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# PNoM 7 Particle Sandbox Intermolecular Forces

 If you download the NetLogo application, this model is included. You can also Try running it in NetLogo Web

## WHAT IS IT?

This model enables students to "sketch" representations of particle systems to explore concepts related to gas behavior and gas particles. It also enables students to simulate an experiment by drawing a particle-to-particle attraction-repulsion graph.

This model is part of the Particulate Nature of Matter (PNoM) Curricular Unit. Most of the models in PNoM use the same basic rules for simulating the behavior of gases. Each model highlights different features of how gas behavior is related to gas particle behavior and adds to new features to the model.

In all of the models, gas particles are assumed to move and to collide, both with each other and with objects such as walls.

In this model, particles can be added, color coded, and sped up or slowed down, by drawing with the mouse cursor in the WORLD. Also, removable and non-removable walls can be added to the model.

## HOW IT WORKS

The particles are modeled as hard balls with no internal energy except that which is due to their motion. Collisions between particles are elastic. The total kinetic energy of the two particles after the encounter is equal to their total kinetic energy before the encounter. Collisions with the wall are not. When a particle hits the wall, it bounces off the wall but does not loose any energy to the wall. It does not gain any energy from the wall, either.

The exact way two particles collide is as follows: 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. In this model, two turtles 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

### Buttons

SETUP - sets up the initial conditions set on the sliders. GO/STOP/ADD ELEMENTS - runs and stops the model. REMOVE/REPLACE RED WALL - Toggles the red walls on and off. REMOVE/REPLACE BLUE WALL - Toggles the blue walls on and off. WATCH-AND-TRACE-A-PARTICLE - highlights a particle and puts its pen down. RESET-PERSPECTIVE used for re-centering the WORLD & VIEW after riding or watching a particle. RIDE-A-PARTICLE - attaches the viewpoint of the observer to a particle. SAVE - saves the current state of the model to an external file (you will need to provide a model name after clicking this button). LOAD - loads a previously saved model state file from the computer (you will need to choose a file after clicking this button).

### SLIDERS

INITIAL-NUM-PARTICLES - the initial number of particles in the model.

### Switches

SHOW-WALL-HITS? - turns visualization of when particles hits the walls (as flashes) ON or OFF. GRAVITY-ON? - turns on the effects on gravity on the particles. The gravitational force (g) is assumed as -9.8 m/s<sup>2</sup>. ATTRACTIVE-FORCE-FROM-GRAPH? - enables the effects of the attraction-repulsion drawing.

### Choosers

VISUALIZE-PARTICLE-SPEED - allows you to visualize particle speeds. For example, selecting "arrows", creates a representation of each particle velocity using a scalar arrow. Selecting "shades" creates representation of each particle speed using a brighter (faster) or darker (slower) shade of the particle's color.

MOUSE-INTERACTION - sets the type interaction the user can do with the mouse in the WORLD & VIEW. Possible settings include: "none - let the particles interact" - particles move about. "draw basic wall" - adds a gray wall under the mouse cursor. "draw red removable wall" - adds a red wall under the mouse cursor which can be alternatively removed and replaced (like a valve) using the REMOVE/REPLACE RED WALL. "draw green removable wall" - adds a green wall under the mouse cursor which can be alternatively removed and replaced (like a valve) using the REMOVE/REPLACE GREEN WALL. "big eraser" - erases all objects (except the yellow box boundary walls) under the mouse cursor. "slow down particles" - reduces the current speed of the particles by 10%. "speed up particles" - increases the current speed of the particles by 10%. "paint particles green" - recolors the particles under the mouse cursor green (other settings include orange and purple). "add green particles" - adds a couple of new particles under the mouse cursor (other settings include orange and purple).

### Adding the effect of gravity

The gray area with X and Y axes next to the model enables you to draw a gravity-height graph and then run the model to simulate the effects of gravity on the whole system.

DRAW ATTRACT/REPEL FORCE GRAPH - Click this button to start drawing on the right side of the view. You can to click any gray point to draw the graph. You need to click this button again once you are done drawing. RESET AXES - Cleans the previous temp-time drawing.

## THINGS TO NOTICE

The mouse interaction can be used while the model is running as well as when it is stopped.

Transparency is used to model flashes of where pressure was transferred to the wall by fading away the color of the flash at locations where a particle hit the wall.

## THINGS TO TRY

Create a model of hot and cold gasses on two sides of the plunger with walls. Explore the effects of temperature on the behavior of the plunger.

Create a model of how the plunger position changes after one of the walls of the plunger is removed.

Simulate the effects of pulling the plunger by putting a lot of particles on both sides of the plunger, closing one side of the syringe with a wall, waiting for the plunger position to stabilize, and finally removing some particles from the closed side of the syringe.

## RELATED MODELS

• GasLab Models
• Connected Chemistry models.

## HOW TO CITE

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

Please cite the NetLogo software as:

To cite the Particulate Nature of Matter curriculum as a whole, please use:

• Novak, M., Brady, C., Holbert, N., Soylu, F. and Wilensky, U. (2010). Particulate Nature of Matter curriculum. http://ccl.northwestern.edu/curriculum/pnom/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL. Thanks to Umit Aslan and Mitchell Estberg for updating these models for inclusion the in Models Library.