## Maxwell's Demon

### Week 1

As an introduction to learning how to model things mathematically, we were asked to model Maxwell's Demon. I chose to use Processing for my model because of its ease in graphic interfacing, and I have recently been learning it for another class, so it helped to give me some good extra practice with it.

To begin, the canvas size is set up, and the array of particle objects is instantiated. Then, I draw the bounding box of the model and the gate that is controlled by the demon. Each particle is then moved based on its speed, and displayed on the screen. The particle object includes a random color, a random position and speed (within constraints), a set diameter, and a boolean used to determine if the particle is passing through the gate. Displaying each particle is as simple as drawing a circle at the correct position.

I then ensure that the particles are staying within the boundaries of the model by flipping the sign of its x- and y-velocity when it encounters a wall in that direction. When the particle approaches the gate in the middle of the model, it checks to see if the gate is closed or if the particle wasn't already in the process of passing through, then bounces or lets the particle pass through accordingly.

To determine if the gate is close or not, the "demon" simply looks at from which side and how fast the particle is approaching the gate. The total speed is calculated by taking the square root of the squares of the x- and y-speed. If that total speed is more than half of the set maximum speed of the particles, then it is allowed to pass from the left side of the model to the right side, but will then be stuck on the right side. If the total speed is less than or equal to half of the max speed, the particle is allowed to pass from the right side to the left side, and will then be stuck there.

Here is the my simulation as it appears run through Processing:

As can be seen, the faster moving particles can pass from left to right, but not from right to left, and the slower moving particles can pass from right to left, but not from left to right. Here is code for my simulation with the same parameters for reference:

int gate_width = 10; // Width of the gate in the middle of the model int numParticles = 30; // Number of particles simulated in the model int fill_white = 255; // Fill canvas with white int fill_black = 0; // Fill gate with black int rgb_min = 0; // RGB from 0 to 255 int rgb_max = 255; int particle_diameter = 30; // Diameter of all particles int max_speed = 4; // Max speed of all particles Particle[] particles; // Declare array of particles void setup() { // Set canvas size, initialize all particles size(1200,600); particles = new Particle[numParticles]; for (int i = 0; i < particles.length; i++) { particles[i] = new Particle(); } } void draw() { // Draw white background, draw gate, move and display all particles noStroke(); fill(fill_white); rect(0,0,width,height); fill(fill_black); rect(((width/2)-(gate_width/2)),0,gate_width,height); for (int i = 0; i < particles.length; i++) { particles[i].move(); particles[i].display(); } } class Particle { // Particle object contains color, x and y positions, x and y speeds, color c; // diameter, and passing variable used for passing through the gate. float xpos; float ypos; float xspeed; float yspeed; float diameter; boolean passing; Particle() { // Give particles random color, set diameter, random float r = random(rgb_min, rgb_max); // position and speed (within constraints), and float g = random(rgb_min, rgb_max); // initialize to not passing. float b = random(rgb_min, rgb_max); c = color(r,g,b); diameter = particle_diameter; xpos = random(diameter, (width-diameter)); ypos = random(diameter, (height-diameter)); xspeed = random(-max_speed, max_speed); yspeed = random(-max_speed, max_speed); passing = false; } void display() { // Draw the particle at its position fill(c); ellipse(xpos,ypos,diameter,diameter); } void move() { // If a particle hits edge of canvas, have it bounce; float radius = diameter/2; // if a particle hits the gate while it's closed, have float newXpos = xpos + xspeed; // it bounce; if not, let it pass through. float newYpos = ypos + yspeed; if (((newXpos + radius) >= width) || ((newXpos - radius) <= 0)) { xspeed *= -1; passing = false; } if (((newYpos + radius) >= height) || ((newYpos - radius) <= 0)) { yspeed *= -1; } if (((newXpos + radius) >= (width/2 - gate_width/2)) && ((newXpos - radius) < (width/2 - gate_width/2)) && (xspeed > 0)) { if (!passing && isGateClosed()) { xspeed *= -1; passing = false; } } if (((newXpos - radius) <= (width/2 + gate_width/2)) && ((newXpos + radius) > (width/2 + gate_width/2)) && (xspeed < 0)) { if (!passing && isGateClosed()) { xspeed *= -1; passing = false; } } xpos = newXpos; ypos = newYpos; } boolean isGateClosed() { // Let fast-moving particles pass from left to right side; boolean gateClosed = true; // let slow-moving particles pass from right to left side. float speed = 0.0; speed = sqrt(sq(xspeed) + sq(yspeed)); if (((speed > max_speed/2) && (xspeed > 0)) || ((speed > 0) && (speed <= max_speed/2) && (xspeed < 0))) { gateClosed = false; passing = true; } return gateClosed; } }