Hi..I've written a little program that places some charged (negative and positive) particles in a box, and then lets their attractions and repulsions determine their behaviour. I really wanted repulsion to take over regardless of the charges if the particles got too close - but for some weird reason when I put this code in none of the particles will draw. See:

http://www.openprocessing.org/visuals/?visualID=6456

I have commented out the offending bit of code. I'm also losing particles out of the edges of the screen, despite my best efforts to stop them.

Any suggestions welcome, thanks.

Yes, you're right. I would have noticed that in the behaviour if it ran with that code in there....which it won't, which I find puzzling.

Great, thanks. Your comments let me fix the issues. I still left the "repulsion at close range for all particles" bit out as it made the behaviour less interesting. You can uncomment it yourself now, if you're curious to see the difference.

I'm just playing around with stuff. I used to be a scientist, but I gave it up and moved into fine arts practice about 9 years ago. So I did all this chemistry/ physics/ maths at university, which I still remember well - and then later on I did a Fine Arts degree... and maybe I'm just starting to bring my science and arts backgrounds together with this coding art. For some reason a lot of the science I learned in the past seems to be surfacing in my mind and I feel like playing around with it.

It's great to create an accurate model and then start playing around with it, changing the laws of physics to create some interesting behaviour.

I'll try out that 1/r^4 idea.... I had the repulsion at 1/r^2, but it was too strong and resulting in a sort of jerky bouncing that I didn't like.

I thought of that. It effectively creates a toroidal surface. As the particles are sort of forming into chains and following each other around, then I wouldn't want that to be broken by the one of the particles wrapping around to the other side. So the distances would have to be calculated on that toroidal surface (which makes my head hurt).

Now interactive - click and drag particles to move them. Click in empty space to create new particles.

Good suggestion....can't seem to get it to work. Here's my code, if you have any suggestions.....(particle class the same as before) - see OpenProcessing link above.

Code:

float force=100; //attraction/repulsion force
particle[] particles;
int numparticles=2;
float dragfactor=0.50; //stop speeds from accumumulating too much
int selectedindex=-1; // index of the currently selected particle -1=none
float[] lastxpos;
float[] lastypos;

void setup()
{
  size(800,800);
  background(0);
  stroke(255);
  strokeWeight(1);
  noFill();
  smooth();
  lastxpos= new float[25]; //to store positions for release velocity
  lastypos = new float[25];
  //frameRate(30);
  particles = new particle[numparticles];
  for(int i=0; i<numparticles; i++)
  {
    if (i%2==0) particles[i] = new particle(random(width/10, 9*width/10), random(height/10, 9*height/10), 1, 0, 0);
    else particles[i]= new particle(random(width/10, 9*width/10), random(height/10, 9*height/10), -1, 0, 0);
  }
  if(mousePressed) //to store positions for release velocity
  {
    for(int i=0; i<lastxpos.length; i++)
    {
	lastxpos[i]=lastxpos[i-1];
	lastypos[i]=lastypos[i-1];
    }
    lastxpos[0]= particles[selectedindex].xpos;
    lastypos[0]= particles[selectedindex].ypos;
  }
}


void draw()
{
  background(0);
  for(int i=0; i<numparticles; i++)
  {
   
    float xaccel=0, yaccel=0;
    for(int j=0; j<numparticles; j++)
    {
	float ijdist = dist(particles[i].xpos, particles[i].ypos, particles[j].xpos, particles[j].ypos);
	float theta = atan2(particles[i].ypos-particles[j].ypos, particles[i].xpos-particles[j].xpos);
	  
	 
	  
	  if(ijdist>20) //attractive or repulsive forces depending on charges
	  {
	    xaccel+=  particles[i].charge * particles[j].charge * (force/(ijdist*ijdist)) * cos(theta); 
	    yaccel+=  particles[i].charge * particles[j].charge * (force/(ijdist*ijdist)) * sin(theta); 
	  }
	  
	  else if(ijdist>0.1) 
	  {
	    xaccel+=  (force/(pow(ijdist,4))) * cos(theta); 
	    yaccel+=  (force/(pow(ijdist,4))) * sin(theta); 
	  } 
	  
	  
	  particles[i].xspeed+=xaccel;
	  particles[i].yspeed+=yaccel;
   }
   particles[i].move();
   particles[i].render();
  }
 
}

void mousePressed()
{
   for(int i=0; i<numparticles; i++)
   {
     if (dist(mouseX, mouseY, particles[i].xpos, particles[i].ypos)<8)
     {
	 selectedindex=i;
	 break;
     }
   }
   
   if (selectedindex==-1)
   {
     float randno= (int)random(0, 2);
     if (randno==0) 
     {
	 particles = (particle[])append(particles, new particle(mouseX, mouseY, 1, 0, 0));
     }
     else particles= (particle[])append(particles, new particle(mouseX, mouseY, -1, 0, 0));
     numparticles++;
   }
   else
   {
     for(int i=0; i<lastxpos.length; i++)
     {
	 lastxpos[i]=particles[selectedindex].xpos;
	 lastypos[i]=particles[selectedindex].ypos;
	//when mouse pressed set all values of arrays to current position
     }
   }
}

void mouseDragged()
{
   if(selectedindex!=-1)
   {
     particles[selectedindex].xpos=mouseX;
     particles[selectedindex].ypos=mouseY;
   }  
}

void mouseReleased()
{
  if(selectedindex!=-1)
  {
    particles[selectedindex].xspeed=(lastxpos[0]-lastxpos[24])/25;
    particles[selectedindex].yspeed=(lastypos[0]-lastypos[24])/25;
  }
  selectedindex=-1;
} 

Yes....that works fine. Thanks.

I was initially trying to use pmouseX and pmouseY - but didn't work for some reason. Then I got even more complicated with storing arrays of past positions, but that didn't work either.