from other-particle
let mass2 [mass] of other-particle
let speed2 [speed] of other-particle

;; since particles are modeled as zero-size points, theta isn't meaningfully
;; defined. we can assign it randomly without affecting the model's outcome.
let theta (random-float 360)

;;; PHASE 2: convert velocities to theta-based vector representation

;; now convert my velocity from speed/heading representation to components
;; along theta and perpendicular to theta
let v1t (speed * cos (theta - heading))
let v1l (speed * sin (theta - heading))

;; do the same for other-particle
let v2t (speed2 * cos (theta - heading2))
let v2l (speed2 * sin (theta - heading2))

;;; PHASE 3: manipulate vectors to implement collision

;; compute the velocity of the system's center of mass along theta
let vcm (((mass * v1t) + (mass2 * v2t)) / (mass + mass2) )

;; now compute the new velocity for each particle along direction theta.
;; velocity perpendicular to theta is unaffected by a collision along theta,
;; so the next two lines actually implement the collision itself, in the
;; sense that the effects of the collision are exactly the following changes
;; in particle velocity.
set v1t (2 * vcm - v1t)
set v2t (2 * vcm - v2t)

;;; PHASE 4: convert back to normal speed/heading

;; now convert my velocity vector into my new speed and heading
set speed sqrt ((v1t ^ 2) + (v1l ^ 2))
set energy (0.5 * mass * (speed ^ 2))
;; if the magnitude of the velocity vector is 0, atan is undefined. but
;; speed will be 0, so heading is irrelevant anyway. therefore, in that
;; case we'll just leave it unmodified.
if v1l != 0 or v1t != 0
[ set heading (theta - (atan v1l v1t)) ]

;; and do the same for other-particle
set speed sqrt ((v2t ^ 2) + (v2l ^ 2))
set energy (0.5 * mass * (speed ^ 2))
if v2l != 0 or v2t != 0
[ set heading (theta - (atan v2l v2t)) ]
]

;; now recolor, since color is based on quantities that may have changed
recolor
[ recolor ]
end

to recolor  ;; particle procedure
ifelse speed < (0.5 * 10)
[
set color blue
]
[
ifelse speed > (1.5 * 10)
[ set color red ]
[ set color green ]
]
end

;;;
;;; drawing procedures
;;;

;; creates initial particles

to make-particles
create-particles number-of-particles
[
setup-particle
random-position
recolor
]
calculate-tick-delta
end

to setup-particle  ;; particle procedure
set speed init-particle-speed
set mass particle-mass
set energy (0.5 * mass * (speed ^ 2))
set last-collision nobody
end

;; place particle at random location inside the box.

to random-position ;; particle procedure
setxy ((1 + min-pxcor) + random-float ((2 * max-pxcor) - 2))
((1 + min-pycor) + random-float ((2 * max-pycor) - 2))
end

to-report last-n [n the-list]
ifelse n >= length the-list
[ report the-list ]
[ report last-n n butfirst the-list ]
end

;; histogram procedure

to draw-vert-line [ xval ]
plotxy xval plot-y-min
plot-pen-down
plotxy xval plot-y-max
plot-pen-up
end

```

There are 15 versions of this model.

Uri Wilensky over 11 years ago Updated version tag Download this version
Uri Wilensky over 11 years ago Updated to version from NetLogo 5.0.3 distribution Download this version
Uri Wilensky over 12 years ago Updated to NetLogo 5.0 Download this version